What you'll learn
In humans, whether you are biologically male or female is decided by a single pair of chromosomes called the sex chromosomes. This guide explains how sex is determined at fertilisation, how to use a genetic diagram (Punnett square) to show the expected ratio of males to females, and how sex-linked inheritance works for genes carried on the X chromosome. For AQA GCSE Biology you need to understand the XX and XY system, be able to predict the probability of offspring being male or female, and explain why some genetic conditions, such as red-green colour blindness, are more common in males. By the end you should be able to draw and interpret genetic crosses involving sex chromosomes.
Key terms and definitions
Sex chromosomes — The pair of chromosomes that determine biological sex; in humans these are the 23rd pair.
X chromosome — The larger sex chromosome; females have two (XX), males have one (XY).
Y chromosome — The smaller sex chromosome that carries the gene triggering male development; only males have one.
Gamete — A sex cell (sperm or egg) containing one set of chromosomes, including one sex chromosome.
Genotype — The combination of alleles an organism has, for example XX or XY.
Phenotype — The physical characteristics that result from the genotype, for example male or female.
Sex-linked characteristic — A feature controlled by a gene carried on a sex chromosome, usually the X chromosome.
Carrier — An individual who has one copy of a recessive allele but does not show the condition; they can pass it on.
Core concepts
The sex chromosomes
Humans have 23 pairs of chromosomes, giving 46 in total. Twenty-two pairs control general body features, and the 23rd pair are the sex chromosomes. Females have two X chromosomes (XX) and males have one X and one Y (XY). The Y chromosome carries a gene that switches on the development of male characteristics; without a Y chromosome, an embryo develops as female.
How sex is determined at fertilisation
When gametes form, the sex chromosome pair separates so each gamete gets just one. A female (XX) can only pass on an X chromosome, so all eggs carry X. A male (XY) can pass on either an X or a Y, so half his sperm carry X and half carry Y. The sex of the child therefore depends on which sperm fertilises the egg:
- X sperm + X egg → XX → female
- Y sperm + X egg → XY → male
Because half the sperm carry X and half carry Y, there is an equal chance of a boy or a girl at each fertilisation.
Using a genetic diagram for sex
A Punnett square shows this clearly. Place the mother's gametes (X and X) along the top and the father's gametes (X and Y) down the side:
| X (egg) | X (egg) | |
|---|---|---|
| X (sperm) | XX | XX |
| Y (sperm) | XY | XY |
The four boxes give two XX (female) and two XY (male), a ratio of 1 female : 1 male, or a 50% chance of each.
What sex-linkage means
Some genes are found only on the X chromosome, with no matching gene on the smaller Y chromosome. These are described as sex-linked. Because a male has only one X chromosome, he has only one copy of each of these genes. If that single copy carries a recessive allele for a condition, he will show the condition, because there is no second X chromosome to mask it. A female has two X chromosomes, so a recessive allele on one X can be masked by a normal allele on the other — she would be a carrier rather than affected.
Why sex-linked conditions are more common in males
Consider a recessive sex-linked condition such as red-green colour blindness. A male has a single X chromosome, so one recessive allele is enough to make him colour blind. A female needs the recessive allele on both of her X chromosomes to be affected, which is much less likely. This is why more males than females have red-green colour blindness and similar X-linked conditions.
How a recessive allele passes through a family
Because the Y chromosome does not carry the gene, a father cannot pass an X-linked condition to his sons — sons always get their single X from the mother. This gives sex-linked conditions a distinctive pattern in a family tree: the condition often seems to "skip" a generation, passing from an affected grandfather, through his unaffected carrier daughters, to some of his grandsons. A daughter of an affected father is always at least a carrier, because she must inherit her father's affected X. Recognising this pattern in a family tree is a common higher-tier task, so it is worth learning how the allele travels: from father only to daughters, and from a carrier mother to half of her sons and half of her daughters.
