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Chapter 12: Sexual reproduction and inheritance.

Prokaryotic organisms, and some eukaryotic ones, reproduce asexually. Usually a single-celled individual splits into two genetically identical individuals. Even in this case, there can be slight genetic differences between the two offspring as a result of mutations, but there is no guarantee. Most eukaryotic organisms, on the other hand, produce offspring by sexual reproduction. Sexual reproduction has ensured that all individuals are at least slightly different. Identical twins may be an exception, but even here, single gene coding mistakes or mutations can result in minor differences.

Without the resulting variability among individuals that sexual reproduction produces, you and your friends would look alike, and you would not be able to distinguish your parents from your neighbors or your dog from any other. All other individuals of each kind of organism would look and function the same. There would be no cultivated food crops because there never would have been a larger tomato, a juicier grapefruit, or higher yielding wheat inflorescence to select as the start of a better crop plant.

Objectives

After completing this chapter, you should be able to:

* Outline the process of meiosis

* Identify where the meiosis process takes place in angiosperms

* Explain how a single-celled zygote is produced

* Describe the differences in the processes of meiosis and mitosis

* Discuss how mutations form

* Recognize the alternation of generations

* Explain double fertilization

* Diagram the inheritance patterns Mendel expressed

* Describe complete dominance and recessive genes

* Demonstrate an understanding of backcrosses

* Draw a Punnett square showing a dihybrid cross, involving two traits

* Discuss the implications of genetics engineering

* Outline how actual genetic code works

* Describe molecular evolution

Key Terms

meiosis

chromosomes

allele pair

bivalent

cytokinesis

interkinesis

anaphase II

telophase II

chiasma

sporophyte

gametophyte

alternation of generations

haploid

microsporogenesis

microgametogensis

megagamete

sporogenous

tapetum

exine

intine

integuments

microphyle

embryo sac

egg cell

synergids

antipoidal cells

endosperm

Gregor Mendel

pure parents

F1

dominant

recessive

hereditary factors

F2

principle of segregation

punnett square

heterozygous

homozygous

backcross

incomplete dominance

monohybrid crosses

dihybrid cross

trihybrid cross

linked

principle of independent assortment

epistasis

pleiotropic

multigenic

genetic engineering

recombinant DNA

protein sequencer

polymerase chain reaction (PCR)

taq polymerase

exons

Meiosis

The key to sexual reproduction is a process of nuclear and cellular division different from that of mitosis (Chapter 7). The critical features of meiosis are the production of daughter cells having only one set of chromosomes, half the number of the parent cells that began the process. Diploid cells have two sets (2n) of chromosomes, half that number, one set (n), is termed the haploid number. The fusion of two haploid cells during fertilization completes the sequence of events in the reproduction cycle, producing a genetically unique organism.

Most sexually reproducing organisms are diploid, which means that all the cells making up the body of the organism, that is, somatic cells, contain a nucleus with two sets of homologous, or similar, chromosomes. These homologues are similar in size, position of their centromere, and most important, their genetic composition. Each of the two chromosomes of a homologous pair contains genes for the same group of traits, and these genes occur in the same sequence from one end of the homologue to the other. It is important to remember that homologues contain genetic instructions for the same traits but not necessarily the same instructions. Thus, the expression of each trait is controlled by two genetic messages; two homologous chromosomes control one gene from each trait. These two genes are called an allele pair or alleles. Alleles are always in the same position (locus) on the two homologous chromosomes. All the functions of each diploid somatic cell, therefore, are genetically encoded twice, once per haploid set of chromosomes. A set of chromosomes is composed of one chromosome from each different homologous pair, as shown in Figure 12-1, and it does not matter which of the two homologues is present.

[FIGURE 12-1 OMITTED]

The following analogy may help you visualize homologous chromosomes. Hold your hands together so that your palms are facing each other and your fingers are touching and pointing in the same direction. Your two thumbs and each of your four fingers are paired, with similar digits across from each other. This is analogous to five homologous chromosome pairs in that each member of each pair is the same size and length, and they look similar. The matching joint creases in your fingers are analogous to the loci for the two genes from the same trait--alleles. Meiosis is a process that occurs only in specific tissues of sexually reproducing organisms. In angiosperms, the total flower is the reproductive structure, but meiosis occurs in the anthers and ovules. In each of these tissues a different sequence of events takes place.

Like mitosis, the process of meiosis is a continuous series of activities, but it has been subdivided into phases for convenience of discussion (see Figure 12-2). Unlike mitosis, meiosis I is the reduction division because it is during this sequence of events that the haploid conditions are produced from a diploid parent cell. The second meiosis division, meiosis II, is similar to mitosis in its activities. For convenience, meiosis is discussed using the subdivision of meiosis I and II with each further subdivision into the phases of nuclear divisions presented for mitosis.

The importance of a reduction division preceding the fusion of two gametes is apparent if you consider what would happen without it. If a diploid organism produced gametes through mitosis, each would be diploid, just like all the somatic cells. Human gametes (egg and sperm) each would have 46 chromosomes, the diploid number for all somatic cells. Fusion of the two gametes would result in a single-celled zygote having 92 chromosomes. Two such individuals, each producing gametes without a reduction division, would have an offspring with 184 chromosomes in each cell. By the fifth generation, the offspring would have 1,472 chromosomes in every cell of every individual. Obviously, this doubling of chromosome number for each generation could not continue for long before there would be an unsolvable space problem. In addition, problems with multiple genetic messages would occur, especially in animal systems.

[FIGURE 12-2 OMITTED]

Most organisms within a given species, therefore, are constant in their chromosome number. As already stated, all humans have a diploid number of 46 chromosomes (23 pairs). By comparison, dogs have 78, horses 74, elephants 56, and chimpanzees 48 chromosomes. The diploid number of plants ranges from a low of 4 in a couple of species to a high of 1,290.

Meiosis I

As in mitosis, the cellular activities of interphase, including G1, S, and G2 phases, prepare the cell for division; these activities include DNA replication to produce chromosome to compose of two genetically identical chromatids united at the centromere. Prophase I is underway when the threadlike chromosomes first become visible. Next, the nuclear membrane starts to break down, and the spindle fibers begin forming. Later, the nucleolus becomes gradually less distinct.

As the chromosome coil becomes more condense and visible, the homologous pairs physically come together. This process of synapsis does not occur in mitosis. Synapsis produces an exact pairing of the two homologues with the same genetic regions side by side for the full length of the two chromosomes. The pairing is evident as soon as the chromosomes become visible. The pairing homologues, each composed of two chromatids, are called a bivalent (bi, "two"; valent, "combined, associated"). Thus, each bivalent contains four chromatids.

