The big idea: Your sex is decided by one pair of chromosomes — the sex chromosomes.
Females are XX (two X chromosomes). Males are XY (one X, one Y).
All other 22 pairs are called autosomes — they are the same in both sexes.
Because the mother is always XX, every egg carries an X. The father is XY, so a sperm carries either an X or a Y — and that is what sets the baby's sex.
- Sex chromosomes
- The one pair of chromosomes that determines sex: XX in females, XY in males.
- Autosomes
- The other 22 pairs of chromosomes, which are the same in males and females.
- X chromosome
- A larger sex chromosome. Females have two; males have one. It carries many genes besides sex.
- Y chromosome
- A small sex chromosome found only in males. It triggers male development and carries very few other genes.
The sperm decides the sex: Every egg carries an X. A sperm carries an X or a Y.
X-sperm + X-egg → XX → a girl.
Y-sperm + X-egg → XY → a boy.
So the father's sperm determines the baby's sex, and there is a roughly 50:50 (1:1) chance of a boy or a girl each time.
| Egg: always X | Egg: always X | |
|---|---|---|
| Sperm: X | XX → girl | XX → girl |
| Sperm: Y | XY → boy | XY → boy |
Why it's about 1:1: Half of all sperm carry X and half carry Y, so across many babies you expect about half girls (XX) and half boys (XY) — a 1:1 ratio.
The X chromosome carries many ordinary genes that have nothing to do with sex — for example the gene for distinguishing red and green.
A gene on a sex chromosome is sex-linked. Most well-known examples are on the X and are recessive — such as red-green colour blindness and haemophilia.
Writing sex-linked genotypes: We write the allele as a small letter on top of the X, because the gene sits on the X chromosome.
Xᴮ = X carrying the dominant (normal) allele.
Xᵇ = X carrying the recessive (colour-blind) allele.
The Y has no matching gene, so we just write Y — there is no second copy of the allele on it.
- Sex-linked gene
- A gene carried on a sex chromosome (usually the X), so its inheritance is tied to the offspring's sex.
- X-linked recessive
- A recessive allele on the X chromosome, e.g. red-green colour blindness or haemophilia.
- Carrier
- A female (XᴮXᵇ) who has one recessive allele but is unaffected, because her other X carries the dominant allele that masks it. She can still pass the allele on.
Possible genotypes for colour vision
- XᴮXᴮ — female, unaffected (homozygous dominant)
- XᴮXᵇ — female, unaffected carrier (heterozygous)
- XᵇXᵇ — female, colour-blind (needs two recessive alleles — rare)
- XᴮY — male, unaffected
- XᵇY — male, colour-blind (just one recessive allele on his single X)
Why a male is colour-blind with only one allele: A male has only one X, and the Y carries no matching allele.
So whatever single allele is on his X shows directly — there is no second copy to mask it.
A single recessive allele (XᵇY) makes him colour-blind. A female would need the recessive allele on both X chromosomes (XᵇXᵇ) to be colour-blind.
Deduce an unaffected female's genotype: If a female is unaffected, she could be XᴮXᴮ (homozygous dominant) or XᴮXᵇ (a carrier).
You usually cannot tell which from her phenotype alone — both look normal. A favourite 1-mark 'deduce' answer is exactly this: homozygous dominant or carrier.
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How this is tested: On Paper 2 a 5-mark Describe asks why red-green colour blindness occurs more often in males than in females — you must link it to males being XY (one X, no matching allele on the Y) so a single recessive allele shows, while females are XX and need two.
On Paper 1 a 1-mark item asks you to deduce the genotype of an unaffected female (answer: homozygous dominant or carrier).
A 2-mark Outline can ask how sex is determined (XX female / XY male; the sperm carries X or Y and decides sex).
IB-style question — explain a sex-linkage cross
A woman who is a carrier for red-green colour blindness (XᴮXᵇ) has children with a man who has normal colour vision (XᴮY). Using a genetic diagram, explain the expected colour-vision outcomes for their daughters and sons, and state why colour blindness is more common in males. [5]
How to score all five marks
- State the genotypes. Mother = XᴮXᵇ (carrier, unaffected); father = XᴮY (unaffected). So the mother's eggs are Xᴮ or Xᵇ, and the father's sperm are Xᴮ or Y.
- Draw the Punnett square. Combining the gametes gives four equally likely offspring: XᴮXᴮ, XᴮXᵇ, XᴮY, XᵇY.
- Daughters. Every daughter gets the father's Xᴮ, so she is either XᴮXᴮ (unaffected) or XᴮXᵇ (a carrier) — no daughter is colour-blind; half of them are carriers.
- Sons. A son gets his X from the mother. So he is XᴮY (unaffected) or XᵇY (colour-blind) — about half the sons are colour-blind.
- Why males more often. A male has only one X and the Y has no matching allele, so a single recessive allele shows; a female needs the recessive allele on both X chromosomes, which is rarer. (Award marks for: genotypes; Punnett/gametes; daughters outcome; sons outcome; the one-X reason.)
Final answer
Mother XᴮXᵇ × father XᴮY → XᴮXᴮ, XᴮXᵇ, XᴮY, XᵇY. No daughter is colour-blind (½ are carriers); about ½ of sons are colour-blind. Males are affected more often because they have only one X (the Y carries no matching allele), so a single recessive allele already shows.
| Father's sperm: Xᴮ | Father's sperm: Y | |
|---|---|---|
| Mother's egg: Xᴮ | XᴮXᴮ — unaffected girl | XᴮY — unaffected boy |
| Mother's egg: Xᵇ | XᴮXᵇ — carrier girl (unaffected) | XᵇY — colour-blind boy |
✓ Why this scores full marks: It shows the gametes and the four offspring (not just states a ratio), then reads off daughters and sons separately, and ends with the cause — males have one X with no masking allele.
The classic slip is to give a ratio without the genetic diagram, or to forget that daughters all inherit the father's Xᴮ so none are affected here.
| Male (XY) | Female (XX) | |
|---|---|---|
| Sex chromosomes | One X and one Y | Two X chromosomes |
| Copies of the gene on the X | Only ONE copy | TWO copies |
| Does the Y carry a matching allele? | No — the Y is short and lacks it | — |
| To be colour-blind you need… | Just ONE recessive allele (on the single X) | TWO recessive alleles (one on each X) |
| Result | More common — a single recessive allele already shows | Rarer — a dominant allele on the other X usually masks it |