Explain sex determination in human beings with a chart.

How to lay out the answer. Follow the frame, then write it in your own words.

1 · Open. Humans have 23 pairs of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes.
2 · Parents and gametes.
- Female 44 + XX: all eggs are 22 + X.
- Male 44 + XY: sperms are 22 + X and 22 + Y in equal numbers.
3 · Draw the chart. [Chart: parents 44 + XX × 44 + XY; their gametes; offspring 44 + XX girl and 44 + XY boy in the ratio 1 : 1]
4 · Result. X sperm + egg → girl; Y sperm + egg → boy; a 50% chance each time.
5 · Link to daily life. The father's sperm decides the sex, so blaming mothers is wrong; sex selection is against the law in India.
6 · Close. So the sex of a child is decided at fertilisation, by chance.

Scoring tip: Show the cross as a chart with arrows from parents to gametes to children. The chart carries most of the marks.