During bivalent formation, the nuclear membrane and nucleolus continues to disappear, the spindle forms, and the paired chromosomes become fully condensed and begin moving toward the center of the cell. Prophase I ends at this point.

Metaphase I

Metaphase I begins as the chromosomes arrive at the equatorial plane of the cell and their centromeres attach to spindle fiber. Typically, the bivalents are doughnut shaped because their centromere regions push away from one another during full chromosome condensation. As each bivalent lines up on the spindle, the two centromeres attach to fibers on opposite sides of the equatorial plane. For example, in a plant having a diploid chromosome number of 6, there are three bivalents attached to the spindle at this point in meiosis. Each of these three bivalents contains a chromosome from each of the two parents that formed the reproducing plant. Which member of the homologous pair lines up on which side of the equatorial plane is a random process.

The next event during meiosis results in the actual reduction in the number of chromosomes each daughter nucleus receives. The centromeres of each of the two homologous chromosomes paired during the first part of meiosis I are pulled apart and move toward opposite poles of the cell--this occurs in anaphase I. Each chromosome is still composed of two chromatids. Half the total number of chromosomes goes one direction and half in the other. It is important to remember that in each resulting group, there is one chromosome from each bivalent. Thus, each pole has a complete set of chromosomes with one genetic message for each trait. In a plant where 2n = 6 represents the diploid condition, n = 3 represents the haploid condition that results from meiosis I.

As the separated homologues reach opposite ends of the parent cell, several events occur at once--what is considered telophase I. During telophase I the uncoiling of the chromosomes, the reformations of the nuclear membrane, the reappearance of nucleoli, and the complete disappearance of spindle fibers occur. Here, it is important to point out that in different organisms these events vary in their level of completion. In some organisms, telophase I events never really occur before the chromosomes begin the sequence of activities leading to the second meiotic division. In others, telophase I proceeds almost to completion prior to the start of meiosis II division. Cytokinesis can also occur completely, partially, or not at all, depending on the organism. In instances when telophase I events do take place to completion, there can be a period termed interkinesis, but it differs from interphase in that there is no DNA replication.

Meiosis II

Each of the two daughter nuclei resulting from meiosis I is haploid, having one set of chromosomes composed of two chromatids each. The events that these two haploid daughter nuclei go through are essentially the same as those in mitosis. In prophase II, the chromosomes become tightly recoiled and are visible as having two chromatids each. The nuclear membrane and the nucleolus gradually disappear, and the spindle fibers begin forming. Once the spindle is formed and the chromosomes are fully condensed, the chromosomes move the equatorial plane between the ends of the spindle. This is metaphase II. Each chromosome acts independently, with centromeres attaching to spindle fibers at the equatorial plane.

The centromeres then divide at the beginning of anaphase II, and the two chromatids separate and move toward opposite poles. As these newly formed chromosomes reach the opposite poles, telophase II events begin. At the end of telophase II, there are four resulting daughter nuclei, each one containing a single complete set of chromosomes and each one being genetically different from the others.

Crossing Over

Seemingly, the four haploid daughter nuclei produced by meiosis should be two pairs that contain genetically identical members. In other words, each of the two daughter nuclei resulting from meiosis I will be genetically different because of the random assortment of homologues, but each chromosome in those two daughter cells should have identical chromatids. These chromatids then separate in meiosis II. But the chromatids then separate are not identical, especially because of the process of crossing over, an exchange of genetic material during the early part of meiosis

This exchange is between adjacent chromatids on the two homologous chromosomes of a bivalent. As homologous chromosomes undergo synapsis during early prophase I, each chromosomes is composed of two genetically identical, or sister, chromatid arms of the bivalent condition adjacent to nonsister chromatid arms, which often cross over each other and associate with the outside nonsister chromatid arms of the bivalent for a portion of their length. As the chromosomes become more condensed and visible, this configuration can be seen as a crossover of chromatid segments and is termed a chiasma. Chromosomes average one chiasma per bivalent, which means there are, on average, as many chiasmata in every meiotic division as the haploid chromosome number.

As bivalents reach their most condensed state, the homologues begin to repel one another and physically separate, except at the crossovers. The crossed-over chromatid segments break from their original strand and fuse with the adjacent chromatid of the homologue. This results in an exchange of genetic material between two of the four chromatids in a bivalent. The exchange produces two chromosomes, each composed of two genetically different chromatids.

When the chromatids separate during meiosis II, therefore, the four resulting nuclei all are genetically different from each other. This genetic variability, combined with the mixing of genetic information that occurs as a result of random assortment and separation of homologues during meiosis I, provides the potential for a phenomenal number of original genetic combinations in gametes. To this genetic diversity, add the random possibilities of which two genetically different gametes will fuse during fertilization, and it becomes clear why no two sexually reproducing individuals are ever alike.

Mutations

Although the replication of genetic information normally occurs flawlessly, coding mistakes, or mutations, do occasionally occur. Most mutations are not lethal, and many are thought to have absolutely no effect at all; however, others produce changes in the appearance or function of an organism. Mutations are another source of variability in sexually reproducing organisms. Scientific dogma argued against such "transposable elements." Dr. Barbara McLintock suggested otherwise and won the 1983 Nobel Prize in Medicine.

Between the random assortment of homologues, chiasmata, mutations, and chance combinations of gametes during the process of sexual reproduction, variability is ensured. Variability is essential for the success of organisms as their environments gradually change over many generations.

The Angiosperm Life Cycle

The actual process of meiosis is essentially the same in all sexually reproducing organisms. What the resulting haploid cells (nuclei) are called and what they do next differ. In animals, the haploid cells usually differentiate to become gametes, which fuse during fertilization into a single-celled diploid zygote. That zygote can then begin dividing mitotically to form a multicellular organism that grows, matures, and reinitiates the entire cycle (see Table 12-1).

Alternation of Generations

In plants, however, different sequences of events usually occur. Plants have two generations: sporophyte and gametophyte.

The sporophyte generation produces spores, each of which develops into a gametophyte, which produces gametes. The gametes fuse to form a sporophyte, and the alteration between these phases continues in successive generation. This is called the alternation of generations in plants, as shown in Figure 12-3. The sporophytes are diploid, producing haploid spores by meiosis. These haploid spores divide mitotically to produce the multicellular, haploid gametophytes. Specific cells produced by the haploid gametophytes are gametes, which may fuse with other gametes to form a diploid zygote. The zygote divides mitotically to form the diploid sporophyte stage of the life cycle.

