Sex determination — AQA GCSE Combined Science
Test yourself on Sex determination with AQA GCSE practice questions.
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Sex determination explained
Chromosomes are thread-like structures of DNA found in the nucleus.
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In ordinary human body cells, chromosomes exist as homologous pairs, giving 23 pairs in total, which is 46 chromosomes. One chromosome of each pair is inherited from the mother and one from the father. The pairs are numbered, and the first 22 pairs are autosomes; the 23rd pair is the sex chromosomes. Body cells are diploid, meaning they have two of each chromosome. Gametes are different: sperm and egg cells are haploid and contain 23 single chromosomes, not 23 pairs. At fertilisation, the sperm nucleus fuses with the egg nucleus, restoring the diploid number of 46 chromosomes, or 23 pairs, in the zygote.
22 pairs control characteristics only, but one of the pairs carries the genes that determine sex.
Of the 23 pairs of chromosomes in a human body cell, 22 pairs are autosomes. These autosomes carry genes that control general characteristics such as eye colour, blood group and height. The remaining pair is the sex chromosomes. In females the pair is XX, and in males it is XY. The sex-determining region of the Y chromosome carries the gene that triggers male development, so the presence or absence of a Y chromosome determines sex. Gametes carry one sex chromosome: all eggs carry X, while sperm carry either X or Y. At fertilisation, an X-bearing sperm produces XX offspring, and a Y-bearing sperm produces XY offspring.
In females the sex chromosomes are the same (XX).
Human body cells are diploid, so each contains 23 pairs of chromosomes. One pair, the sex chromosomes, determines sex. In females this pair is homologous: both chromosomes are X, written XX. Because the two are the same size and carry the same genes, they pair normally during meiosis. A female therefore produces eggs that all carry one X chromosome. When an X-bearing egg is fertilised by an X-bearing sperm the resulting zygote is XX, so the offspring is female. This explains why the mother's contribution does not determine sex; the sperm's sex chromosome does. For example, a genetic diagram crossing XX with XY gives XX and XY offspring in roughly equal numbers.
In males the chromosomes are different (XY).
Human body cells are diploid with 23 pairs of chromosomes. The sex chromosomes form one pair. In males the two sex chromosomes are not the same: one is a large X chromosome and the other is a much smaller Y chromosome, written XY. Because X and Y are different in size and gene content, they are not truly homologous, although they still pair and separate during meiosis. A male therefore produces two types of sperm in roughly equal numbers: half carry X and half carry Y. If an X sperm fertilises an egg the zygote is XX and female; if a Y sperm fertilises an egg the zygote is XY and male. This is why the father's sperm determines the sex of the offspring.
Students should to be able to carry out a genetic cross to show sex inheritance.
In humans, sex is determined by the sex chromosomes inherited at fertilisation. Females have two X chromosomes (XX) and males have one X and one Y (XY). A genetic cross is a diagram that tracks how alleles or chromosomes separate in gametes and recombine at fertilisation. To show sex inheritance, write the parental genotypes, XX for the mother and XY for the father. Each parent produces gametes containing one sex chromosome: the mother makes only X eggs, while the father makes X and Y sperm in equal numbers. Draw a Punnett square with the mother's gametes along one side and the father's along the other. The four boxes give XX, XX, XY and XY. This shows a 1:1 ratio of female to male offspring, so each fertilisation has a 50% chance of producing a female and a 50% chance of producing a male.
Students should understand and use direct proportion and simple ratios in genetic crosses.
Genetic crosses predict the proportions of offspring genotypes and phenotypes. Direct proportion means that as the number of fertilisations increases, the expected number of offspring of a particular type increases in step with it. A simple ratio compares the expected numbers of different outcomes, such as 3:1 for a monohybrid cross between two heterozygotes or 1:1 for sex inheritance. To use these ideas, complete the cross, count the boxes for each outcome, then express the counts as a ratio in simplest form. For example, a cross giving three tall and one short outcome has a 3:1 ratio, so the probability of tall is 3 ÷ 4 = 0.75 or 75%. If 200 offspring are expected, the predicted number of tall plants is 0.75 × 200 = 150. Ratios describe expected proportions, not guaranteed results in every small family or sample.
Your focus
- State that ordinary human body cells contain 23 pairs of chromosomes, giving 46 chromosomes in total.
- Describe the origin of each chromosome in a pair as maternal or paternal.
