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Modern Genetic Analysis Chapter 4. The Inheritance of Genes
Solved Problem

1.

You have four mice. Numbers 1 (male), 2 (female), and 3 (female) are agouti in color ("mousy" gray), and number 4 (male) is black. The following crosses were made and progeny obtained:

a. Which phenotype is dominant?
b. List the genotypes of all four mice.
c. Predict the progeny types and proportions of a cross of 3 × 4.
See answer

Solution
a. It is a good idea to look over all the data before attempting a solution. Are there any big clues? The most informative is the cross of 1 × 3 because here we learn that some black mice are born to agouti parents; therefore, agouti must be the dominant phenotype, and black recessive. This cross also establishes that the two phenotypes are determined by alleles of one gene because the 3:1 ratio is a single-gene ratio. The cross 2 × 4 is also informative and confirms the dominance of agouti.
b. Now filling in the genotypes is relatively straightforward. Let A be the allele for agouti and a the allele for black. We can deduce that 1 and 3 must both be A/a, because the 3:1 progeny ratio is produced only by a cross between heterozygotes. Number 2 must be homozygous A/A because if it were A/a, it would have produced some black progeny in the cross with 1. Since number 4 is black, he must be homozygous recessive (a/a) because black is recessive. The cross of 2 × 4 confirms that the genotypes are A/A and a/a, because all the progeny are expected to be A/a (agouti), which was observed. In summary,
c. The cross of 3 × 4 is A/a × a/a, and since the heterozygote will produce equal numbers of A and a gametes, the progeny are predicted to be 1/2 A/a and 1/2 a/a.
See question

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