The gametophyte (haploid) stage or generation is the dominant form for lower (nonvascular) plants--the sporophyte begins less evident and smaller. With increasing complexity from the lower to the higher (vascular) plant forms, this relationship gradually changes in the angiosperms (the highest plants), where the sporophyte is the dominant, long-lived, independent group and the gametophytes are small short lived dependent groups of specialized cells that differentiate within the flowers of the sporophyte tissues of the flower, one in the anthers and the other in the ovules. In the anthers, microsporogenesis (the making of small spores) is followed by microgametogensis (the making of small gametes). In the ovules, megasporogenesis is followed by megagametogenesis. The spores produced in the ovules are larger than the spores produced in the anthers and are therefore termed megaspores. Likewise the egg (in the ovule) is larger, a megagamete.

Microsporogenesis and Microgametogensis

In the developing anther of a flower, there are four areas of sporogenous (fertile) cells, one in each of the four sacs comprising the anther. These cells initiate the development sequence to produce pollen grains. The tapetum is a layer of nutritive tissue that provides nourishment for the sporogenous cells as they divide. The tapetum, as well as the sporogenous cells, are diploid. The cells in the sporogenous areas that undergo meiosis are called microspore mother cells, or microsporocytes. Each diploid microspore mother cell produces four haploid microspores. In monocots, cytokinesis normally occurs as each of the two meiotic divisions occurs, whereas in dicots the four haploid nuclei are produced first, then cytokinesis forms all four microspores simultaneously at the end of meiosis.

[FIGURE 12-3 OMITTED]

Each of the four haploid microspores enlarges and develops into a pollen grain, as shown in Figure 12-4. The outer coat of pollen grain, the exine, is a tough layer that makes pollen one of the most persistent plant parts over the entire geologic fossil record of higher plants. The inside layer of the pollen grain, the intine, is not as resistant and is composed of cellulose and pectin produced by the microspore cell. Pollen grains vary in size from less than 20 [micro]m to over 200 [micro]m in diameter and in external morphology from smooth to very ornamented, with ridges, spines, and surface undulation. There are even pollen grains with winglike projections of the exine that aid in wind dispersal.

[FIGURE 12-4 OMITTED]

Microgametogenesis, which occurs inside the developed pollen grain, comprises the gametophyte stage, the gamete-producing organism in the alternation of generations. The haploid microspore nucleus divides mitotically to produce two haploid nuclei: the tube nucleus and the generative nucleus. In some species, this binucleate microgametophyte is the mature pollen. In most species, however, the generative nucleus divides mitotically to produce two haploid gametes, or sperm nuclei, prior to pollen release.

If the pollen is released in the binucleate condition, the generative nucleus divides mitotically sometime during or soon after pollination to produce the two nonmotile sperm nuclei, since both must be present for double fertilization to occur. The tube nucleus is so called because after the pollen grain lands on a receptive stigma, the tube nucleus directs pollen germination through one of the pores in the exine and forms a pollen tube down through the tissue of the style. The tube nucleus produces an enzyme that dissolves intercellular tissue of the style and forms a small tube through which the tube nucleus and two sperm nuclei travel to the ovary.

Megasporogenesis and Megagametogenesis

The ovary of a flower may produce from one to hundreds of ovules, depending on the species. Each ovule is connected to the placental tissue of the ovary by stalklike funiculus. The ovule develops two outer layers, the integuments, which enclose the nucellus tissue within. There is a small opening, microphyle, at the end where the integuments come together. Through this opening, the pollen tube containing the sperm nuclei enters the ovule prior to fertilization.

In the nucleus tissue, there is a single diploid megaspore mother cell that undergoes meiosis and cytokinesis to produce four haploid megaspores (see Figure 12-5). At the conclusion of meiosis, these four megaspores are usually aligned in a chain. Normally the three cells nearest the micropyle degenerate, leaving only one functional megaspore to develop into the megagametophyte.

[FIGURE 12-5 OMITTED]

This single haploid megaspore undergoes three consecutive mitotic divisions to produce two, then four, and finally eight haploid nuclei within a single cell in the ovule. This cell, the embryo sac, is the mature megagametophyte. The eight nuclei are normally arranged with four at the micropyle end and four at the opposite end of the embryo sac. Subsequently one nucleus from each group of four migrates to the middle of the embryo sac. These are the two polar nuclei. Cell walls are synthesized around the remaining three cells at the micropylar end, resulting in a middle egg cell flanked by two synergids. The three nuclei at the other end of the embryo sac also form cell walls and are called the antipoidal cells. Both polar nuclei remain in the center of the original cell, becoming a binucleated central cell. This eight-nucleate, seven-cell condition is the megagametophyte, and single-egg cell is the gamete. Each ovule in a mature ovary contains a megagametophyte or gamete. Each ovule in a mature ovary contains a megagametophyte or gamete "plant" just as each pollen grain contains a two- (or three-) nucleate microgametophyte.

Fertilization

The culmination of sexual reproduction in angiosperms is a unique form of fertilization occurring in the ovule. The pollen tube normally enters the ovule through the micropyle opening. Once adjacent to the egg, it forms a pore in its wall to allow the two sperm cells to exit. Fusion between haploid egg and sperm nuclei occurs, and a diploid zygote cell is formed. This is true sexual fertilization. Angiosperms are said to have double fertilization because the other sperm nucleus fuses with the two polar to form a triploid cell (three sets of chromosomes). This second fertilization is one feature that makes angiosperms unique among plant groups (see Figure 12-6).

[FIGURE 12-6 OMITTED]

The zygote forms the embryo of the seed through repeated mitotic divisions, whereas the triploid endosperm tissue of the seed forms mitotically from the triploid nucleus. The seed coat develops from the ovule's integuments. The new individual becomes relatively dormant in its seed, ready to be dispersed, germinate, and grow to maturity to initiate the reproductive cycle again. It is worth repeating that the most important consequence of sexual reproduction is variability.

Inheritance

Sexually reproducing diploid organisms have two genetic expressions for each trait, one provided by each parent. These two alleles are located at the same position, or locus, on homologous chromosomes. The study of genetics entails an understanding of how these alleles are passed from one generation to the next and how their messages control the appearance of an organism. The appearance of an organism is termed its phenotype; the genetic information controlling it is the genotype. Only the phenotype can be seen; thus, the study of inheritance stems largely from studying the phenotypes of individual traits making up the organisms.

The first such extensive study in which the results were carefully recorded and published was in 1866 by an Austrian monk named Gregor Mendel. Mendel studied a number of traits of the common pea plant (Pisum sativum), each of which had two different expressions. Between 1856 and 1868, he made hundreds of carefully recorded crosses and observations about the inheritance of these traits through several generations. Mendel made these observations and drew sound conclusions about the control of inheritance without knowing about chromosomes and meiosis; these were first described later in the century.