The idea of probability in genetics
Genetic diagrams give the probability of each outcome, not a guarantee. A 50% chance of a boy means that, on average, half of a large number of births would be boys — but any individual family might have several children of the same sex by chance. Each fertilisation is an independent event, so the probability is the same every time regardless of what happened before. Understanding probability this way stops you making the common error of thinking the odds "even out" within one small family.
Worked examples
Example 1: Probability of a girl
A couple already have three boys. What is the chance their next child is a girl? Each fertilisation is independent, and half the father's sperm carry X and half carry Y. The chance of a girl is 50% (or 1 in 2), regardless of the sex of previous children. Past outcomes do not change the probability.
Example 2: Drawing the sex cross
Show, using a genetic diagram, why boys and girls are equally likely. Mother is XX, so all eggs are X. Father is XY, so sperm are X or Y. A Punnett square gives XX, XX, XY, XY — two female and two male, a 1:1 ratio. This means a 50% probability of each sex.
Example 3: A carrier mother
A woman is a carrier for a recessive sex-linked condition (X^N X^n, where N is normal and n is the recessive allele). Her partner is unaffected (X^N Y). What is the chance a son is affected? The mother passes X^N or X^n; the father passes X^N or Y. Sons receive the Y from the father and one X from the mother. Half the sons get X^n Y (affected) and half get X^N Y (unaffected). So there is a 50% chance that a son is affected. Daughters cannot be affected here, because they always receive a normal X^N from the father.
Example 4: Interpreting a ratio
A genetic cross for a sex-linked gene predicts, among the sons, a 1:1 ratio of affected to unaffected, but none of the daughters are affected. Explain this pattern. The father can only pass a normal allele on his single X to his daughters, so no daughter is affected. Sons receive their only X from the carrier mother, half of whom carry the recessive allele, giving the 1:1 ratio among sons.
Common mistakes and how to avoid them
A very common error is thinking the mother determines the baby's sex. She cannot — she only provides X chromosomes. It is the father's sperm, carrying either X or Y, that decides the sex of the child.
Students sometimes write that the chance of a boy changes if a family already has several boys. It does not. Each fertilisation is an independent event with a 50% chance of each sex.
When drawing genetic diagrams for sex-linkage, always write the allele on the chromosome, for example X^N or X^n, not just N or n. Sex-linked alleles are carried on the X chromosome, and the diagram must show this or you will lose marks.
Another mistake is forgetting that males cannot be carriers of X-linked conditions. With only one X chromosome, a male either has the condition or does not — there is no second X to hide the allele. Only females can be carriers.
Finally, do not confuse "carrier" with "affected". A carrier has the allele but shows no symptoms; an affected individual shows the condition.
Exam technique for "Sex determination and sex-linked inheritance"
For sex determination questions, draw a clear Punnett square with the mother's X and X across the top and the father's X and Y down the side. State the ratio (1 male : 1 female) and convert it to a percentage or probability if asked. Always name the gametes explicitly.
For sex-linked crosses, use superscript notation such as X^N and X^n and show the Y chromosome as carrying no allele. Work through the sons and daughters separately, because the pattern is usually different for each. Explaining why a condition is more common in males is a frequent extended question: link it to males having only one X chromosome, so a single recessive allele is enough to cause the condition.
Practise expressing outcomes three ways — as a ratio, as a fraction, and as a percentage — because questions may ask for any of them.
Quick revision summary
- Humans have 23 pairs of chromosomes; the 23rd pair are the sex chromosomes: XX in females, XY in males.
- The father determines the child's sex, because his sperm carry either X or Y; the mother always provides X.
- A genetic diagram gives a 1:1 ratio of males to females, a 50% chance of each.
- Sex-linked genes are carried on the X chromosome; males have only one X, so a single recessive allele causes the condition.
- Females can be carriers of X-linked conditions; males cannot.
- X-linked recessive conditions such as red-green colour blindness are more common in males.