Final answer:
In an X-linked inheritance pattern in Drosophila, the expected proportion of yellow flies in the F3 is calculated based on the possible matings of F2 individuals. Considering the recessiveness of the yellow allele, the overall proportion of yellow flies in the F3 generation is estimated to be 37.5%.
Step-by-step explanation:
Drosophila Genetics and Expected Proportions in F3 Generation
When discussing the expected proportion of yellow flies in the F3 generation following a cross in Drosophila, it is important to understand the basics of X-linked inheritance. A yellow female (XyXy) crossed with a gray male (X+Y) will produce F1 offspring that are heterozygous females (XyX+) and yellow males (XyY). When these F1 individuals are intercrossed, the F2 females will be 50% yellow (XyXy) and 50% gray (X+Xy), while F2 males will be 50% yellow (XyY) and 50% gray (X+Y).
Considering that yellow (Xy) is recessive to gray (X+), when the F2 generation is allowed to mate randomly, the expected proportion of yellow flies in the F3 is calculated by considering all possible matings. Yellow females can only pass down the Xy allele, while heterozygous gray females will pass down Xy 50% of the time. Gray males will pass down X+ 100% of the time, and yellow males will pass down Xy 100% of the time.
Thus, the probability of F3 offspring being yellow will be the sum of probabilities where they inherit Xy from both parents or Xy from mother and Y from the father. The detailed genetics of these crosses would reveal that 25% of the females and 50% of the males would be yellow, summing to an overall proportion of 37.5% yellow flies in the F3 generation.