A fish of genotype a/a ; B/b is crossed with a fish whose genotype is A/a ; B/b. What proportion of the progeny will be heterozygous for at least one of the genes

Proportion of heterezygous progeny = 3/4

Explanation:

According to Mendel's law of independent assortment which states that the alleles of two (or more) different genes get sorted into gametes independently of one another. This means that the allele a gamete receives for one gene does not influence the allele received for another gene.

Gametes produced by the fishes assuming independent assortment:

Fish 1 = aB, ab, aB, ab

Fish 2 = AB, Ab, aB, ab

Progeny from cross:

aBAB, aBAb, aBaB, aBab

abAB, abAb, abaB, abab

aBAB, aBAb, aBaB, aBab

abAB, abAb, abaB, abab

Proportion of heterezygous progeny = 12/16 = 3/4

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