Introduction to Drosophila Sex Determination

Drosophila melanogaster utilizes a unique mechanism of sex determination based on the ratio of X chromosomes to the number of sets of autosomes (X: A). It differs from systems where the presence of a Y chromosome determines maleness, such as in mammals. In Drosophila, this ratio read by cellular machinery determines sexual phenotype.

Calculating the X:A Ratio for XXXXY / AA

• The fly’s genotype is XXXXY with two sets of autosomes (AA indicating diploid autosomes).

• The number of X chromosomes = 4 (XXXX)

• Number of sets of autosomes = 2 (diploid)

• Thus, X:A ratio = 42=224=2

Since the ratio is greater than 1, the fly is categorized as a meta female or super female.

Phenotypic Outcome of XXXXY / AA

• The elevated X:A ratio leads to overexpression of female-determining genes encoded on the X chromosomes relative to male factors on autosomes.

• These flies develop female sexual characteristics despite the presence of a Y chromosome because Drosophila Y chromosome influences fertility but does not determine sex.

• The “meta female” generally has abnormal development due to the atypical chromosome count but is phenotypically female.

Summary

A Drosophila with genotype XXXXY / AA will be a meta female (super female) due to the X chromosomes to autosome set ratio exceeding 1. This confirms the genic balance theory where sex is dictated by the X:A ratio rather than Y chromosome presence.

Final Answer:
(1) Meta female