- Following statements were made about sex determination in Drosophila melanogaster
A. It is achieved by a balance of female determinants on the X-chromosome and male determinants on the autosomes.
B. A Drosophila with 0.66 value of X:A ratio would develop intersex type.
C. Due to non-involvement of Y chromosome in sex determination process, XO Drosophila develop as normal fertile male.
D. The high value of X:A ratio facilitates activation of feminizing switch gene Sex – lethal (sxl).
E. Sex specific expression of sxl causes selective activation of dosage compensation genes in
female Drosophila.
Select the option with combination of all correct statements.
(1) A, B, E (2) A, B, D
(3) B, C, E (4) B, C, DOverview of Sex Determination in Drosophila
Sex determination in Drosophila is controlled by a genic balance mechanism where the ratio of the number of X chromosomes to the number of autosomal sets (X:A ratio) directs sexual fate:
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An X:A ratio of 1.0 (e.g., XX with two sets of autosomes) results in female development.
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An X:A ratio of 0.5 (e.g., XY or XO with two sets of autosomes) results in male development.
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Intermediate ratios such as 0.66 can lead to intersex phenotypes, exhibiting characteristics of both sexes.
The Y chromosome plays no role in sex determination but is essential for male fertility.
Important Statements About Drosophila Sex Determination
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A. Balance of female determinants on X chromosome and male determinants on autosomes:
This reflects the genic balance theory where the interplay between factors on X chromosomes and autosomes determines sex. -
B. An X:A ratio of 0.66 leading to intersex phenotype:
Correct, representing an intermediate ratio causing a blend of male and female features. -
C. XO flies develop as normal fertile males:
Incorrect. XO flies are sterile males because they lack the Y chromosome necessary for male fertility, although in sex determination they are male due to the ratio. -
D. High X:A ratio activates the feminizing switch gene Sex-lethal (Sxl):
True. The presence of two X chromosomes relative to autosomes activates Sxl expression, initiating female development. -
E. Sex-specific expression of Sxl causes selective activation of dosage compensation genes in females:
Sxl in females prevents dosage compensation machinery from upregulating X chromosome expression. This statement is true as Sxl regulates dosage compensation to balance gene expression between sexes.
Correct Combination of Statements
Considering the explanations:
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A, B, and D are correct.
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C is incorrect (XO are sterile males, not normal fertile males).
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While statement E is mostly accurate about Sxl regulating dosage compensation, the question’s best option based on given combinations is (2) A, B, D.
Summary
Sex determination in Drosophila involves an intricate balance determined by the X:A ratio with pivotal genes like Sex-lethal (Sxl) acting as the feminizing switch. Intersex phenotypes correspond to intermediate ratios, and the Y chromosome does not determine sex but contributes to male fertility. The critical and correct statements about this system are reflected in option (2) A, B, and D.
Final Answer:
(2) A, B, D -
