Sex Chromosomes
Sex Chromosomes
Humans have 23 pairs of chromosomes. The 23rd pair determines sex:
Female
Male
Biology glossary
- What sex chromosomes do females and males have?
- Females: XX (two X chromosomes). Males: XY (one X and one Y chromosome). These are the 23rd pair of chromosomes in humans.
Humans have 23 pairs of chromosomes. The 23rd pair determines sex:
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Memory Aids
"XX or XY — Y decides": The presence of a Y chromosome makes someone male. No Y = female. The Y chromosome is the deciding factor — remember "Y for Yes, it's a boy."
Sperm type determines sex: "Mum gives X, always. Dad gives X or Y — that's the gamble." Mothers are not responsible for the sex of the baby because they can only contribute X.
Sex-linked mnemonics — "Males are more exposed": Males (XY) have only one X chromosome, so a recessive allele on that X has no partner to mask it. Females (XX) need two copies of the recessive allele. Think of males as having an "exposed" X.
Punnett square for sex determination: Always use X and Y as the symbols. Place the father's gametes (X and Y) along the top of the grid, and the mother's gametes (both X) along the side. This gives XX, XY, XX, XY — the standard 50:50 result.
Quick Check: Draw a genetic diagram to show how the sex of a child is determined, and explain why the theoretical probability of having a boy is 50%.
Mother (XX) x Father (XY). Gametes: all eggs carry X; sperm carry either X or Y in equal numbers. Punnett square: XX, XY, XX, XY — two female (XX) and two male (XY) outcomes. Probability of boy = 2/4 = 50%. The probability is 50% because the father produces equal numbers of X-bearing and Y-bearing sperm during meiosis. Any egg fertilised by a Y-bearing sperm will be XY (male); any fertilised by an X-bearing sperm will be XX (female).
Quick Check: Haemophilia is an X-linked recessive disorder (X^H is normal, X^h causes haemophilia). A carrier mother (X^H X^h) and an unaffected father (X^H Y) have children. What proportion of their sons will have haemophilia? Explain your reasoning.
Punnett square with mother X^H X^h and father X^H Y: gametes are X^H, X^h (mother) and X^H, Y (father). Offspring: X^H X^H (unaffected female), X^H X^h (carrier female), X^H Y (unaffected male), X^h Y (haemophiliac male). Of the two male offspring, one (X^h Y) has haemophilia. Therefore 50% (1 in 2) of sons will have haemophilia. Males are affected by X-linked recessive conditions more often than females because they only have one X chromosome — if it carries the recessive allele, there is no dominant allele to mask it.
Quick Check: A couple already have three daughters. Evaluate the claim that they are "more likely" to have a son next time.
The claim is incorrect. Each conception is an independent event with a 50% probability of being male and a 50% probability of being female. Having three daughters in a row does not change the probability for the fourth child — the sex of previous children has no influence on the outcome of future ones. This is the same as flipping a fair coin three times and getting heads each time: the fourth flip still has a 50% chance of each outcome. The sex of the child is determined by which type of sperm (X or Y) fertilises the egg, and this is a random event each time.
Now try it yourself
Quiz · Question 1 of 26
What determines the sex of a human baby?
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