Question

Sex determination in humans takes place by
A. Sex chromosomes of father
B. Measurement of sperm
C. Measurement of ovum
D. Sex chromosomes of mother

Answer 1

Hint: Humans have a total of 46 chromosomes where each cell nucleus contains 23 pairs of chromosomes. While the first 22 pairs of chromosomes are known as autosomes, the $23^{rd}$ pair is known as allosomes or sex chromosomes that play a deciding factor in sex determination in humans.

Complete Answer:
- Sex chromosomes are either a pair of chromosomes which decide whether a person is male or female.
- Human sex chromosomes are classified as the X-chromosome and Y-chromosome. Most females have two X chromosomes while the males have an X chromosome and a Y chromosome.
- During meiosis the male sex-chromosome pair XY separates and transfers either an X or a Y to separate gametes. Thus half of sperms have the X chromosome while the other half has the Y chromosome.
- All female egg cells generally carry a single X. After fusion of the male and female gametes the zygote would carry either XX or XY depending on whether the sperm carrying X or Y fertilised the ovum. If the eggs are fertilized by an X-bearing sperm, it results in a female (XX), whereas if they are fertilized by Y-bearing sperm, it results in a male (XY).

Thus, the correct answer is option A i.e, Sex chromosomes of father.

Note: During coitus, the sperm is delivered to the intersection of the isthmus and ampulla, where the ovum is fertilised by the sperm leading to the formation of the diploid zygote. The presence of X or Y chromosomes in the sperm decides the sex of the embryo.