Question
Explain sex determination in humans using the XX-XY system. What determines whether a baby is male or female, and what is the probability?
Solution — Step by Step
Humans have 46 chromosomes (23 pairs). 22 pairs are autosomes (non-sex chromosomes). Pair 23 are the sex chromosomes.
Females: Two X chromosomes (XX) — homogametic sex
Males: One X and one Y chromosome (XY) — heterogametic sex
The Y chromosome carries the SRY gene (Sex-determining Region Y), which triggers male development. Without SRY, the default developmental pathway leads to female characteristics.
During meiosis:
Mother (XX): Both X chromosomes can go into eggs. All eggs carry one X chromosome.
Father (XY): The two sex chromosomes separate. Half the sperm carry X, and half carry Y.
- X-bearing sperm → fertilizes egg → XX → female
- Y-bearing sperm → fertilizes egg → XY → male
Cross: Mother (XX) × Father (XY)
| X (from mother) | X (from mother) | |
|---|---|---|
| X (sperm) | XX (female) | XX (female) |
| Y (sperm) | XY (male) | XY (male) |
Probability: 50% XX (female) : 50% XY (male)
Since the mother contributes only X chromosomes, and the father contributes either X or Y:
The sex of the child is determined by the type of sperm that fertilizes the egg.
An X-bearing sperm produces a daughter (XX); a Y-bearing sperm produces a son (XY).
This is why blaming the mother for the sex of children is biologically incorrect — the father’s contribution determines it.
Why This Works
The XX-XY system is a classic example of chromosomal sex determination. The SRY gene on the Y chromosome is a master switch: it activates a cascade of gene expression leading to testes development and, consequently, male anatomy and physiology.
In XX embryos (no SRY), the gonad defaults to developing into ovaries, and female anatomy follows. The female developmental pathway is sometimes called the “default” pathway, though this is a simplification.
Alternative — Other Sex Determination Systems
Humans use XY, but other systems exist:
- ZW system (birds, some snakes): females are ZW, males are ZZ
- XO system (grasshoppers): females have two sex chromosomes (XX), males have only one (XO)
- Environmental sex determination (crocodiles, some turtles): temperature during egg incubation determines sex
Common Mistake
Students write “the mother determines the sex of the child” — the exact opposite of the correct answer. Since the mother always contributes an X chromosome, she has no influence on whether the child is male or female. It is the father who contributes either X (daughter) or Y (son). This is a factually important point often tested in CBSE and NEET.