Sex in humans is determined by the sex chromosomes: females are XX and males are XY. The Y chromosome carries far fewer genes than X, so genes on the X without Y equivalents are called sex-linked. Males have only one copy of these genes and always express a recessive sex-linked allele.
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:
| X (egg) | X (egg) | |
|---|---|---|
| X (sperm) | XX (female) | XX (female) |
| Y (sperm) | XY (male) | XY (male) |
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)
| Genotype | Sex | Phenotype |
|---|---|---|
| X^H X^H | Female | Unaffected |
| X^H X^h | Female | Carrier (unaffected but carries the allele) |
| X^h X^h | Female | Haemophilia (rare) |
| X^H Y | Male | Unaffected |
| X^h Y | Male | Haemophilia |
Worked inheritance diagram — carrier mother × unaffected father:
| X^H (from father) | Y (from father) | |
|---|---|---|
| X^H (from mother) | X^H X^H (unaffected female) | X^H Y (unaffected male) |
| X^h (from mother) | X^H X^h (carrier female) | X^h Y (haemophilia male) |
- 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?
| Condition | Chromosome | Type | Description |
|---|---|---|---|
| Haemophilia A | X | Recessive | Clotting factor VIII deficiency |
| Red-green colour blindness | X | Recessive | Cannot distinguish red from green wavelengths |
| Duchenne muscular dystrophy | X | Recessive | Progressive muscle wasting |
| Fragile X syndrome | X | Dominant with variable expression | Learning difficulties |
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?
- Identify the alleles using superscript notation: X^A (dominant) and X^a (recessive).
- Write the parental genotypes, including the Y chromosome for males (e.g. X^A Y, not just X^A).
- Show gamete formation — females produce two types of egg (X^A and X^a), males produce X^A sperm and Y sperm.
- Complete the Punnett square with all four combinations.
- 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.
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