Sex Linkage

Spec Mapping — OCR H420 Module 6.1.2 — Patterns of inheritance, content statements covering sex linkage, the distinctive inheritance patterns of X-linked recessive disorders, and the use of pedigree analysis (refer to the official OCR H420 specification document for exact wording). This lesson extends Mendelian principles to genes on the sex chromosomes — the asymmetry between males (XY hemizygous) and females (XX) produces the most distinctive of all inheritance patterns and is a high-utility exam topic.

The story of sex linkage starts in the Drosophila room of Columbia University in 1910. Thomas Hunt Morgan (Nobel 1933, paraphrased) was sceptical of Mendel's theory and looked for exceptions. He found a single white-eyed male in a culture of wild-type red-eyed Drosophila melanogaster. When he crossed this white-eyed male with red-eyed females he found that all F1 were red-eyed (red is dominant) — but in the F2, all the white-eyed flies were male. This was the first proof that a specific phenotypic trait is physically linked to a specific chromosome (the X), the empirical foundation of the chromosomal theory of inheritance. Alfred Sturtevant (1913, paraphrased), then a young undergraduate in Morgan's lab, used Drosophila linkage data to construct the first chromosome map — the basis of all subsequent genetic mapping.

Not all genes are inherited equally. Those on the sex chromosomes show distinctive inheritance patterns because males and females carry different combinations of sex chromosomes. Sex-linked inheritance is particularly important because several well-known human genetic disorders, including haemophilia, red-green colour blindness and Duchenne muscular dystrophy, are X-linked recessive conditions that affect males much more frequently than females. OCR A-Level Biology A specification module 6.1.2(c) requires you to understand sex linkage and be able to analyse sex-linked pedigrees and crosses.

Key Definitions:

• Sex linkage — the inheritance of a gene carried on a sex chromosome.
• X-linked — carried on the X chromosome.
• Y-linked — carried on the Y chromosome.
• Autosome — any chromosome that is not a sex chromosome.
• Carrier (heterozygote) — a female heterozygous for a recessive X-linked allele who does not usually show the trait but can pass it on.
• Hemizygous — having only one copy of a gene (e.g. males for all X-linked genes).

---

Sex Determination in Humans

Humans have 23 pairs of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes (the 23rd pair).

• Females are XX — two X chromosomes.
• Males are XY — one X and one Y.

The Y chromosome is much smaller than the X and carries fewer than 100 genes. The X chromosome carries over 800. Therefore:

• A female has two copies of every X-linked gene — she can be homozygous or heterozygous.
• A male has only one copy of every X-linked gene (hemizygous), so whatever allele he carries on his single X will be expressed. There is no "carrier" state for males.

Men pass their X to daughters and their Y to sons. Women pass one of their two Xs to every child regardless of sex. This asymmetry produces the characteristic sex-linked inheritance pattern.

---

X-Linked Recessive Inheritance

Most sex-linked diseases are X-linked recessive. A male with the allele is affected; a female needs two copies to be affected, and a female with one copy is a carrier (usually unaffected but able to pass it on).

Key observations about X-linked recessive conditions

• Affected males far outnumber affected females.
• A carrier mother has a 50% chance of passing the allele to each child. Half her sons will be affected; half her daughters will be carriers.
• An affected father passes his X (with the allele) to every daughter (all become carriers) and his Y (no allele) to every son (all unaffected, unless they inherit it from their mother).
• Affected individuals often "skip" generations, appearing in grandsons via unaffected carrier daughters.

Punnett square: carrier mother × unaffected father

Let Xᴬ be the normal allele and Xᵃ the recessive disease allele.

	Xᴬ	Y
Xᴬ	XᴬXᴬ	XᴬY
Xᵃ	XᴬXᵃ	XᵃY

Offspring: 1 unaffected daughter : 1 carrier daughter : 1 unaffected son : 1 affected son. Among sons alone, half are affected. Among daughters alone, half are carriers.

