Genetic codes: ever wonder why your trusty lab is the color it is?
Regardless of his opinions and practices, have you ever wondered bow all this works? How does a breeder (purposely) produce Labs of various colors?
Physical traits are determined by codes in the chromosomes inherited 'from parents. A dog has 39 pairs of chromosomes,. for a total of 78.. One chromosome in each pair comes from the sire, and the other from the darn.
Imagine a chromosome as a little X-shaped bundle of cells. If you tugged on a "leg" of the X, you would see it is a tightly wound coil of DNA. Imagine the DNA as a twisted helical ladder with distinct sections having rungs labeled with letters. Each section of the ladder is a gene corresponding to a particular trait. The gene can. have different codes, based on the order of the letters on the ladder rungs. These different codes of the gene are called alleles.
COLOR ALLELES As if all of this weren't complicated enough, there is more than one place (or locus) on a Lab's chromosomes where genes influence color. There are two such places, which geneticists call the "B" locus and the "E" locus.
Because the chromosomes come in pairs, every dog has four alleles (gene codes) for color--one from the sire and one from the dam at both locus points. Each of these four alleles can be either dominant (B, E) or recessive (b, e). So, a Lab has eight possible combinations of color alleles: BBEE, BbEE, BbEe, and so on. (Yes, for you statisticians, I'm aware there are technically 16 possible combinations, but Bb and h13 have identical results, as do cE and Ee. So, for simplicity, let's say eight.)
Dominant alleles at the B locus (B) correspond to black hair and skin color, and recessive alleles at the B. locus (b) correspond to chocolate hair and skin color. Dominant alleles at the E locus (E) render the E locus irrelevant, and the dog's color will be determined by the B locus. Recessive alleles at the E locus (e) cause yellow hair color. In that case, the color of the yellow dog's skin features (nose, eyelids, etc.) will be determined by the B locus.
Of course, both alleles at a given locus must be recessive in order for the recessive trait to be manifested. In other words, a dog must have a "bb" combination at the B locus in order to be chocolate, and a dog must have an "cc" combination at the E locus in order to be yellow.
To summarize, here are the possible allele combinations (genotypes) that Labs can have, and their resulting coat colors:
* BBEE = Black
* BbEE = Black
* BbEe = Black
* bbEE = Chocolate
* bbEe = Chocolate
* BBee = Yellow with black skin features
* Bbec = Yellow with black skin features
* bbee = Yellow with brown skin features
BREEDING IMPLICATIONS Let's examine the "mystifying" occurrence in which two black Labs produce a litter containing pups of all three colors. To see how this can happen, we'll have to reach into our middle school science books and pull out the Punnett Square.
The secret is in the genotypes of the parents. Although they are black in color, they carry recessive genes at both locus points. They are what many people call "chocolate- or yellow-factored" black Labs. Here's how it works:
Obviously, it's highly unlikely this pair will produce a litter of 16 pups. But the Punnett Square illustrates the odds of any color emerging. Black has a nine-in-16 chance of occurring; yellow has a four-in-I6 chance; chocolate has a three-in-16 chance.
Granted, it's possible a litter of eight pups could all be black. But it. wouldn't be surprising if a litter of eight had four or five blacks, a couple yellows and a chocolate or two.
We could construct Punnett Squares all day with various genotypes, and here are a few things we'd learn:
* Yellow-to-yellow breedings can only produce yellow puppies.
* Chocolate-to-chocolate breedings cannot produce black pups, but may result in an occasional yellow.
* Chocolate or yellow pups can result from a breeding only if both parents carry a recessive allele at the appropriate locus. Hence the frustration of many backyard breeders who have bred a yellow or chocolate dam to a black sire (or vice versa) and never had any yellow or chocolate pups.
I can't explain fully how to determine a dog's genotype is this short of a column, but the best way is to draw from the dog's appearance and pedigree. For example, consider a yellow dog with a black nose. Just looking at him, we know he is B_ee. We're not sure about the second allele at the B locus. But if he had a chocolate parent, we know he is Bbee, because a "b" allele is all that parent had to give.
Or consider a black bitch with a yellow sire and black dam. We know she is B_Ee. How? She isn't yellow, so she must have a dominant allele at one of the E loci. The other E loci must be a recessive allele (e), because that's all her sire had to give. And she's black, so she must have at least one dominant allele at the B loci (B).
If her breeder says there have been no chocolates in her bloodline, she's probably BBEe. She'll produce a majority of black puppies, but could rear some yellows if bred with a yellow or yellow-factored black.
Red Labs ("fox red") are registered as yellows, and silver Labs are not officially recognized. Silver genetics are highly disputed. But that's as much as I'll say, because my mailbox is entirely too small.
Black dam with BbEe genotype can donate any of these allele pairs... Black sire with BbEe genotype can donate any of these allele pairs... BE Be bE be BE BBEE (black pup) BBEe BbEE (black pup) BbEe (black pup) (black pup) Be BBEe (black pup) BBee BbEe (black pup) Bbee (yellow pup, (yellow black nose) pup, black nose) bE BbEE (black pup) BbEe bbEE (chocolate bbEE (chocolate (black pup) pup) pup) be BbEe (black pup) Bbee bbEe (chocolate Bbee (yellow pup. (yellow pup) brown nose) pup, black nose)
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|Date:||Jun 1, 2014|
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