Sex determination — AQA GCSE Biology

In human genetics, biological sex is determined by the 23rd pair of chromosomes, where human females carry two X chromosomes (XX) and human males carry one X and one Y chromosome (XY). Because half of all sperm carry an X chromosome and half carry a Y chromosome, every fertilisation event has an independent 50% probability of producing male or female offspring.

Sex determination in humans describes how genetic material inherited during fertilisation establishes biological sex. Out of the 23 pairs of chromosomes found in normal human diploid body cells, 22 pairs are autosomes controlling general characteristics, whilst the 23rd pair constitutes the sex chromosomes.

Understanding this topic links directly to meiosis, gamete formation, and monohybrid genetic crosses within AQA GCSE Biology. It highlights the mechanism of random fertilisation, reinforcing why biological sex ratios naturally approximate 1:1 across human populations.

• →Human body cells contain 23 pairs of chromosomes, with the 23rd pair designated as the sex chromosomes.
• →Females have two X chromosomes (XX) and produce egg cells that exclusively carry one X chromosome.
• →Males have one X and one Y chromosome (XY) and produce sperm carrying either an X or a Y chromosome in equal proportions.
• →Fertilisation is a random event, yielding a 1:1 phenotypic ratio (50% male to 50% female) for every single pregnancy.

• In males, the X and Y sex chromosomes are not fully homologous.
• Ordinary human body cells are diploid, containing 23 pairs (46 chromosomes total).
• 22 pairs are homologous autosomes; the 23rd pair are sex chromosomes (XX or XY).
• Gametes are haploid (23 single chromosomes); fertilisation restores the diploid number.
• The 22 pairs of autosomes carry genes that control characteristics other than sex, such as eye colour and blood group.
• The 23rd pair are the sex chromosomes and carry the genes that determine whether an individual is male or female.
• Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY).
• The Y chromosome carries the gene that causes male development, so the presence of a Y chromosome determines maleness.
• All egg cells carry an X chromosome, while sperm cells carry either an X or a Y chromosome.
• Sex is determined at fertilisation by the sperm: an X-bearing sperm gives a female (XX) and a Y-bearing sperm gives a male (XY).
• Human body cells contain 23 pairs of chromosomes, including one pair of sex chromosomes.
• In females the sex chromosomes are the same, both X, so the pair is written XX.
• In males the sex chromosomes are different, one X and one Y, so the pair is written XY.
• The Y chromosome carries the gene that triggers testes development, so XY normally produces a male phenotype.
• Egg cells all carry one X chromosome, while sperm cells carry either an X or a Y chromosome.
• Sex is determined at fertilisation by whether an X-bearing or a Y-bearing sperm fuses with the egg.
• Write the parental genotypes as XX for the mother and XY for the father.
• Show the gametes as X from the mother and X or Y from the father.
• Complete the grid so that each cell combines one gamete from each parent.
• Identify the offspring genotypes as XX and XY in equal numbers.
• State the expected ratio of female to male offspring as 1:1, equivalent to a 50% chance of each sex.
• Conclude that the sperm, not the egg, determines the sex of the offspring.
• Identify the expected ratio of offspring types from the completed genetic cross.
• Add the parts of the ratio to find the total number of parts.
• Divide the total number of offspring by the total number of parts to find the value of one part.
• Multiply the value of one part by each number in the ratio to predict expected numbers.
• Recognise that expected numbers are proportions, so actual results may differ due to chance, especially with small sample sizes.
• Use direct proportion to scale predictions from a small expected sample to a larger one.

