Sex Determination and Sex-Linked Inheritance: GCSE Biology

How is sex determined in humans?

Humans have 23 pairs of chromosomes in each body cell. Twenty-two of these pairs are autosomes — the same in males and females. The 23rd pair is the sex chromosomes:

• Females: XX (two X chromosomes)
• Males: XY (one X and one Y chromosome)

During meiosis, gametes are produced. Every egg cell contains one X chromosome. Sperm cells contain either an X or a Y chromosome.

Crossing diagram:

This shows a 1:1 ratio of male to female offspring — a 50% chance of each sex in any pregnancy. The sex of the offspring is determined by which sperm fertilises the egg; the mother's contribution is always an X.

What makes a gene sex-linked?

A sex-linked gene is one carried on a sex chromosome — most commonly on the X chromosome (X-linked). Because the Y chromosome is very small and carries few genes (mostly those involved in male development), the Y chromosome does not carry matching alleles for most X-linked genes.

This creates an asymmetry:

• Females (XX) have two copies of every X-linked gene and can be homozygous or heterozygous.
• Males (XY) have only one copy of every X-linked gene (carried on their single X chromosome). This is called being hemizygous.

The consequence: a male with a recessive allele on his X chromosome will always express the trait — there is no second allele to mask it. A female needs two copies of the recessive allele to show the trait.

What is haemophilia and how is it inherited?

Haemophilia A is an X-linked recessive condition in which the blood lacks a clotting factor (Factor VIII). People with haemophilia bleed for much longer than normal after injury.

Notation:

• X^H = dominant allele (clotting factor present, unaffected)
• X^h = recessive allele (clotting factor absent, haemophilia)

Worked inheritance diagram — carrier mother × unaffected father:

• Probability of a son having haemophilia: 1 in 2 (50%)
• Probability of a daughter being a carrier: 1 in 2 (50%)
• Probability of any child being affected: 1 in 4 (25%)

Why are males more commonly affected by X-linked recessive conditions?

Males have only one X chromosome. If that X carries a recessive allele (such as X^h), there is no second X to provide a dominant allele to mask it. The male always expresses an X-linked recessive allele, regardless of whether it is "dominant" or "recessive" in a conventional sense — those terms only apply when there are two alleles to compare.

Females require two copies of the recessive allele (X^h X^h) to be affected. Since this requires inheriting a recessive allele from both parents, it is statistically less likely — especially for rarer conditions. A female with one X^h allele is a carrier: she does not show symptoms but can pass the allele to her children.

What other conditions are X-linked?

Red-green colour blindness is much more common in males (~8% of males, ~0.5% of females) for exactly the same reason as haemophilia — males only need one X carrying the recessive allele to be colour-blind.

How do you draw a sex-linked inheritance diagram?

1. Identify the alleles using superscript notation: X^A (dominant) and X^a (recessive).
2. Write the parental genotypes, including the Y chromosome for males (e.g. X^A Y, not just X^A).
3. Show gamete formation — females produce two types of egg (X^A and X^a), males produce X^A sperm and Y sperm.
4. Complete the Punnett square with all four combinations.
5. State the probabilities for each phenotype in both males and females separately.

Common exam mistake: writing the male genotype without the Y chromosome (e.g. just X^A instead of X^A Y). Always include the Y to make sex-linkage clear and to show that males are hemizygous.

Frequently asked questions

What does it mean to be a carrier of a sex-linked condition?

A carrier is a female (XX) with one dominant allele and one recessive allele for an X-linked condition (e.g. X^H X^h for haemophilia). She does not show symptoms because the dominant allele provides enough of the functional protein. However, she can pass the recessive allele to her children: a son who inherits it will be affected; a daughter who inherits it will be a carrier herself. Males cannot be carriers — if they have the recessive allele, they will show the condition.

Why can't fathers pass X-linked conditions to their sons?

A father passes his Y chromosome to his sons (XY → Y to son) and his X chromosome to his daughters (XY → X to daughter). Therefore, X-linked conditions cannot be passed from father to son. A man with haemophilia will not have affected sons — unless the sons' mother is also a carrier or affected. He will, however, pass the condition to all of his daughters (they all inherit his X^h), making all of them carriers.

How is red-green colour blindness different from haemophilia in its inheritance?

Both are X-linked recessive conditions and follow the same inheritance rules. The difference is in frequency: the recessive allele for red-green colour blindness is more common in the population (~8% of all X chromosomes), so it appears in about 8% of males and about 0.6% of females. Haemophilia is much rarer. Both conditions are more common in males because males are hemizygous for X-linked genes — one copy of the recessive allele is sufficient for expression.

Can females ever be affected by X-linked recessive conditions?

Yes, but it is rare. A female must inherit the recessive allele on both X chromosomes (X^a X^a) to show the condition. This can only happen if her father is affected (X^a Y) and her mother is at least a carrier (X^A X^a). For common X-linked conditions like colour blindness, this does happen — about 0.6% of females are colour-blind. For rarer conditions like haemophilia, homozygous affected females are extremely uncommon.

For genetics questions that build from first principles — without memorising patterns — explore Professor Darwin at aitutors.me.