Drosophila that are homozygous for a loss-of-function mutation of swallow develop normally.If a male that is homozygous for this mutant allele mates with a female that is homozygous for the normal allele,offspring of both sexes develop normally and survive to adulthood.In the reciprocal cross,offspring die before completing early embryonic development because their heads do not develop normally.From this information,one can infer that swallow is most likely a
A) maternal effect gene.
B) Hox gene that acts as a transcription factor to influence the expression of many other genes.
C) segment polarity gene.
D) gap gene that responds to concentrations of a maternal effect gene like bicoid or nanos.
E) pair rule gene.

Question

Quizplus · Accepted Answer

The fact that offspring only die when the mother carries the mutant allele suggests that swallow is a maternal effect gene, meaning that the gene product (RNA/protein) deposited in the egg by the mother is necessary for proper embryonic development. The normal function of the gene is therefore masked in the homozygous mutant offspring from either parent, but will only cause a phenotype if missing from the mother's contribution to the egg. Hox genes and segment polarity genes are required for embryonic development but do not fit the pattern of inheritance observed in this scenario. Gap genes and pair rule genes are involved in patterning the embryo but are not maternal effect genes.

Drosophila That Are Homozygous for a Loss-Of-Function Mutation of Swallow