In Drosophila subobscura, the presence of a recessive gene called grandchildless (gs) causes the offspring of homozygous females, but not those of homozygous males, to be sterile. Can you offer an explanation as to why females and not males are affected by the mutant gene?

The grandchildless gene is recessive, which means that it is only expressed when an individual has two copies of the gene (one from each parent). If a heterozygous individual (carrier) has both a dominant and a recessive allele, the dominant allele will be expressed, and the individual will not be affected by the grandchildless gene. When two carriers mate, there is a 25% chance that their offspring will inherit two recessive alleles and be affected by the gene.

In Drosophila, as in many other organisms, male and female reproductive systems are different. Males produce sperm, while females produce eggs. During reproduction, the male's sperm fertilizes the female's eggs. The differences between male and female reproductive systems may contribute to the observed effect of the grandchildless gene only affecting the offspring of homozygous females.

One possible explanation for the grandchildless gene affecting the offspring of homozygous females and not homozygous males could be related to the gene's effect on the egg production or development in homozygous females. Since females contribute not only their genetic material but also the cytoplasm and other organelles to their offspring, it is plausible that the expression of the grandchildless gene leads to abnormalities in the egg's cytoplasm, which in turn impacts the development of the offspring and leads to sterility. In contrast, males contribute only genetic material in the form of sperm, and if the grandchildless gene has no effect on sperm development or function, the offspring of homozygous males would not be affected.

In conclusion, the offspring of homozygous females carrying the recessive grandchildless gene are sterile, whereas the offspring of homozygous males are not. This can be attributed to the different roles of male and female reproductive systems, where the gene could have a detrimental effect on the egg's development in homozygous females. On the other hand, the lack of effect in offspring of homozygous males suggests that the mutant gene does not impact sperm development or function.