A male fly that is homozygous for recessive alleles at two linked autosomal loci is mated with a wild-type female fly that is homozygous dominant for both alleles. All F₁ offspring are then randomly mated. If the loci are so close as to have no recombination, what proportion of offspring will be homozygous for both mutations in the F₂ generation?

Question

Quizplus · Accepted Answer

Since the two loci are linked and no recombination occurs, they will always be inherited together as a block. In the F1 generation, all offspring will be heterozygous for both mutations (S1U1B1S1U1B0). When these F1 offspring are mated randomly, we can use a Punnett square to determine the possible genotypes of the FS1U1B12S1U1B0 offspring:
```
      S1U1B1  S1U1B0
S1U1B1  FS1U1B11S1U1B1  FS1U1B1S1U1B0
S1U1B0  FS1U1B1S1U1B0   FS1U1B10S1U1B0
```
From this, we can see that 1/4 of the FS1U1B12S1U1B0 offspring will be homozygous for both mutations (FS1U1B10S1U1B0).

A Male Fly That Is Homozygous for Recessive Alleles at Two