MadSci Network: Genetics |

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Dear Deborah

Thanks for your question. While it is theoretical, that does not make it unimportant.

There is indeed a "very very small chance" that a cross between a poodle and a poodle x chihuahua hybrid would produce "a dog that is genetically a poodle". All the would need to happen would be for the poodle x chihuahua to produce gametes (doesn't matter whether they are egg or sperm) that receive only the poodle contributions from the parent. And there is obviously a very very small chance that this could occur.

More interesting, I think, is the whole question of what constitutes a genetic poodle, and how one might distinguish a genetic poodle from a dog that looks exactly like a poodle. In other words, what is the relationship between the phenotype and the genotype for dog breeds?

There are abstracts to a couple of interesting scientific papers on the subject here and here.

Moderator's note: Ignoring recombination of chromosomes during meiosis (which makes the math more complicated), you can get a rough estimate of the frequency of all poodle gametes produced by the poodle x chihuahua hybrid as follows. The probability of the poodle x chihuahua producing an all poodle gametes is the probability that when each chromosome pair is segregated during meiosis, a given gamete receives the poodle parental chromosome each time. Since chromosomes segregate randomly and independantly (mendel's laws), this is simply (1/2) to the power of n, where n is the number of chromosomes. Dogs have 39 chromosome pairs (humans have 47 pairs), thus the probability of getting a pure poodle gamete is (1/2)^39 = one in 2^39 or one in 5.5 x 10^11, or pretty small. Considering that dogs generally have much much less than 10^11 offspring, this is pretty unlikely to happen.

For more about meiosis (how these gametes are formed and visuals on the chromosomes segregating), see here

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