- Compare the chromosome number in body cells with that in gametes and explain the role of fertilisation in restoring 23 pairs.
Show all 18 objectives
- Distinguish the 22 pairs of autosomes from the pair of sex chromosomes.
- Describe the sex chromosome genotypes XX and XY and explain how the Y chromosome determines male sex.
- Construct a genetic cross to show the inheritance of sex and interpret the resulting probabilities.
- State that human females have two X sex chromosomes, written XX.
- Describe how X-bearing eggs are produced by meiosis in females.
- Use a genetic diagram to show that XX offspring result from an X egg fertilised by an X sperm.
- State that human males have one X and one Y sex chromosome, written XY.
- Explain how meiosis in males produces X-bearing and Y-bearing sperm in roughly equal numbers.
- Use a genetic diagram to show that XY offspring result from an X egg fertilised by a Y sperm.
- Construct a genetic cross diagram using XX and XY parental genotypes.
- Identify the sex chromosomes present in eggs and sperm.
- Interpret the offspring of a sex inheritance cross as a 1:1 ratio with a 50% probability for each sex.
- Derive a simple ratio from the outcomes of a genetic cross.
- Convert a ratio into a probability and use it to predict expected offspring numbers.
- Explain why observed offspring numbers may differ from expected ratios in small samples.
Sex determination exam tips
Marking Points
- State that ordinary human body cells are diploid, containing two copies of each chromosome.
- Identify the total number of chromosomes as 46, arranged as 23 homologous pairs.
- Explain that one chromosome in each pair comes from the mother and one from the father.
- Distinguish body cells, which have 23 pairs, from gametes, which have 23 single chromosomes.
- Link the restoration of 23 pairs at fertilisation to the fusion of haploid sperm and egg nuclei.
- Identify the 22 pairs of autosomes as carrying genes that control characteristics other than sex.
- Identify the 23rd pair as the sex chromosomes, XX in females and XY in males.
- Explain that the Y chromosome carries the gene that determines male sex, so its presence results in a male.
- Describe gametes as carrying one sex chromosome, with eggs always X and sperm either X or Y.
- Use a Punnett square or genetic cross to show the 50:50 chance of XX or XY offspring at fertilisation.
- Human body cells contain 23 pairs of chromosomes, one pair being the sex chromosomes.
- In females the two sex chromosomes are both X, so they are the same and written XX.
- The X chromosomes are homologous, carrying the same genes in the same order, so they pair during meiosis.
- All normal eggs produced by a female carry a single X chromosome.
- Fertilisation of an X egg by an X sperm gives an XX zygote, which develops into a female.
- Because a female has only X chromosomes, she cannot determine the sex of her offspring; the sperm does.
- Human body cells contain 23 pairs of chromosomes, including one pair of sex chromosomes.
- In males the sex chromosomes are different: one X and one smaller Y, written XY.
- The X and Y chromosomes differ in size and the genes they carry, so they are not a matching homologous pair.
- During meiosis the X and Y separate, so a male produces roughly equal numbers of X-bearing and Y-bearing sperm.
- Fertilisation by a Y sperm gives an XY zygote, which develops into a male.
- The father's sperm, not the mother's egg, determines whether the offspring is male or female.
- Parental genotypes correctly stated as XX for the female and XY for the male.
- Gametes correctly identified: eggs all carry X; sperm carry either X or Y in equal proportions.
- A completed Punnett square or equivalent cross showing the four possible offspring combinations XX, XX, XY and XY.
- Offspring genotypes interpreted correctly as two XX (female) and two XY (male).
- A correct conclusion that the expected ratio of female to male offspring is 1:1, giving a 50% probability of each sex.
- Use of correct genetic terminology such as gamete, fertilisation, genotype and chromosome.
- Correctly counting the outcomes in a completed genetic cross before forming a ratio.
- Expressing the outcome as a simple ratio in its lowest terms, for example 3:1 or 1:1.
- Converting a ratio into a probability by dividing the number of favourable outcomes by the total number of outcomes.
- Using direct proportion to predict expected numbers, for example multiplying the probability by the total number of offspring.
- Recognising that expected ratios describe probability and may not match small observed samples exactly.
- Maintaining correct genotype and phenotype labels when interpreting the ratio.
Examiner Tips
- 💡Use the phrase '23 pairs' precisely, and add '46 chromosomes in total' to show the numerical link.