In spite of the stability of gene position as described by classic Mendelian inheritance, it is possible for a gene or groups of genes to be transferred from one chromosome to another.

Mendel's Experiments

"Pure Parents" are those that produce offspring bearing only one expression for a given trait. When Mendel crossed plants that were pure for one phenotype of a given trait (for example, yellow seeds) with plants pure for the other expression of trait (for example, green seeds), the results were probably a bit unexpected. It seems logical that the offspring from such a cross would have some plants with green and some with yellow seeds. But, as Table 12-2 shows, the F1, or first filial (filial, "offspring"), generations for every such cross had only one of the two possible expressions appear. These results were consistent regardless of which parent variety provided the pollen and which provided the ovule. Mendel termed the expression of the trait that appeared in the F1 plants the dominant character, the other he called recessive character. Thus, yellow is dominant and green is recessive for pea color. This means that if the hereditary factors for both yellow and green are present, the expression of the trait will be yellow. The term gene was not proposed until much later, but Mendel's factors are equivalent to genes. Note that the seed characters are expressions of the genotype of the parent plants not that of the embryo (next generation plant) contained therein.

When Mendel let the F1 plants self-pollinate, he noticed that in the next generation approximately one-fourth of the plants had green seeds. As can be seen from these results, the hereditary factor for the recessive expression of the traits reappeared in the F2 generation after being absent in the F1 generation. So the recessive factor can be carried through a generation unchanged even when it is not expressed. This phenomenon held true for each of the other six character combinations studied; the recessive expression was totally absent from all F1 plants that were allowed to self-pollinate. Most important, these recessive characters appeared in the F2 generation in approximately a 25% frequency in every case.

Mendel concluded from these results that each plant has two hereditary factors for each trait studied. When gametes are formed, these two factors segregate and only one factor of each pair is included in each gamete. The formation of a new individual following fertilization has two of these factors coming together, one from each parent, to direct the expression on the trait in question. Mendel's principle of segregation explains how a recessive character can be hidden, but not lost, from one generation to the next.

By combining what is now known about genes, meiosis, and the separation of homologous chromosomes with Mendel's experimental crossing results, an explanation is possible as to how these traits are inherited and controlled. Here, letters represent the two possible alleles of a given trait, the capital letter for the dominant allele and the lowercase of the same letter for the recessive. The mathematical possibilities can be traced for the experimental crosses to show what happens to these genes. Y represents the allele (hereditary factor of Mendel) for yellow, y the allele for green. A pure parent producing yellow seeds would have two alleles for yellow, one on each of a homologous pair of chromosomes and are the same loci on those chromosomes. The genotype for this yellow-seed parent would have a YY. Similarly, a pure parent for green seed would have a yy genotype. During meiosis in each of these two parents, the two alleles for seed color segregate into different gametes produced containing one of the y alleles, 50% the other. In a yellow (yy) x green (yy) cross, therefore, all offspring produced during fertilization will have a Y allele from one parent and a y allele from the other. All the F1 plants thus produced would have a Yy genotype and a yellow phenotype. Although only the phenotype can be seen, the genotype of the F1 is known because the genotypes of the pure parents were known.

This cross also shows how the yellow allele, Y, has complete dominance over the allele for green seed color, y. When there is one of each of these alleles, the phenotype is always yellow. A Punnett square visually demonstrates how alleles for a given trait from two parents segregate. Fifty percent of the gametes receive one of them, and 50% receive the other. Joining each possible gamete type of one parent with each possible gamete type of the other parent is done by connecting each of the two letters across the top of the square with each of the letters down the side of the square. It can be seen that each union of gametes produces a Yy genotype. Four out of four genotype are Yy. And this is termed a heterozygous genotype (hetero, "different"). Both YY and yy genotypes are called homozygous (homo, "same"); YY is homozygous dominant and yy homozygous recessive. Only the homozygous recessive genotype, yy, will produce genotypes that do not include a dominant allele for yellow. (see Figure 12-7).

[FIGURE 12-7 OMITTED]

When Mendel allowed the F1 plants to self-pollinate, he allowed plants all having the heterozygous genotype Yy to interbreed. Each F2 plant produces 50% Y and 50% y gametes because of the gene segregation during meiosis. The resulting F2 genotype frequencies are 1YY:2Yy:1yy. These genotypes produce a 3 yellow:1 green phenotypic ratio because YY and Yy will yield the same phenotype--yellow seed. This 3:1 ratio was approximated by all of Mendel's crosses. Of course, a Punnett square established the theoretical mathematical frequencies for each possibility, and the larger the number of crosses, the closer the observed ratio should approximate the expected ratio. Mendel's results are considered to be amazingly close to the expected, possible frequencies. Remember that Punnett squares reflect ratios, not the actual number of offspring.

Backcrosses

When an offspring is crossed with one of the two parental types that gave rise to it, backcross has occurred. For instance, if one of the F1 plants has a Yy genotype and was backcrossed to the homozygous recessive parent with green seeds (yy), what would be the genotype and phenotypic ratios produced? A Punnett square setting up the backcross would indicate that the resulting progeny should have a 50:50 ratio of Yy and yy genotypes. Since all Yy's will be yellow and all yy's will be green, this will result in a 50:50 yellow/green phenotype yy ratio. The backcross between an F2 (Yy) with the yellow seeds of the F2 offspring and one of its parents (all Yy because they are F1 plants) is also possible; however, there is a problem with visually interpreting the results because there are two possible genotypes, YY and Yy, for yellow seed. All that can be seen is yellow seeds: The genotype cannot be seen directly. In this situation, there is a way to determine the genotype for a plant with a dominant phenotype (in this case yellow seed)--a testcross is performed.

Testcrosses

When the phenotype for a given trait is the dominant expression of that trait, the genotype could be either homozygous dominant or heterozygous. For the yellow-versus-green seeded pea plants just discussed, yellow could be either YY or Yy. To determine which genotype a plant possesses, a testcross is performed. Since the yellow seeded plant has the "unknown" genotype, a cross must be made with a plant with a known genotype. This makes possible offspring with different phenotype, depending on which of the two dominant genotypes the unknown plant possesses. A plant with the homozygous recessive genotype, a green-seeded plant, is used for the known genotype.

