Chromosomes: autosomes and sex chromosomes
Chromosomes are thread-like structures in the nucleus made of DNA and protein. Humans have 46 in each body cell, in 23 pairs. Of these, 22 pairs are autosomes (the same in males and females) and 1 pair is the sex chromosomes.
Females have two matching sex chromosomes: XX. Males have one X and one smaller Y: XY. The Y carries a gene that makes the baby develop as a male.
How is sex determined in human beings?
When gametes form, each gets only one chromosome from every pair (23 in all).
- Mother (XX): every egg carries 22 + X.
- Father (XY): half the sperm carry 22 + X, half carry 22 + Y.
| Sperm X | Sperm Y | |
|---|---|---|
| Egg X | XX girl | XY boy |
So the chance of a girl or a boy is 1 : 1 (50% each) in every pregnancy, and the sex depends on which sperm fertilises the egg, i.e. it is decided by the father's contribution, by chance.
Sex determination in other animals
Not every animal uses sex chromosomes. In some reptiles, the temperature at which eggs are kept decides the sex of the young: for several turtles, warmer nests give more females, while in some lizards and crocodiles the pattern is different. Some snails can even change sex during their life. So sex determination is genetic in humans but can depend on the environment in other species.
Board exam focus
- Draw the XX × XY cross and state the 1:1 ratio (3 marks).
- 'Is the mother responsible for the sex of the child?' Explain with reasons.
- Difference between autosomes and sex chromosomes; number of chromosomes in a gamete (23).
Revise how genes pass on in Heredity and Mendel's laws.
Key formulas and definitions
- Human body cell: 46 chromosomes = 23 pairs = 22 pairs autosomes + 1 pair sex chromosomes
- Female: 44 + XX; Male: 44 + XY
- Egg: 22 + X (always); Sperm: 22 + X or 22 + Y
- X egg + X sperm = XX girl; X egg + Y sperm = XY boy
- Chance of girl : boy = 1 : 1
Worked examples
1. How many chromosomes does a human sperm carry?
A gamete has one chromosome from each pair: 23 (22 autosomes + X or Y).
2. A couple has three daughters. What is the chance that their fourth child is a son?
Each pregnancy is independent. The sperm is X or Y with equal chance, so the chance is still 1/2 (50%). Earlier children do not change it.
3. Show with a cross why the ratio of boys to girls is 1:1.
Mother XX gives eggs X, X. Father XY gives sperm X, Y. Combinations: XX, XY, XX, XY → 2 girls : 2 boys = 1 : 1.
4. Why can't the mother decide the sex of the child?
All her eggs carry X. Only the father makes two kinds of sperm (X and Y), so the sex depends on which sperm fertilises the egg.
Common mistakes
- Saying a sperm has 46 chromosomes. Gametes have 23.
- Writing the mother's gametes as 'X and Y'. The mother has no Y chromosome.
- Thinking earlier children change the chances for the next child. Each time it is 50:50.
- Calling all 46 chromosomes sex chromosomes. Only one pair is; the other 22 pairs are autosomes.