Sex determination in humans, birds and honey bee
Sex is decided by chromosomes. Human males are XY (male heterogamety), female birds are ZW (female heterogamety), and honey bee males grow from unfertilised eggs.
How the sex chromosome was found
In 1891, Henking followed a special nuclear structure through sperm formation in some insects. Half the sperm received it and half did not. He called it the X body; it was later named the X chromosome. Chromosomes that decide sex are sex chromosomes; all the others are autosomes. In grasshoppers, females are XX and males have only one X, so they are XO. Here, sperm with an X give daughters and sperm without one give sons.
Sex determination in humans (XX-XY type)
- 1Female: 22 pairs of autosomes + XX. Every egg carries 22 + X; the female is homogametic.
- 2Male: 22 pairs of autosomes + XY. He makes two kinds of sperm in equal numbers: 22 + X and 22 + Y; the male is heterogametic.
- 3Egg (X) + sperm (X) → zygote XX → girl.
- 4★Egg (X) + sperm (Y) → zygote XY → boy.
- 5So every pregnancy has a 50% chance of a boy and 50% of a girl, and the sperm decides which.
★ key step
Three mechanisms compared
| Feature | Humans (XX-XY) | Birds (ZW-ZZ) | Honey bee (haplodiploid) |
|---|---|---|---|
| Female | XX | ZW | Diploid (32 chromosomes), from a fertilised egg |
| Male | XY | ZZ | Haploid (16 chromosomes), from an unfertilised egg |
| Heterogametic sex | Male | Female | Not decided by sex chromosomes |
| Who decides the sex of offspring | Sperm (X or Y) | Egg (Z or W) | Whether the egg is fertilised or not |
The honey bee: haplodiploid sex determination
In honey bees, sex depends on the number of chromosome sets. A fertilised egg grows into a female (a queen or a worker), which is diploid with 32 chromosomes. An unfertilised egg grows into a male (a drone) by parthenogenesis, which means development without fertilisation. So the drone is haploid, with 16 chromosomes. Drones make sperm by mitosis, not meiosis. A drone has no father and cannot have sons, but he has a grandfather and can have grandsons.
How to draw the human sex determination chart
- Top line: Female 44 + XX × Male 44 + XY, labelled Parents.
- Next line: gametes in circles: eggs 22 + X (one kind only); sperm 22 + X and 22 + Y.
- Draw arrows from each sperm to the egg.
- Bottom line: zygotes 44 + XX (female) and 44 + XY (male).
- Write the ratio 1 : 1 (50% each) below, and the title 'XY type of sex determination in humans'.
Because the father's sperm decides whether a baby is a boy or a girl, blaming a mother for not having a son is scientifically wrong. Sex selection before birth is also illegal in India.
2 marks: “Why is it wrong to blame a mother for giving birth to a daughter?” All eggs carry X. The father makes X-bearing and Y-bearing sperm in equal numbers, and the sperm that fertilises the egg decides the sex. So the father's contribution decides, and it is a matter of chance.
3 marks: “Compare sex determination in humans and birds.” Humans: female XX, male XY; male heterogamety; sperm decides. Birds: female ZW, male ZZ; female heterogamety; egg decides. In both, the two kinds of gametes are made in equal numbers, so the sex ratio is about 1 : 1.
3 marks: “How is sex determined in the honey bee? Why does a drone have a grandfather but no father?” Fertilised eggs give diploid females (32); unfertilised eggs give haploid males (16) by parthenogenesis. A drone comes from an unfertilised egg, so no father; his mother came from a fertilised egg, so she had a father, his grandfather.
XY: the guY's sperm decides. ZW: the Woman (hen) decides. Bees: fertilised = female.