A
The ratio of number of X-chromosomes to the sets of autosomes
B
X and Y chromosomes
C
The ratio of pairs of X-chromosomes to the pairs of autosomes
D
Whether the egg is fertilized or develops parthenogenetically
The answer is A.
Solution
In Drosophila, sex is determined by the genic balance mechanism proposed by C. B. Bridges. Sex depends on the ratio of the number of X chromosomes (X) to the number of sets of autosomes (A), i.e. the X/A ratio. The Y chromosome does not decide sex (it is needed only for male fertility). X/A = 1 (e.g. XX + 2A): female. X/A = 0.5 (e.g. XY + 2A): male. X/A between 0.5 and 1: intersex. X/A greater than 1: metafemale; less than 0.5: metamale. Determination by fertilised or unfertilised egg (haplodiploidy) occurs in honeybees. So in Drosophila, sex is determined by the ratio of the number of X chromosomes to the sets of autosomes.
ConceptX to autosome ratio in Drosophila
Principles of Inheritance and Variation in past papers
198 questions from this chapter have appeared across 27 exam years.
