Twenty-two of your chromosome pairs are matched. The twenty-third may not be, and that single asymmetry produces two of the most reliably examined ideas in genetics: why the father determines a child’s sex, and why haemophilia affects far more males than females.
📘 What you need to know
Sex is determined by an entire chromosome pair, not by a single gene. Females are XX, males are XY (pair 23 in humans).
Pairs 1 to 22 are autosomes and have no influence on sex.
Only the father can pass on a Y, so the sperm that fertilises the egg determines the sex — giving a 50:50 ratio.
The X is much larger than the Y and carries around 16 times more genes, including genes with nothing to do with sex, such as certain blood clotting factors.
The Y carries the SRY gene, involved in the development of testes and the production of testosterone.
A sex-linked gene is present on one sex chromosome and not the other. Males have only one copy of an X-linked gene, so a single recessive allele is enough to give the condition.
Haemophilia is caused by a recessive allele of the X-linked gene for factor VIII. Females can be carriers; males cannot.
Sex-linked alleles are written as a superscript on the X: XFXf for a carrier female, XfY for an affected male.
The one pair that does not match
Most characteristics are determined by one gene or a handful of genes. Sex is different: it is determined by which whole chromosome you inherit as the twenty-third pair. Two X chromosomes give a female, an X and a Y give a male.
Worth noting, because questions occasionally probe it: the XX and XY rule applies to mammals, not to all species. Birds, some insects and some reptiles use entirely different systems.
Why the father determines the sex
The mother is XX, so every egg she produces carries an X. There is no alternative. The father is XY, so meiosis gives him two kinds of sperm in equal numbers: half carrying his X, half carrying his Y. Whichever sperm happens to reach the egg first decides the outcome.
The last line is the one to remember. A father passes his X only to daughters, so an X-linked condition never goes from father to son.
What the X and Y actually carry
The X chromosome is considerably larger than the Y and has its centromere more centrally placed. As a result the Y codes for far fewer genes — the X carries roughly sixteen times as many.
Crucially, many of the genes on the X have nothing to do with sex. Certain blood clotting factors are coded for on the X and not on the Y, which is where sex-linked disease comes from. The Y, meanwhile, carries genes for male characteristics, including the SRY gene, which is involved in the development of testes in male embryos and in the production of testosterone. Females do not receive these genes, so ovaries develop instead and female sex hormones are expressed.
The gene bands are schematic, but the size difference is not. Everything about sex-linked inheritance follows from the Y being too small to carry partners for most X genes.
Sex-linked disorders: haemophilia
A sex-linked gene sits on one sex chromosome and not the other, so the sex of an individual affects which alleles they can pass on. If the gene is on the X, a male (XY) has only one copy while a female (XX) has two.
That asymmetry gives females three possible phenotypes — normal, carrier, or affected — but males only two: normal or affected. There is no such thing as a male carrier for an X-linked condition.
Haemophilia is the standard example. A gene on the X codes for a protein called factor VIII, which is needed for blood to clot. The dominant allele F codes for normal factor VIII; the recessive allele f results in a lack of it. A person with only the recessive allele cannot produce factor VIII and their blood does not clot normally.
Notice there is no box containing a male carrier. A male has one X, so he either has the working allele or he has the condition — there is nothing in between.
The notation
Write the chromosome as an upper case X or Y, with the allele as a superscript next to it. So XfXf is a homozygous female who has haemophilia, XFXf is a heterozygous female who is a carrier, and XfY is a male who has haemophilia. The Y never takes a superscript, because the gene is not on it.
Why males are affected more often. A female needs two copies of the recessive allele; a male needs only one, because he has no second X to carry a working version. Stating both halves of that comparison is what earns the mark — saying only “males have one X” is half an answer.
Worked examples
WORKED EXAMPLE
A man with haemophilia and a woman who is not a carrier have children. Determine the phenotypes of their sons and daughters. [3]
Write the parental genotypesfather X(f)Y, mother X(F)X(F)List the gametesfather gives X(f) or Y; mother gives X(F) onlyCombinedaughters: X(F)X(f) — all carriers, none affectedsons: X(F)Y — all completely unaffectedAll daughters are carriers; all sons are normalthe sons get the father’s Y, so they cannot inherit his faulty allele at all
WORKED EXAMPLE
Explain why haemophilia is far more common in males than in females. [3]
Locate the genethe factor VIII gene is on the X chromosome and is not present on the YCompare the two sexes directlya male is XY, so he has only one copy; a single recessive allele is expressed because there is no dominant allele to mask ita female is XX, so she needs the recessive allele on both X chromosomes to be affected3 marks: gene located on X, males have one copy so it is expressed, females need two copiesthe phrase “no second X to mask it” is doing the real work here
💡 Exam tips
Show the gametes. Completing a grid is not enough — examiners want to see that you worked out what each parent could produce.
Always give the phenotype as well as the genotype of the offspring, including whether a female is a carrier.
Read carefully whether the probability asked for is out of all children or out of one sex. These differ by a factor of two.
Use the X-superscript notation for sex-linked genes and ordinary letters for autosomal genes. Mixing them up signals confusion.
Never write a superscript on the Y for an X-linked gene. The gene is not there at all.
⚠️ Common mix-ups
Saying a male can be a carrier of an X-linked condition. He has one X, so he either has the condition or he does not.
Claiming the mother determines the sex. Every egg carries an X; only the father can contribute a Y.
Thinking father-to-son transmission of an X-linked allele is possible. Sons receive the father’s Y, never his X.
Treating sex determination as a single gene. It is an entire chromosome pair, which is why it is written differently.
Assuming XX and XY applies to every organism. It is the mammalian system; other groups differ.
Confusing autosomal recessive with X-linked recessive. If affected males and females appear in roughly equal numbers, suspect autosomal.
Up next: Pedigree Charts — how to look at a family tree and deduce, without being told, whether an allele is dominant or recessive and whether it sits on an autosome or the X.
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