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    Sex determination — AQA GCSE Biology

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    Sex determination explained

    In males, the X and Y sex chromosomes are not fully homologous.

    Read the full explanation

    Initial cellular context is established: ordinary human body cells are diploid with 23 pairs or 46 chromosomes. Standard processing applies to autosomes: 22 pairs are homologous. The final transformation: the 23rd pair determines sex. Females (XX) have a homologous pair, but males (XY) do not. Body cells divide by mitosis to maintain 23 pairs. Gametes are haploid, containing 23 single chromosomes. Fertilisation restores the diploid state. For example, a sperm and egg fuse to form a zygote with 46 chromosomes.

    Your focus

    1. State the number of chromosomes in human body cells and gametes.
    2. Identify that male sex chromosomes (XY) are not fully homologous.
    3. Explain how fertilisation restores the diploid chromosome number.

    Sex determination exam tips

    Quick Revision Summary (Key Takeaway)

    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.

    Topic Overview

    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.

    Key Concepts
    • →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.
    Marking Points
    • 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.
    Examiner Tips
    • 💡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.
    • 💡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.
    Common Mistakes
    • 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.
    • 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).
    Revision Plan
    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.
    Exam Question Types
    • 📋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.
    Command Word Expectations (AQA)
    Explain

    Provide biological reasons why something occurs; use connective words like 'because', 'therefore', and 'as a result' (e.g. explain why the sperm determines sex).

    Determine

    Use the provided data, pedigree chart, or genetic cross to establish a definitive genotype, phenotype, or probability.

    Calculate

    Perform a mathematical step to reach a numerical answer, showing workings (e.g. calculating percentage probability of having a female offspring).

    How Students Lose Marks (Examiner Pitfalls)
    Pitfall: Assuming previous births alter the biological probability of subsequent children (the gambler's fallacy).
    ❌ Weak Answer (Loses Marks):The couple already has three daughters, so their next baby is much more likely to be a boy to balance it out.
    Example improved answer:Each fertilisation is an independent genetic event. There is always a 50% (or 0.5 / 1 in 2) probability that the fourth child will be male, because 50% of the father's sperm carry an X chromosome and 50% carry a Y chromosome.
    Examiner Tip: Always state that fertilisation is an 'independent event' and quote the ratio (1:1), fraction (1/2), or percentage (50%) when asked for probability.
    Pitfall: Omitting clear labels for parental genotypes, gametes, and offspring phenotypes when drawing a Punnett square.
    ❌ Weak Answer (Loses Marks):Draws a 2x2 grid with X, X, X, Y without stating which corresponds to parents, gametes, or resulting children.
    Example improved answer:Parental genotypes: Mother = XX, Father = XY. Gametes: Mother = X, X; Father = X, Y. Punnett square genotypes: 2 x XX, 2 x XY. Offspring phenotypes: 50% female, 50% male.
    Examiner Tip: Write out four distinct stages in any genetic cross: Parental phenotypes, Parental genotypes, Gametes, and Offspring genotypes linked to their resulting phenotypes.
    Step-by-Step Worked Solutions

    Question: A couple plans to have a child. Construct a genetic cross to determine the probability that the child will be male. Express your answer as a percentage.

    1. 1.Step 1: Identify the parental genotypes. The mother is female (XX) and the father is male (XY).
    2. 2.Step 2: Determine the gametes produced. The mother can only pass on an X chromosome (gametes: X, X). The father passes on either an X or a Y chromosome (gametes: X, Y).
    3. 3.Step 3: Construct a 2x2 Punnett square crossing the gametes: row headers X, Y (father); column headers X, X (mother). Resulting squares: XX, XX, XY, XY.
    4. 4.Step 4: Analyse the offspring genotypes. Two out of four squares are XX (female) and two out of four are XY (male).
    5. 5.Step 5: Calculate the percentage probability: (2 / 4) * 100 = 50%.
    Final Answer: The probability of the child being male is 50% (or 0.5 / 1 in 2).

    Question: Explain why the father's gametes determine the biological sex of the offspring, rather than the mother's gametes.

    1. 1.Step 1: State the chromosomal makeup of female gametes. All human egg cells (ova) contain a single X chromosome because females are XX.
    2. 2.Step 2: State the chromosomal makeup of male gametes. Human sperm cells contain either an X chromosome or a Y chromosome in a 1:1 ratio because males are XY.
    3. 3.Step 3: Relate fertilisation combinations to sex outcomes. If an X-carrying sperm fertilises the egg, the zygote is XX (female). If a Y-carrying sperm fertilises the egg, the zygote is XY (male).
    4. 4.Step 4: Conclude clearly. Therefore, biological sex depends entirely on whether an X or Y sperm fertilises the ovum.
    Final Answer: Human females are homogametic (all ova carry X), while human males are heterogametic (50% of sperm carry X, 50% carry Y). The sex of the zygote is determined by whether an X-carrying or Y-carrying sperm fertilises the ovum.
    Active Recall Memory Test
    What are the sex chromosome combinations for human biological females and males?
    Key Fact: Females are XX; males are XY.
    How many total chromosomes and how many sex chromosomes are present in a human egg cell?
    Key Fact: 23 total chromosomes, containing exactly 1 sex chromosome (which is always an X chromosome).
    Why does the father's gamete determine the biological sex of the offspring?
    Key Fact: Because eggs only carry an X chromosome, while sperm can carry either an X or a Y chromosome.
    What is the expected phenotypic ratio of male to female offspring from any human pregnancy?
    Key Fact: 1:1 (or 50% male : 50% female).
    Frequently Asked Questions
    Why is the biological sex ratio in humans 1:1?
    During meiosis in males, homologous chromosomes separate so that exactly half of the resulting sperm receive an X chromosome and the other half receive a Y chromosome. All female eggs carry a single X chromosome. When random fertilisation occurs, there is an equal 50% chance of forming an XX zygote (female) or an XY zygote (male), maintaining an overall 1:1 ratio.
    Does the mother have any genetic influence over the sex of the baby?
    Genetically, no. Because the mother has the genotype XX, all viable ova produced during meiosis carry an X chromosome. The biological sex of the embryo is determined solely by whether the fertilising sperm cell carries an X chromosome (producing a girl) or a Y chromosome (producing a boy).
    Why are human sex chromosomes considered a pair if X and Y look different?
    Although the X chromosome is substantially larger and carries far more genes than the smaller Y chromosome, they act as homologous partners during meiosis. They possess small pseudoautosomal regions that pair up during cell division, ensuring that one sex chromosome segregates into each gamete.
    If a couple has three girls in a row, is their fourth child guaranteed to be a boy?
    No, it is never guaranteed. Fertilisation is an independent probability event, meaning previous outcomes have no physical or biological influence on future conceptions. At every single pregnancy, the chance of conceiving a boy remains exactly 50% (or 0.5).
    How do sex chromosomes differ from autosomes in GCSE Biology?
    Autosomes are the 22 pairs of chromosomes in human body cells that control general somatic characteristics such as eye colour and blood group. The 23rd pair consists of the sex chromosomes (X and Y), which determine the biological sex of the individual alongside carrying specific sex-linked genes.

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