- 💡When a question asks about gametes, contrast their haploid number with the diploid number of body cells.
- 💡Sketch a simple cell with 23 paired lines and label one chromosome from each parent to support your written answer.
- 💡Label a genetic cross clearly with parental genotypes, gametes and offspring genotypes to show sex determination.
- 💡Use the terms autosome and sex chromosome accurately when describing the 23 pairs.
- 💡When asked about probability, state that there is a 50% chance of each sex, based on equal numbers of X-bearing and Y-bearing sperm.
- 💡Use a Punnett square or genetic diagram with parental gametes X and X for the mother and X and Y for the father to show the 1:1 ratio of XX to XY.
- 💡Always write sex chromosomes as XX or XY, not as single letters, and label gametes clearly as eggs and sperm.
- 💡Link the answer back to meiosis: homologous X chromosomes separate so each egg receives one X.
- 💡Draw a genetic diagram with parental genotypes XX and XY, showing gametes X, X and X, Y and the resulting XX and XY offspring.
- 💡State the expected ratio of male to female offspring as approximately 1:1 and link it to the equal numbers of X and Y sperm.
- 💡Use the terms 'sex chromosome', 'gamete' and 'fertilisation' accurately to gain credit for explanation.
- 💡Always show the parental genotypes, the gametes and the completed cross, because working can earn credit even if the final ratio is wrong.
- 💡Label the Punnett square clearly with female and male gametes so the examiner can follow your reasoning.
- 💡State the outcome as both a ratio and a probability, for example 1:1 and 50%, to cover the wording of the question.
- 💡Write the ratio and then convert it to a fraction or decimal before calculating predicted numbers.
- 💡Show each step of a proportion calculation so method marks can be awarded even if the final value is wrong.
- 💡Check that the ratio is in simplest form and that the order of the ratio matches the order requested in the question.
Common Mistakes
- Writing that body cells contain 23 chromosomes rather than 23 pairs; correct this by stating 23 pairs, which equals 46 chromosomes.
- Confusing diploid and haploid numbers; correct this by remembering body cells are diploid (2n = 46) and gametes are haploid (n = 23).
- Claiming that all 23 pairs are identical; correct this by noting that the pairs are homologous but may carry different alleles, and the 23rd pair differs between males and females.
- Stating that the 22 pairs control only sex; correct this by saying the 22 pairs are autosomes controlling other characteristics, while the 23rd pair determines sex.
- Believing that females are XY and males are XX; correct this by recalling females are XX and males are XY.
- Thinking that the egg determines sex; correct this by explaining that sperm carry either X or Y, so the sperm determines the sex of the offspring.
- Writing that females have 'two X chromosomes but they are different' — the error is confusing sex chromosomes with non-homologous pairs; the correction is that the two X chromosomes are homologous and the same type.
- Stating that females produce both X and Y eggs — the error is mixing up the parents; the correction is that females produce only X eggs, while males produce X and Y sperm.
- Saying sex is decided by the mother's chromosome — the error is ignoring the father's contribution; the correction is that the sperm's sex chromosome determines the sex of the offspring.
- Writing that males have 'two different chromosomes called XY but they are homologous' — the error is treating X and Y as a matching pair; the correction is that they differ in size and gene content.
- Saying males produce only Y sperm — the error is forgetting meiosis separates X and Y; the correction is that half the sperm carry X and half carry Y.
- Claiming the egg determines sex — the error is ignoring which sperm fertilises the egg; the correction is that an X egg plus a Y sperm gives a male, while an X egg plus an X sperm gives a female.
- Writing the father's gametes as XY rather than separating them into X and Y sperm; correction: gametes carry only one chromosome from each pair, so each sperm carries either X or Y.
- Showing the mother producing X and Y eggs; correction: the mother is XX, so all her eggs carry an X chromosome.
- Claiming that a particular family must have equal numbers of boys and girls; correction: the 1:1 ratio is a probability for each fertilisation, and small samples can differ by chance.
- Reversing the ratio, such as writing 1:3 instead of 3:1; correction: state clearly which value refers to which phenotype and keep the order consistent.
- Treating a 3:1 ratio as a guarantee that three-quarters of actual offspring must show the dominant phenotype; correction: it is an expected probability, and chance variation is larger in small samples.
- Adding the parts of a ratio incorrectly when finding a probability; correction: the total number of outcomes is the sum of all parts, so 3:1 gives four equal parts.