Thus, a testcross would be a green-seeded plant (yy) x a yellow-seeded plant (either YY or Yy). Establish two Punnett squares and work through the two possible crosses: yyxYY and yyXYy. You should find that the first cross of homozygous recessive x homozygous dominant would result in 100% yellow-seeded offspring. The second cross of homozygous recessive x heterozygous would result in a phenotypic ratio 50:50, yellow- and green-seeded offspring. Depending on the results of the testcross, therefore, the genotype of the yellow-seeded plant can be determined. Such a cross between plants with dominant and recessive phenotypes will work for any trait that is controlled by complete dominance, as were the seven pea plant traits studied by Mendel.

Incomplete Dominance

In all seven of the pea plant traits studied by Mendel, one allele always showed complete dominance over the other allele for that trait. For other traits, however, this is not always the case: The expression of many traits is controlled by the alleles always blending their contributions. Such genetic controls are said to be examples of incomplete dominance. In Mendel's pea plants, red flowers were completely dominant over white. A heterozygous genotype, therefore, would have a red flower. The genotype frequencies for incomplete dominance crosses are based on the same principle of segregation; only the phenotype expression of heterozygous individuals will differ. In a plant with incomplete dominance for flower color, such as snapdragon, heterozygous plants would be pink. Two pink-flowered plants cross-pollinating would produce 25% red-, 50% pink-, and 25% white-flowered offspring. The 1:2:1 genotypic ratio of homozygous dominant/heterozygous/ homozygous recessive is directly reflected by the phenotype in plants with incomplete dominance, as shown in Figure 12-8.

[FIGURE 12-8 OMITTED]

Dihybrid Inheritance

To this point, we have discussed the inheritance of only one trait at a time, or monohybrid crosses. To consider only a single trait is convenient and serves to demonstrate the basic mechanisms of Mendelian genetics, but it is not a realistic approach. A dihybrid cross involves inheritance patterns for two traits considered simultaneously, a trihybrid cross involves three simultaneous traits, and so on. There are two different situations possible when more than one trait is being studied. The alleles controlling one trait can be on a different chromosome pair from the alleles controlling the other trait, or both allele pairs can be on the same chromosome pair. In the latter case, the allele pairs can be on the same chromosome pair. In the latter case, the two traits are said to be linked. We discuss linkage later in this chapter, but first let us consider dihybrid inheritance of traits located on different chromosome pairs (unlinked traits).

If we were to select two of the morphological traits studied by Mendel, we would study their inheritance patterns together much as we did with a single trait. If a plant that is pure, or homozygous, for round yellow seed is crossed with a plant that has wrinkled green seed, we would have a cross similar to the single trait of seed color examined earlier in the chapter. Each parent plant would be homozygous--one homozygous dominant for both traits, for seed color, the letters R for round r for wrinkled will also be used. The homozygous dominate parent, then, would have a YY/RR genotype with the two Y's representing two alleles for yellow and the two R's representing two alleles for round. The other parent will be homozygous recessive for each trait and would have a genotype of yy/rr. Homologous chromosomes line up and separate during meiosis I independently of other pairs (see Figure 12-9). This is called the principle of independent assortment and is very important in the production of genetic variability.

[FIGURE 12-9 OMITTED]

The more homozygous pairs there are the greatest number of possible alignments. The formula for determining the possible independent alignments is 2n, where n = the number of bivalents involved. Using the formula for figuring the possible combinations in a dihybrid cross, we can see that there are two or four possible combinations of alleles going to different gametes. Three bivalent (2n = 6) gives 23 or 8 possible alignments. There would be eight possible combinations when using a second color for each one of the homologues. Even though each gamete has the same genetic composition for this cross, it is important that the Punnett square be set up using all possible combinations. Of course, only one of the total number of alignments that could occur actually happens in each parent cell undergoing meiosis.

All the F1 plants resulting from this cross are genotypically heterozygous for both traits and all have yellow and round seeds, since both yellow and round expressions are completely dominant over their recessive counterparts. At this point, the dihybrid cross is essentially identical to a sample monohybrid cross, except with two traits. The next step, however--crossing F1 among them--is a different situation. The principle of independent assortment applies in determination of the possible genetic combinations of this cross.

The resulting phenotype is in a 9:3:3:1 ratio of yellow round/ yellow-wrinkled/green-round/green wrinkled. Again, these are ratios of expected phenotypes: The total of 16 does not represent the number of F2 offspring produced.

As complicated as this dihybrid cross was, imagine such a cross with characters displaying incomplete dominance or a trihybrid cross (three nonlinked traits). This use of the Punnett square becomes a bit unwieldy beyond dihybrid crosses, and even trying to represent visually the possible phenotypes in an incomplete dominance dihybrid cross is more confusing than clarifying. The point is that when the study of inheritance includes the more realistic situations involving multiple traits or expressions other than complete dominance, the complexity of such a study is much greater.

Linkage

It should be obvious that many genes occur on a single chromosome, and therefore at telophase I they must segregate as a group. Such genes or alleles are said to be linked. Linkage serves to stabilize the genome, allowing coordinated genes to continue acting together. If all genes sorted independently, there would be a significantly greater chance for unsuccessful recombinations to occur, resulting in poorly adapted individuals. Thus, linkage is important in the balance between genetic stability within a generation and the production of new recombinations and variability. Two linked traits will not produce the typical 9:3:3:1 ratio in the F2 generation of a dihybrid cross (as previously discussed) because the genes are not subject to independent assortment during meiosis. Until the mechanism of crossing over was elucidated by a geneticist working on fruit flies (Drosophila), such varied crossing results were difficult to understand. Now it is easy to see how, the linked traits can become "unlinked" by a crossover. The farther apart physically such genes are, the greater the likelihood that they will cross over; the closer together, the stronger the linkage. However, genes that are very far apart on a long chromosome arm may actually have a higher linkage frequency than others closer together because two crossovers on the same arm can relink them.

Chromosome Mapping

A genetic linkage map shows the relative locations of specific DNA markers along the chromosome. Each marker is like a mile marker along a highway. Any inherited physical or molecular characteristic that differs among individuals and is easily detectable is a potential genetic marker. Markers can be expressed DNA regions (genes) or DNA segments that have no known coding function but whose inheritance pattern can be followed. DNA sequence differences are especially useful markers because they are plentiful and easy to characterize precisely. Markers must be polymorphic to be useful in mapping; that is, alternative forms (alleles) must exist among individuals so that they are detectable among different members in the mapping population.

A mapping population is the group of individuals that will be evaluated for their "score" at a set of markers. This raw mapping data is analyzed by software that constructs the map by observing how frequently the alleles at any two markers are inherited together. The closer the markers are, the less likely it is that a recombination event (a crossover during meiosis) will separate the alleles, and the more likely it is that they will be inherited together. Thus, unlike other types of maps, the distance between points on a genetic map is not measured in any kind of physical unit; it is a reflection of the recombination frequency between those two points. This genetic map unit is measured in terms of centimorgans (cM, named after the geneticist Thomas Hunt Morgan). Two markers are said to be 1 cM apart if they are separated by recombination 1% of the time. The genetic distance tells you little about the physical distance--the actual amount of DNA separating the markers. This genetic to physical distance relationship varies between species, and varies between different spots within the genome of a single species.