• 💡Specify that only 22 pairs are always homologous (autosomes).
• 💡Always give the total chromosome count (46) alongside the pair count (23) to show the diploid state.
• 💡Use a Punnett square with X and Y gametes to show the 50:50 chance of male or female offspring.
• 💡State clearly that all eggs carry X and sperm carry X or Y, then link this to the sex of the zygote.
• 💡Distinguish autosomes from sex chromosomes by number: 22 pairs versus 1 pair.
• 💡Use the exact symbols XX and XY when describing the sex chromosome pairs.
• 💡State clearly that gametes are haploid and carry only one sex chromosome.
• 💡Link the Y chromosome to the development of testes and the male phenotype rather than saying Y simply means male.
• 💡Label the parental genotypes, the gametes and the offspring clearly so the logic of the cross is easy to follow.
• 💡Check that every offspring cell contains exactly two sex chromosomes, one from each parent.
• 💡Finish by stating the expected ratio or percentage chance of female and male offspring.
• 💡Write the ratio clearly before doing any calculation so the parts are easy to identify.
• 💡Show each step of the calculation, including the total number of parts and the value of one part.
• 💡Use words such as expected or on average when stating predicted numbers, because actual results may vary.
• 💡Always clearly distinguish between chromosomes (X, Y) and alleles (such as dominant 'B' or recessive 'b') to avoid confusing sex determination with monohybrid disease inheritance.
• 💡When asked for probability in AQA mark schemes, you can write 50%, 0.5, or 1/2, but avoid writing ratios like 1:1 if the question asks explicitly for probability or percentage.
• 💡Remember that the Y chromosome is significantly smaller than the X chromosome and carries the SRY gene responsible for triggering testes development.

• Saying all 23 pairs are homologous; correction: male XY chromosomes are not fully homologous.
• Saying body cells contain 23 chromosomes; correction: body cells contain 23 pairs, which is 46 chromosomes.
• Saying gametes contain 23 pairs; correction: gametes contain 23 single chromosomes, so they are haploid.
• Saying the mother determines the sex of the child; correction: the father determines sex because sperm carry either X or Y, while all eggs carry X.
• Writing sex chromosomes without the correct pairing, such as saying males are YY; correction: males are XY and females are XX.
• Thinking the 22 autosome pairs also determine sex; correction: only the 23rd pair, the sex chromosomes, carries the genes that determine sex.
• Writing the male pair as YX rather than XY; the conventional order is X first, so correct it to XY.
• Thinking that females have two identical chromosomes in every pair; only the sex chromosome pair is XX, while the other 22 pairs are autosomes.
• Believing the mother determines the sex of the child; the egg always contributes X, so the sperm's X or Y chromosome determines sex.
• Putting two gametes from the same parent into one offspring cell; each cell must contain one gamete from each parent.
• Writing the father's gametes as XX and XY instead of X and Y; gametes are haploid and carry only one sex chromosome.
• Forgetting to state the ratio or probability after completing the grid; the cross should end with a conclusion about expected offspring.
• Adding the ratio parts incorrectly, for example treating 1:1 as three parts; the total is found by adding all parts, so 1:1 gives two parts.
• Dividing by one part of the ratio instead of the total number of parts; always divide by the sum of the ratio.
• Expecting observed offspring numbers to match predictions exactly; chance variation means real results can differ from expected proportions.
• Thinking the mother's egg determines the sex of the baby. In reality, all eggs carry an X chromosome; sex is decided by whether the fertilising sperm carries an X or a Y chromosome.
• Believing that having several consecutive children of one sex increases the probability of the opposite sex next time. Each fertilisation event is entirely independent, maintaining a 50% probability every time.
• Confusing the number of sex chromosomes with the total chromosome number. Human gametes have 23 individual chromosomes (1 sex chromosome), while diploid body cells have 46 individual chromosomes (2 sex chromosomes).

1. 1Day 1: Review human karyotypes and memorize that 22 pairs are autosomes and the 23rd pair are sex chromosomes (XX vs XY).
2. 2Day 2: Practice drawing fully labelled Punnett squares showing the cross between XX and XY parents.
3. 3Day 3: Answer exam questions calculating probabilities and explaining why sex determination is an independent event for each child.
4. 4Day 4: Attempt mixed pedigree tree questions where you must identify the sex and genotypes of individuals across generations.

• 📋Punnett Square Construction: Complete a genetic cross grid showing parental gametes, offspring genotypes, and phenotypes.
• 📋Probability Calculations: State the probability, percentage, or ratio of conceiving a male or female child.
• 📋Extended Explanations: Explain why males determine offspring sex or why the sex ratio in large human populations is approximately 1:1.
• 📋Pedigree Tree Interpretation: Analyse family pedigree charts to deduce sex chromosome distribution across multiple generations.