Aa. Aa. AA. Zebra mussels have the following genotypes and phenotypes. Which cross would produce the greatest number of heterozygous. A) AA x aa. B) Aa x Aa. C) AA x AA. D) as x Aa

A (AA x aa)

Explanation:

In option (A) both of the parent mussels are homozygotes, for alleles "a" and "A" respectively. Therefore, every single one (100%) of their offspring would be expected to be Aa heterozygotes. Combinations (B) and (D) yield a lower proportion of heterozygotes (50%), and combination (C) yields none at all. Punnett squares follow:

(A)

  |  A      A        

a |  aA    aA      

a |  aA    aA      

(B)

  |  A      a      

A |  AA   Aa    

a |  aA    aa      

(C)

  |   A      A        

A |  AA    AA    

A |  AA    AA    

(D)

  |  A      a        

a |  aA    aa      

a |  aA    aa