A genetic map helps us understand the structure, function, and evolution of the genome. It can be an important tool for agricultural crop improvement. Recent work has shown that the genetic maps of many closely related species (for example, the grains) are quite similar with respect to the content and location of genes, and scientists are trying to determine how the genetic map of one species may be applied to others.

Gene Interaction

Up to this point, inheritance has been discussed in terms of a single pair of alleles controlling one trait. For purposes of demonstrating the mechanism of genetic control and inheritance traits, a one-gene/ one-trait model is easy to understand. However, probably very few traits are exclusively controlled by a single allele pair. It is very likely that most genes do not act alone; rather, interactions of nonallelic pairs of genes control the expression of the phenotype. It is truly remarkable that Mendel was lucky enough to have chosen those seven traits in the garden pea that exhibit complete dominance, no linkage problems, and apparently single allelic control. Apparently they are single-gene traits because it is very possible that other gene interactions also are involved in the control or expression of these traits, even though there is no overt incidence for such action.

Epistasis

One of the most common examples of gene interactions is one gene having a masking effect on the expression of another, nonallelic gene. It is worth noting that this is not a case of dominance because epistasis genes influence nonallelic genes. Dominance is the effect of one allele over the other in an allele pair.

An interesting example of epistasis has been discovered in white clover (Trifolium repens). Hydrocyanic acid (HCN) is present in high concentrations in some strains of white clover, whereas other strains contain none. Some experimental crosses between positive (with HCN) and negative strains have resulted in an unusual F2 ratio for a species previously thought to have a single gene pair control of HCN presence with a positive dominance. For such a plant, the expected ratio of positive to negative in the F2 would be 3:1. When the following results were obtained, it was difficult for the researchers to accept a single-gene pair control:
P    POSITIVE x NEGATIVE
F1   100% positive
F2   351 positive:256 negative


The 351:256 results more closely resemble a 9:7 ratio, which would indicate a modified 9:3:3:1 ratio of a dihybrid cross, indicating two pairs of genes. Epistasis can operate in any cross involving two or more pairs of genes. The results of epistatic action would be a reduction in the number of expected phenotypic expressions in which two or more of the classes become indistinguishable from each other.

It was discovered that white clover has two different enzymes that affect the production of HCN in the following way:
GENE A     GENE B
Enzyme a   Enzyme b


Precursor compound [right arrow] Cyanogenic glucoside [right arrow] HCN

An individual, therefore, must have at least one dominant of each of two pairs of genes, A and B, to produce HCN from the chemical precursors available in all the plants. Without epistasis, a 9:3:3:1 ratio of enzymes a and b/enzyme a only/enzyme b only/neither enzyme would be normal for a dihybrid cross in the F2 generation. The 9:7 ratio would be enzymes a and b (HCN production)/no HCN because one or the other enzyme is absent, and biochemically all the other results are the same due to epistatic influence.

There are other examples of epistasis in corn, mice, chicken, and other animals. The actual effects of the epistatic genes often vary from the example just given, but the general control of such genes is predictable within any genetic system studied.

Pleiotropy

Not only do genes interact to control the expression of a single trait, but many genes affect more than one trait. Single genes that can affect the expression of several traits are said to be pleiotropic, because of the nature of all gene action--the production of proteins and resulting enzymes--it is probable that most genes have some influence on other traits during the development of the organism. Thus, not only does more than one pair often influence the phenotypic expression of a single trait, but also single genes can be involved in the control of many different characteristics.

Multigenic Inheritance

Most of the genetic controls of phenotypic characteristics discussed thus far have been discrete or discontinuous. There is no blending; rather, the phenotype represents one expression or the other. Flowers are red or white, seeds are yellow or green, and seed coats are round or wrinkled. Even considering the effects of incomplete dominance, epistasis, and pleiotropy as modifiers of genetic control, the possible phenotypes change only in degree or number of discrete possibilities. For a trait to vary continuously there must be multigenic or polygenic control.

Multigenic control is one of the combined additive effects of two or more allele pairs of genes. Height, color, and shape of plants and plant parts are typical examples of traits with continuous variations of expression. Fruit or inflorescence size in many plants, and human skin, hair, and eye color also vary in a continuous fashion. The more allele pairs involved in the multiple gene control of a trait, the more continuous is the blending of possible phenotypes. Of course, it is unrealistic to assume that all allele pairs affecting the same trait additively does so with equal influence. Environmental changes and modifier genes also can come to bear on final phenotypic expression. In spite of how little is really known about multigenic inheritance, we can assume that many traits are controlled in this manner, and considerable research effort has been directed at better understanding the phenomenon of multiple gene control of heredity.

Molecular Genetics

Since Mendel's pioneering work, the revelation of DNA structure must be ranked as the most significant discovery in the area of genetics. Since the mid-1970s, the area of molecular genetics has seen incredible breakthroughs in understanding gene action. Researchers study inheritance at the nucleotide level. They have learned to cut, splice, and insert genes into organisms and study the effects. Most of this genetic engineering started with bacteria and viruses but has progressed to the point that many plants have been transformed. For example, corn is now transformed with Bacillus thunguriensis to keep the corn earworms at bay. Roundup soybeans, are resistant to Roundup so the crop can be sprayed with the herbicide. This is only a few of the examples of the use of genetic engineering.

The engineered genes have been used to produce insulin, interferon, human growth hormones, various vaccines, a hormone that makes dairy cows produce more milk, and cloned sheep, calves, and cats. The genes that have been painstakingly produced for these and other experimental uses are often termed recombinant DNA. Basically, any genetic material that is produced synthetically or in a different organism and then introduced into the test organism is called recombinant DNA. The two areas given greatest attention by molecular geneticists are the production of new medicines and the improvement of domesticated plants and animals. At present, the world market for genetic engineering in agriculture alone could easily be $400 billion. Although the potential for introduction of new genes into food crops and livestock to enhance their productivity is great, these applications will be somewhat slower to develop than in areas of medicines. The safety of these products for human consumption has to be tested. Acceptance by the public will be the cause of the slowdown.
PUBLISHING GENOME SEQUENCES FOR PLANTS

DNA was first sequenced in 1977. The first free-living organism to
have its genome completely sequenced was the bacterium Haemophilus
influenzae, in 1995. Then in 1996, baker's yeast (Saccharomyces
cerevisiae) was the first eukaryote genome sequence to be released,
and in 1998, the first genome sequence for a multicellular
eukaryote, a free-living nematode about 1 mm long (Caenorhabditis
elegans), was released.


