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The A locus and the D locus are so tightly linked that no recombination is ever observed between them. If Ad/Ad is crossed with aD/aD and the F1 is intercrossed, what phenotypes will be seen in the F2 and in what proportions

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Answer:

F2: 1/4 phenotype Ad (genotype Ad/Ad), 1/2 phenotype AD (genotype Ad/aD): 1/4 phenotype aD (genotype aD/aD)

Explanation:

In this case, A and D loci are strongly linked and there is little or no recombination between them. Therefore, Ad/Ad and aD/aD individuals will produce only one type of gametes (i.e., individuals Ad/Ad only can produce Ad gametes, while aD/aD individuals only can produce aD gametes), thereby all F1 will be Ad/ aD. Finally, Ad/aD individuals can produce two types of gametes: Ad and aD, thereby the F2 will be 1/4 Ad/Ad (phenotype Ad), 1/2 Ad/aD (phenotype AD) and 1/4 aD/aD (phenotype aD)  

Parental cross:   Ad/Ad x aD/aD

F1 generation:  Ad/aD

F2 generation:        

                Ad            aD

Ad         Ad/Ad       Ad/aD

aD         Ad/aD       aD/aD

F2 generation: 1/4 Ad/Ad (phenotype Ad), 1/2 Ad/aD (phenotype AD) and 1/4 aD/aD (phenotype aD)

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