8 min readConcept sheetPart II (2 marks) on types; Part III (4 marks) on structure or sex determination

Chromosomes and sex determination in humans

A chromosome is a thread of DNA and protein with two chromatids joined at the centromere. Humans have 23 pairs; the X and Y chromosomes decide sex.

Human chromosomes46 (23 pairs): 22 pairs of autosomes + 1 pair of sex chromosomes

Female44 + XX

Male44 + XY

Sex of the child is decided byThe father's sperm, which carries either X or Y

Parts of a chromosome

Chromatids: the two identical sister threads of a chromosome.
Centromere (primary constriction): the point where the two chromatids are joined.
Secondary constriction: a second narrow part seen in some chromosomes.
Satellite: a small knob beyond the secondary constriction.
Telomere: the end of each chromosome; it keeps the chromosome stable.
A chromosome is made of DNA and proteins. It carries the genes.

Types of chromosomes by the position of the centromere

Type	Centromere	Shape
Telocentric	At the very tip	Rod
Acrocentric	Very near one end; one arm is very short	Rod
Sub-metacentric	A little away from the middle; arms unequal	J or L
Metacentric	In the middle; arms equal	V

Autosomes vs sex chromosomes (allosomes)

AutosomesControl body characters, not sex. Humans have 22 pairs (44). They are the same in males and females.

Sex chromosomes (allosomes)Decide sex. One pair: XX in females, XY in males. The Y chromosome is smaller than the X.

How sex is decided in humans

1
The mother (44 + XX) makes only one kind of egg: 22 + X.
2
The father (44 + XY) makes two kinds of sperm in equal numbers: 22 + X and 22 + Y.
3★
Egg (22 + X) + sperm (22 + X) → 44 + XX: a girl.
4★
Egg (22 + X) + sperm (22 + Y) → 44 + XY: a boy.
5
So each child has a 50% chance of being a boy or a girl.
6
The father's sperm decides the sex. The mother has no part in this choice.

★ key step

How to draw the structure of a chromosome

1
Draw two long, side-by-side rods touching at one point. Label chromatids.
2
Mark the narrow joining point. Label centromere (primary constriction).
3
Label the part above it short arm and the part below long arm.
4
On one arm, draw a second small narrowing. Label secondary constriction.
5
Beyond it, draw a small round knob. Label satellite.
6
Mark the ends of the arms. Label telomere.

In real life

Blaming a mother for not having a son is wrong: the father's sperm decides the sex, by chance. In India, testing a baby's sex before birth for sex selection is against the law, and every girl child is equally precious.

Exam angle

2 marks: “What are autosomes and allosomes?” Autosomes: the 22 pairs that control body characters. Allosomes: the one pair of sex chromosomes (XX or XY) that decides sex.

4 marks: “Explain how sex is determined in human beings.” Female 44 + XX makes 22 + X eggs; male 44 + XY makes 22 + X and 22 + Y sperms; X sperm → girl, Y sperm → boy; 50 : 50 chance; the father decides; chart.

4 marks: “Describe the structure of a chromosome with a diagram.” Chromatids, centromere, arms, secondary constriction, satellite, telomere; four types by centromere position; labelled diagram.

Remember it

Mothers give X; fathers give X or Y. 'Y makes a guY.'