The "Gene Machine"

The production of a synthesized gene once took four to eight months of tedious biochemical manipulations. Since the advent of gene synthesizers in 1982, functional gene segments of one's choosing can be automatically reproduced in a single day; by simple typing in the desired genetic sequence on the gene machine's keyboard, geneticists can splice the appropriate nucleotides automatically, and a gene fragment is ready to be introduced into the DNA of an experimental organism. Existing sequences can be duplicated or modified, or researchers can even design totally new sequences.

By using this jointly with a protein sequencer, a machine that can read out the exact sequence of amino acids in a protein, researchers can now carry out sophisticated experiments rapidly. The protein sequencer can analyze even tiny amounts of a useful enzyme (protein) in only a few hours. Which amino acids are in the protein, and in what order, can next be programmed into the gene synthesizer. This machine will produce a genetic fragment to be inserted into bacteria to direct the production of large quantities of the desired protein. There are many new instruments used in genetic engineering to simplify each step in the use of DNA production, and the polymerase chain reaction (PCR) is one of those.

Who would have thought a bacterium hanging out in a hot spring in Yellowstone National Park would spark a revolutionary new laboratory technique? The PCR, now widely used in research laboratories and doctor's offices, relies on the ability of DNA-copying enzymes to remain stable at high temperatures. No problem for Thermus aquaticus, the sultry bacterium from Yellowstone that now helps scientists produce millions of copies of a single DNA segment in a matter of hours.

In nature, most organisms copy their DNA in the same way. The PCR mimics this process, only it does it in a test tube. When any cell divides, enzymes called polymerases make a copy of the entire DNA in each chromosome. The first step in this process is to "unzip" the two DNA chains of the double helix. As the two strands separate, DNA polymerase makes a copy using each strand as a template.

The four nucleotide bases, the building blocks of every piece of DNA, are represented by the letters A, C, G, and T, which stand for their chemical names: adenine, cytosine, guanine, and thymine. The A on one strand always pairs with the T on the other, whereas C always pairs with G. The two strands are said to be complementary to each other.

To copy DNA, polymerase requires two other components: a supply of the four nucleotide bases and something called a primer. DNA polymerases, whether from humans, bacteria, or viruses, cannot copy a chain of DNA without a short sequence of nucleotides to "prime" the process, or get it started. So the cell has another enzyme called a primase that actually makes the first few nucleotides of the copy. This stretch of DNA is called a primer. Once the primer is made, the polymerase can take over making the rest of the new chain.

A PCR vial contains all the necessary components for DNA duplication: a piece of DNA, large quantities of the four nucleotides, large quantities of the primer sequence, and DNA polymerase. The polymerase is the Taq polymerase, named for Thermus aquaticus, from which it was isolated.

The three parts of the polymerase chain reaction are carried out in the same vial, but at different temperatures. The first part of the process separates the two DNA chains in the double helix. This is done simply by heating the vial to 90-75[degrees]C (about 165[degrees]F) for 30 seconds.

But the primers cannot bind to the DNA strands at such a high temperature, so the vial is cooled to 55[degrees]C. At this temperature, the primers bind or "anneal" to the ends of the DNA strands. This takes about 20 seconds.

The final step of the reaction is to make a complete copy of the templates. Since the Taq polymerase works best at around 75[degrees]C (the temperature of the hot springs where the bacterium was discovered), the temperature of the vial is raised.

The Taq polymerase begins adding nucleotides to the primer and eventually makes a complementary copy of the template. If the template contains an A nucleotide, the enzyme adds on a T nucleotide to the primer. If the template contains a G, it adds a C to the new chain, and so on to the end of the DNA strand. This completes one PCR cycle.

The three steps in the polymerase chain reaction-the separation of the strands, annealing the primer to the template, and the synthesis of new strands--take less than two minutes. Each is carried out in the same vial. At the end of a cycle, each piece of DNA in the vial has been duplicated.

But the cycle can be repeated 30 or more times. Each newly synthesized DNA piece can act as a new template, so after 30 cycles, 1 billion copies of a single piece of DNA can be produced! Taking into account the time it takes to change the temperature of the reaction vial, 1 million copies can be ready in about three hours.

PCR is valuable to researchers because it allows them to multiply unique regions of DNA so they can be detected in large genomes. Researchers in the Human Genome Project are using PCR to look for markers in cloned DNA. Many medically and industrially important substances are proteins. The availability of this technology will allow researchers to work much more efficiently and carefully toward ethically acceptable uses for recombinant DNA in genetic engineering applications. One of the major applications is in the criminal justice system. Many people serving time in jail have been proven not guilty using the DNA matches from evidence. Now it is possible to use modifications of existing genes, or copies of them, to be put into organisms in which such traits would be desirable. Existing genes and even completely new genes are used in these efforts. A less expensive method for inserting the gene construct into the plant's DNA can be done using Agrobacterium tumefaciens (a bacterium) to insert the gene constructs into the DNA of the desired plant.

Genetic Gibberish

The ability to biochemically disassociate the individual nucleotides of a gene and establish their sequence led to one of the most astonishing discoveries in genetics since the discovery of DNA. Molecular geneticists have found that the complete sequence of nucleotides in the DNA is not translated into the messenger RNA. Mixed in with the actual genetic code for a given protein are segments of genetic gibberish or strings of nucleotides within the nucleus-a long strand of heterogeneous nuclear RNA. This long RNA becomes edited to delete the gibberish segments, leaving only the sensible segments that are spliced together to form mRNA in the existing nucleus. The long RNA is a direct, one-to-one copy of the DNA message, including the gibberish material.

It is provable that the sensible portions are actually much shorter than the gibberish interruptions. The sensible segments have been called exons because they are expressed, whereas the gibberish parts are called introns because they are intragene segments. Data indicate that exons average about 100 to 300 genetic letters, whereas introns average 1,000, with some having up to 10,000 letters within a single gene.

A number of interesting hypotheses suggest why there are any intron segments at all, let alone so many more than exon segments. These long spaces improve the odds that, when errors occur or when fragments or genes are exchanged or lost during recombination, the breaks will occur in intron segments, leaving the exon intact. Thus, all the necessary genetic code for the production of a specific protein will be available in the form of mRNA once all the introns are cut out and the previously interrupted exons are spliced together.

