Answer to Question #319826 in Genetics for aa4

Question

The Hamster Pet Farm ordered a pair of black haired hamsters for breeding. The first generation were all black hamsters. When the Pet Farm mated hamsters of the first generation, one of the offspring turned out to be white. The Pet Farm said this must have been caused by a "hidden gene." Why did this happen? Explain "hidden gene' and how one of the offspring could have been white in this family of hamsters. Explain how you would keep track of the different generations. How could the owner get pure black hamsters. You must use these words to explain the situation or you lose points. Punnett Square, hybrid, pure, dominant, recessive, predict or chance, and pedigree. This is worth 13 points so it should be a paragraph NOT one sentence. 

Answer 1

Suppose that two alleles of gene A are responsible for hair color in hamsters: 

A - black haired;

a - non-black haired (i.e. white haired).

The type of dominance is complete.

Let's investigate the monohybrid cross of hamsters:

F1: AA (black haired) + AA (black haired) + Aa (♀, black haired) + Aa (♂, black haired)

↓

F2:

Based on the Punnett Square above, a "hidden gene" is a recessive allele of the A gene, which in the dominant state is responsible for the black hair color in hamsters.

I would keep track of the different generations by crossing individuals and making pedigrees.

The owner could get pure black hamsters by crossing both two pure black hamsters and pure black hamster with heterozygous black hamster, as well as by crossing two heterozygotes.

Among all offsprings of the first generation, it is possible to predict the birth of a recessive homozygote with a probability of 1/6:

F1: 1st AA (black haired) + 2nd AA (black haired) + 3rd Aa (black haired) + 4th Aa (black haired):

1. 1st AA (black haired) + 2nd AA (black haired) → no white;
2. 1st AA (black haired) + 3rd Aa (black haired) → no white;
3. 1st AA (black haired) + 4th Aa (black haired) → no white;
4. 2nd AA (black haired) + 3rd Aa (black haired) → no white;
5. 2nd AA (black haired) + 4rd Aa (black haired) → no white;
6. 3rd Aa (black haired) + 4th Aa (black haired) → white is present.

Respectively, in a pure black hamster and a heterozygote, the chance of having a recessive homozygote in the second generation is:

1/6*1/4 = 1/24.

If we made a pedigree according to the colors of the hair of the hamsters, then only one cell would be white while all the other cells would be black.