Sex determination in human beings is based on which system?

Human body cells have 46 chromosomes, arranged in 23 pairs. Of these, 22 pairs are autosomes, which are alike in males and females. The 23rd pair consists of the sex chromosomes, which decide whether a person is male or female.

In humans:

Females have two X chromosomes (XX). So all the egg cells (ova) produced by a woman carry one X chromosome. Females are called homogametic.

Males have one X and one Y chromosome (XY). So half of the sperm cells carry an X chromosome and half carry a Y chromosome. Males are called heterogametic.

When an egg is fertilised by a sperm carrying an X chromosome, the child is a girl (XX). When it is fertilised by a sperm carrying a Y chromosome, the child is a boy (XY). Therefore, the sex of the child is decided by the father's sperm, not by the mother. There is an equal (50:50) chance of a boy or a girl in each pregnancy.

The Y chromosome carries a gene called SRY (sex-determining region Y), which triggers the development of testes and male characteristics.

Other systems of sex determination in nature:

XX-XO: females are XX and males have only one X (XO). This occurs in grasshoppers and some other insects.

ZW-ZZ: females are heterogametic (ZW) and males are homogametic (ZZ). This occurs in birds and some reptiles and moths.

Haplodiploid: males develop from unfertilised eggs (haploid) and females from fertilised eggs (diploid), as in honeybees, ants and wasps.

Temperature-dependent: in many turtles and crocodiles, the incubation temperature of eggs decides the sex.

In India, sex-selective practices are illegal under the PCPNDT Act, 1994.

Why not the other options?

• ✗Haplodiploid occurs in honeybees.
• ✗Good to know
• ✗XX-XO occurs in grasshoppers.
• ✗ZW-ZZ occurs in birds.