Other molecular geneticists theorize that these intron segments are not just spacers but are actually involved in regulation, possibly instructing the DNA as to which protein should next be coded into the RNA strand. Still others would condone that exons and introns provide the genetic flexibility necessary to allow for the occasional production of novel new genes. In this way, point mutations and genetic recombination would be supplemented significantly as sources for new genetic variability. Since genetic variability is the raw material for evolutionary change, this area of molecular biology has been termed molecular evolution.

Undoubtedly, few fields of biology have ever moved forward as quickly or generated as much interest as the area of molecular biology. Especially intriguing is genetic control and the potential for manipulating that control.

Summary

1. Sexual reproduction ensures variability among offspring. The process of meiosis is the key to producing that variability. Meiosis produces haploid cells and forms a diploid cell. The fusion (fertilization) of two haploid cells produces a new, single-celled zygote with a unique combination of genes. In angiosperms, meiosis occurs in the anthers and ovules of the flower.

2. Meiosis is two sequential nuclear divisions: The first is a reduction division producing nuclei with only a single set of chromosomes (haploid); second is like mitosis, further producing genetically different cells. The sequence of nuclear and cellular events is different cells or similar to that of mitosis in both meiosis I and II, except that in the former, homologous chromosomes pair up and then separate.

3. Crossing over, the formation of chiasmata, results in additional genetic recombination, thus, increasing variability. Mutations may also result in genetic variability.

4. All plants have alternation of generations between a haploid spore-producing (sporophyte) stage and a gamete-producing (gametophyte) stage. In angiosperms, both stages occur within the tissue of the flower. In the anthers, microsporogenesis is followed by microgametogenesis. In the ovules, megasporogenesis and then megagametogenesis occur. The resulting gametes fuse in a unique double fertilization to produce a new sporophyte stage in the form of a diploid zygote that will develop into the embryo of the seed, complete with a triploid nutritive tissue.

5. All sexually reproducing, diploid organisms have two genetic expressions (alleles) for each trait. The study of inheritance patterns in peas by Gregor Mendel helps elucidate the ideas of gene control of phenotypic expression in offspring. Traits with complete control of dominance can be homozygous dominant, homozygous recessive, or heterozygous dominant genotype. Experimental crosses between pure patterns producing F1 and then between F1 plants producing F2 offspring, can demonstrate the patterns of gene control.

6. Backcrosses between offspring and parent plants, especially a testcross between a dominant plant and a homozygous recessive plant, enables research to study gene control. Some traits have incomplete dominance, allowing for a blending of genetic control and intermediate phenotypic traits.

7. Dihybrid crosses involve two traits. When the alleles controlling those traits are on the same chromosome, they are said to be linked. Other gene interactions include the modifying effects of epistasis and pleiotropic and the complex control of multigene influence.

8. Molecular genetics is an area of science that studies the modification of genes. Genetic engineering has allowed the artificial production of recombinant DNA. Technology advancement in the form of gene synthesizers and protein sequences are enabling scientists to study gene function much more rapidly than ever before.

9. The actual genetic code found in messengers, RNA is now known to have genetic gibberish or intron segments interspersed between the exon segments that actually carry the protein-producing genetic sequence to the ribosomes. The study of molecular evolution is a promising new approach to the understanding of gene function and control.

Something to Think About

1. Define sexual propagation.

2. Diagram a complete flower and show how pollination and fertilization takes place; label all parts.

3. Compare and contrast meiosis and mitosis; determine each phase.

4. Describe what takes place in each phase.

5. What is crossing over?

6. Diagram and label the sexual life cycle of angiosperms; indicate where mitosis and meiosis occur.

7. Diagram and label the sexual life cycle of gymnosperms; indicate where meiosis and mitosis occur.

8. What is a Punnett square?

9. Make a Punnett square with pure parents producing F1s and then between F1 plants producing F2 offspring.

10. Who was the father of genetics?

11. What is dominance in genes, recessive genes, and intermediate genes?

12. Write the abbreviations for homozygous dominance, homozygous recessive, and incomplete dominance.

13. Define molecular genetics and explain how it is being used in your life.

14. What is a genome and why is it important not only in plants but also in animals and humans?

15. Explain how you feel about genetic engineering.

Suggested Readings

Charles, D. 2002. Lords of the harvest: Biotech, big money and the future of food. New York: Perseus Book Group.

Howell, S. 1998. Molecular genetics of plant development. Cambridge, MA: Cambridge University Press.

Leyser, O., and S. Day. 2002. Mechanisms in plant development. Malden, MA: Blackwell Publishing Professional.

Slater, A., N. W. Scott, and M. R. Fowler. 2003. Plant biotechnology: The genetic manipulation of plants. New York: Oxford University Press.

Steeves, T. A., and J. M. Sussex. 2004. Patterns in Plant Development (2nd ed.). Cambridge, MA: Cambridge University Press.

Internet

Internet sites represent a vast resource of information. The URLs for Web sites can change. Using one of the search engines on the Internet such as Google, Yahoo!, Ask.com, or MSN Live Search, find more information by searching for these words or phrases: plant sexual life cycle, plant genetics, meiosis, mitosis, Gregor Mendel, F1 generation, F2 generation, phenotype, genotype, microsporogenesis, megasporogenesis, dihybrid crosses, molecular genetics, PCR, and recombinant DNA.
Table 12-1
Mendel's Dihybrid Cross
F2 Results

                  Number
Traits         of plants *

Yellow-round       315
Yellow-wrinkled    110
Green-round        108
Green wrinkled      32

* 315.110.108.32 is approximately a
(9:3:3:1 ratio).

Table 12-2
Mendel's Experimental Crosses of Traits and Results

Parental traits                         F1 Result

Yellow x green seeds          100% yellow
Round and wrinkled seed       100% round
Red x white flowers           100% red
Inflated x constricted pods   100% inflated
Long x short stems            100% long
Axial x terminal flowers      100% axial
Green x yellow pods           100% green

                                                                F2
Parental traits                         F2 Result             Ratios

Yellow x green seeds          6022 yellow; 2001 green         3.0:1
Round and wrinkled seed       5474 round; 1850 wrinkled       2.96:1
Red x white flowers           705 red; 224 white              3.15:1
Inflated x constricted pods   882 inflated; 299 constricted   2.95:1
Long x short stems            787 long; 277 short             2.84:1
Axial x terminal flowers      651 axial; 207 terminal         3.14:1
Green x yellow pods           428 green; 152 yellow           2.82:1
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Title Annotation:PART 4: Evolution and Diversity
Publication:Fundamentals of Plant Science
Date:Jan 1, 2009
Words:10354
Previous Article:Chapter 11: The control of growth and development.
Next Article:Chapter 13: Genetic engineering and biotechnology.
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