Question

Sex determination in Drosophila melanogaster is based on
A. XY mechanism
B. Genic balance between X chromosomes and autosomes
C. Pseudoalleles
D. Environment-chromosome interaction

Answer 1

The XX-XY sex-determination system is present in both mammals as well as in the fruit fly, Drosophila melanogaster. But the mechanism of sex determination is different in both.

Complete answer:
XY mechanism: Incorrect Answer
Human males contain the XY chromosomes. The Y chromosome contains the SRY gene which is the male determining gene. It is responsible for the development of external and internal male genital organs.

Genic balance between X chromosomes and autosomes: Correct Answer
Sex determination in Drosophila melanogaster is based on a balance of female determinants on the X chromosome and male determinants on the autosomes. Usually, flies have either one or two X chromosomes and two sets of autosomes. If in a diploid cell there is one X chromosome (1X:2A), the fly is a male. If there are two X chromosomes in a diploid cell (2X:2A), the fly is female. In flies, the Y chromosome is not involved in determining sex. However, it contains genes responsible for forming sperm in adults. Thus, XO Drosophila is sterile males, XY chromosomes containing flies are fertile males and the XX chromosomes containing flies are fertile females.

Pseudoalleles: Incorrect Answer
When two genes with similar functions are closely placed on a chromosome, they tend to get inherited together always. Such pairs of genes are called pseudoalleles. These are structurally dissimilar genes and are not involved in sex determination.

Environment-chromosome interaction: Incorrect Answer
In certain organisms, the environment plays a role in sex determination. For example, the sex of an alligator depends on the temperature of the nest.

Hence, the correct answer is option (B).

Note: According to the Genic Balance theory of sex determination proposed by C. B. In 1922, if the ratio of X chromosome to a total number of sets of autosomes (X/A) is >1, the organism will be super female. If 0 <(X/A) < 0.50, the organism will show the intersex phenotype. When the X/A value is 1.0 for a normal female and 0.5 for a normal male.