Sex Determination

Sex determination means how the baby’s gender (boy or girl) is decided when the egg from the mother meets the sperm from the father.

The biological mechanism by which the sex (male or female) of an individual is established based on genetic or chromosomal factors, is called sex determination.

• Each parent has 23 pairs of chromosomes.
• One pair is called “sex chromosomes”.
• Females have two X chromosomes (XX).
• Males have one X and one Y chromosome (XY).

Henking's Observation (1891):

In 1891, the German scientist H. Henking studied spermatogenesis in squash bugs (insects) and made a critical observation:

• He traced a specific nuclear structure through the entire process of spermatogenesis.
• 50% of sperms received this structure after spermatogenesis; the other 50% did not.
• He named it the "X body" but could not explain its significance​
• Later scientists confirmed that this "X body" was in fact a chromosome → renamed the X chromosome.

• The mother's egg always has an X chromosome.
• The father's sperm can have either an X or a Y chromosome.
• If the sperm with an X joins the egg (X+X), the baby is a girl (XX).
• If the sperm with a Y joins the egg (X+Y), the baby is a boy (XY).
• Half of the father's sperm have X, half have Y. So, the chances of having a boy or a girl are equal.

Sex determination systems vary widely across the animal kingdom. Four major systems are recognised:

• Y sperms are lighter and sometimes swim faster, so a few more boys might be born than girls.
• Before conception, it is not possible to predict the child’s sex.
• The common belief that the mother decides the child's sex is incorrect.

• Sex determination: It is the mechanism by which an organism develops into a male or a female based on genetic factors.
• Types of organisms: Organisms may be bisexual (hermaphrodite), having both sex organs, or unisexual (dioecious), like humans, with separate sexes.
• Discovery: Henking (1891) discovered the X-body, later identified as the X chromosome involved in sex determination.
• XX–XY system: Females are XX (homogametic) and males are XY (heterogametic), seen in humans and Drosophila.
• ZW–ZZ system: Females are ZW (heterogametic) and males are ZZ (homogametic), seen in birds and some reptiles.
• Haplodiploidy: In honeybees, unfertilized eggs develop into haploid males and fertilised eggs into diploid females.

• Sex determination is decided at fertilisation when sperm meets the egg.
• Females have XX chromosomes, while males have XY chromosomes.
• The mother always provides an X chromosome, while the father provides either X or Y.
• X + X results in a girl, and X + Y results in a boy.
• The father’s sperm determines the sex of the child, and the chances of a boy or a girl are equal.