Question

In Drosophila, the two genes p and m are so tightly linked on an autosome that crossing-over between the genes has never been observed. If a double heterozygote with the alleles for these two genes in the trans configuration is mated with a strain that is homozygous recessive for both genes, then

A. All the progeny will be pm⁺ p m
B. All progeny will be p⁺ m+ / p m.
C. 50% of the progeny will be pt m/pm and 50% will be p⁺ m / p m.
D. Cannot be answered without additional information.
E. None of the above.

Answer 1

When two tightly linked genes on an autosome in Drosophila are crossed with a homozygous recessive strain, all progeny will display the parental genotype due to the absence of recombination.

Step-by-step explanation:

In Drosophila, if the two genes p and m are tightly linked on an autosome and a double heterozygote in the trans configuration (pm+ / p+ m) is mated with a strain that is homozygous recessive for both genes (p m / p m), the progeny will always have the parental genotype. This is because no crossing-over between the genes has been observed. Therefore, the option that best fits this situation is B. All progeny will be p+m+ / p m. This outcome is expected due to the lack of recombination in tightly linked genes, which results in offspring having the same combination of alleles as the parents, otherwise known as parental genotypes.