Introduction

Sex determination varies widely among organisms. Two well-studied systems are Drosophila melanogaster (fruit fly) and humans, each with different key determinants of sex. Understanding these distinctions is crucial for properly interpreting genetic and developmental biology concepts about sex differentiation.

Role of Y Chromosome in Drosophila

• The Y chromosome in Drosophila does not determine sex. Instead, it carries genes essential for male fertility but not for the initial sex determination process.

• Sex determination in Drosophila depends on the X:A ratio, where the ratio of the number of X chromosomes to sets of autosomes determines female or male development.

• For example, XXY flies develop as females because the X:A ratio is 1, despite the presence of a Y chromosome.

Role of Y Chromosome in Humans

• In humans, sex determination is largely governed by the presence or absence of the Y chromosome.

• The Y chromosome contains the SRY gene (sex-determining region Y), which triggers the formation of testes, resulting in male development.

• Humans with XXY genotype are male (Klinefelter syndrome), and XO individuals are female (Turner syndrome), demonstrating the essential role of the Y chromosome in human male sex determination.

X:A Ratio vs Presence of Y Chromosome

• In Drosophila, the X chromosome to autosome sets ratio (X:A ratio) is the critical determinant, not the Y chromosome.

• In humans, the presence of the Y chromosome dictates male development, while the number of X chromosomes influences secondary characteristics but is not the primary sex-determining factor.

Clarifying the Statements

• (1) Y chromosome does not play any role in Drosophila sex determination — True.

• (2) Y chromosome is sex determinant in humans — True.

• (3) In humans sex determination is based on number of X chromosome to sets of autosomes — False.

• (4) In Drosophila sex determination is based on number of X chromosome to sets of autosomes — True.

Conclusion

The statement that is NOT true is:

(3) In humans sex determination is based on number of X chromosome to sets of autosomes.

Human sex determination relies on the presence of the Y chromosome, not the X:A ratio.

Final Answer:
(3) In humans sex determination is based on number of X chromosome to sets of autosomes.