One chromosome pair out of 23 decides biological sex in humans — and because that pair is mismatched in males, a whole family of conditions is inherited differently in males and females. This is the page where genetics stops being symmetrical.
📚 What you need to know
Pair 23 is the sex chromosomes: females are XX, males are XY. Pairs 1 to 22 are autosomes.
All eggs carry X. Half the sperm carry X and half carry Y, so the sperm decides the sex of the child, giving a 50 : 50 ratio.
The Y carries the SRY gene, which triggers testis development and testosterone production. No SRY, and ovaries develop instead.
The X is much larger and carries many genes that have nothing to do with sex; the Y carries very few.
A sex-linked gene is on the X and has no matching allele on the Y, so males have only one copy.
That is why males are affected by X-linked recessive conditions such as haemophilia far more often than females.
Two chromosomes, very different sizes
The X chromosome is large and carries hundreds of genes. The Y is short and carries only a small number, most of them to do with male development. That mismatch is the whole reason sex linkage behaves the way it does.
A female with XX has two copies of every gene on the X. A male with XY has just one copy, and no partner allele on the Y to mask it. Whatever allele a male has on his single X will show in his phenotype — dominant or recessive.
Genes for things like blood clotting sit on the X and have no partner on the Y. In a male there is no second copy to fall back on.
Why the father decides the sex
During meiosis the sex chromosomes separate like any other pair. A female has XX, so every egg gets an X. A male has XY, so half his sperm get the X and half get the Y.
Whichever sperm reaches the egg first decides it. X sperm + X egg = XX, a girl. Y sperm + X egg = XY, a boy. The expected ratio is 1 : 1, so roughly half of all children are male.
Gametes
X (from father)
Y (from father)
X (from mother)
XX — female
XY — male
X (from mother)
XX — female
XY — male
Ratio 1 : 1. 50% chance of a girl, 50% chance of a boy — and, as always, the odds reset with every pregnancy. This is the one Punnett square where you write chromosomes in the boxes instead of alleles.
Careful with wording: the father does not “choose” anything. Say that half his sperm carry X and half carry Y, so the sex of the child depends on which type of sperm fertilises the egg.
Sex-linked inheritance: haemophilia
Haemophilia is caused by a recessive allele of a gene on the X chromosome. That gene codes for factor VIII, a protein needed for blood to clot. Without working factor VIII, bleeding does not stop properly.
Two alleles: XF gives working factor VIII, Xf does not.
Now count the possibilities.
Genotype
Sex
Phenotype
XFXF
Female
Normal clotting
XFXf
Female
Normal clotting, but a carrier
XfXf
Female
Haemophilia (rare)
XFY
Male
Normal clotting
XfY
Male
Haemophilia
Females have three possible phenotypes: normal, carrier or affected. Males have only two: normal or affected. There is no such thing as a male carrier for an X-linked gene, because he has no second X to carry it on.
That is why the condition is far more common in males. A male needs just one faulty allele. A female needs two — which means her father must be affected and her mother must be at least a carrier.
📚 Writing sex-linked genotypes
Always write the chromosome letter (X or Y) with the allele as a superscript: XFXf, not Ff.
The Y gets no superscript — the allele simply is not there.
Male gametes are XF or Y, so always two different kinds.
Dropping the X and Y is the single most common way to lose marks on these questions.
Worked examples
WORKED EXAMPLE 1
A carrier female is married to a male with normal clotting. Draw the cross and state the expected phenotypes of their children.
Step 1: parental genotypes
Carrier female XFXf × normal male XFYStep 2: gametes
Mother: XF or Xf Father: XF or Y
Step 3: combine the four boxesXFXF, XFXf, XFY, XfY1 normal girl : 1 carrier girl : 1 normal boy : 1 boy with haemophiliaNo daughter is affected here, because every daughter receives a working XF from her father.
Gametes
XF (father)
Y (father)
XF (mother)
XFXF — normal female
XFY — normal male
Xf (mother)
XFXf — carrier female
XfY — male with haemophilia
WORKED EXAMPLE 2
Using the same cross, calculate the probability that a child is a boy with haemophilia, and the probability that a boy has haemophilia.
Step 1: read the question carefullyThese are two different questions. One counts all children; the other counts only boys.Step 2: probability that a child is an affected boy
1 box out of 4 → 1/4 = 25%Step 3: probability that a boy is affected
Only 2 boxes are boys, and 1 of those is affected → 1/2 = 50%25% of all children; 50% of the sons“Of the boys” narrows the sample down to two boxes. Underline that phrase in the question.
💡 Exam tip
Always show your gametes for sex-linked crosses — it is usually a separate mark.
Give the sex with the phenotype. “Affected” is not enough; write “male with haemophilia”.
Check whether the question asks about all children or one sex. The answer usually doubles.
The Y carries no allele of the gene — say that explicitly when explaining why males are affected more often.
Remember affected daughters are possible but rare, and require an affected father plus a carrier or affected mother.
Colour blindness works exactly the same way — same reasoning, different protein.
⚠ Common mix-up
Writing sex-linked genotypes as Ff. Without the X and Y the examiner cannot tell the sex, and the mark goes.
Putting a superscript on the Y. The Y has no copy of the gene, so nothing goes on it.
Saying males are carriers. A male with the faulty allele has the condition.
Saying the mother determines the sex. All her eggs carry X.
Assuming an affected father passes haemophilia to his sons. He gives his sons the Y, so he cannot — but all his daughters become carriers.
Treating sex-linked and autosomal conditions the same. Autosomal conditions hit both sexes at the same rate.
Up next: Pedigree Charts — how to read a family tree and work out whether a condition is dominant, recessive or sex-linked.
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