"XX or XY โ Y decides": The presence of a Y chromosome makes someone male. No Y = female. The Y chromosome is the deciding factor โ remember "Y for Yes, it's a boy."
Sperm type determines sex: "Mum gives X, always. Dad gives X or Y โ that's the gamble." Mothers are not responsible for the sex of the baby because they can only contribute X.
Sex-linked mnemonics โ "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 โ 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 โ 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 โ 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 โ 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.
This memory aid covers Memory Aids within Sex Determination for GCSE Biology. Sex chromosomes and sex determination mechanisms It is section 7 of 11 in this topic. Use it for quick recall, then test yourself straight afterwards so the memory aid becomes usable in an answer.
Practice questions for Sex Determination
What determines the sex of a human baby?
Use a genetic diagram to show how sex is inherited in humans.