Punnett square: affected father × unaffected non-carrier mother

	Xᴬ	Xᴬ
Xᵃ	XᴬXᵃ	XᴬXᵃ
Y	XᴬY	XᴬY

All daughters are carriers; all sons are unaffected.

---

Named Examples

1. Haemophilia A

• Caused by a recessive allele on the X chromosome in the gene for clotting factor VIII.
• Affected males bleed excessively from minor injuries and suffer from internal joint bleeding.
• The "royal haemophilia" that spread through European royal families from Queen Victoria is the most famous historical example — Victoria was a carrier and passed the allele to several of her descendants.
• Incidence: around 1 in 5,000 males. Affected females are extraordinarily rare.

2. Red-Green Colour Blindness

• Caused by recessive alleles on the X chromosome in the genes for red or green photopigments in the cone cells of the retina.
• Affected individuals cannot distinguish red from green.
• Affects around 8% of males and only ~0.5% of females of European ancestry — the most common X-linked condition.

3. Duchenne Muscular Dystrophy (DMD)

• Caused by a loss-of-function mutation in the enormous DMD gene, which encodes dystrophin, a protein that connects muscle fibres to their membranes.
• Without dystrophin, muscle cells degenerate, leading to progressive weakness and early death if untreated.
• Affects about 1 in 3,500 boys; extremely rare in girls.

---

Worked Example: Royal Haemophilia

Queen Victoria was a carrier of haemophilia (XᴴXʰ) even though she was unaffected herself. Prince Albert was unaffected (XᴴY). What were the chances for each of their children?

	Xᴴ	Y
Xᴴ	XᴴXᴴ	XᴴY
Xʰ	XᴴXʰ	XʰY

• Daughters: 1/2 unaffected (XᴴXᴴ), 1/2 carriers (XᴴXʰ).
• Sons: 1/2 unaffected (XᴴY), 1/2 haemophiliac (XʰY).

Their son Leopold was indeed haemophiliac, and at least two of their daughters (Alice and Beatrice) were carriers who passed the allele to several royal families across Europe.

---

Pedigree Analysis

In a pedigree (family tree diagram), circles represent females, squares represent males, and filled-in shapes represent affected individuals. Clues that an inheritance is X-linked recessive:

• Far more affected males than females.
• All daughters of an affected father are carriers (and may appear unaffected).
• Affected males are often connected through carrier females on the mother's side.
• The trait can "skip" generations via unaffected carrier females.

Clues that it is not X-linked recessive:

• Affected individuals in every generation → probably autosomal dominant.
• Unaffected parents with affected daughter → probably autosomal recessive (not X-linked, because her father would have had to be affected).

---

Y-Linked and Rare Patterns

A few genes are Y-linked, such as SRY (the testis-determining factor). These are passed exclusively from father to son — no female carriers are possible. X-linked dominant conditions also exist (e.g. vitamin D-resistant rickets) but are much rarer than X-linked recessive.

---

Why Can X-linked Recessive Conditions Affect Females?

A female can be affected if she inherits the recessive allele from both parents — i.e. her father is affected and her mother is a carrier (or affected). This is rare but not impossible, and becomes more likely in families where the allele is common. X-inactivation in females can also allow a carrier female to show mild symptoms if, by chance, a high proportion of her cells inactivate the X carrying the normal allele.

---

Exam Tip

When writing genotypes for sex-linked traits, always use the X and Y chromosomes explicitly and put the allele as a superscript: XᴴY for a normal male, XʰY for a haemophiliac male, XᴴXᴴ for a homozygous normal female, XᴴXʰ for a carrier, and XʰXʰ for an affected female. Do not write Hh for a sex-linked trait — that loses the information about which chromosome the allele is on. Draw the Punnett square carefully and label each offspring by sex.

---

Common Exam Mistakes

• Writing sex-linked genotypes without the X and Y. You must show the chromosome.
• Forgetting that males cannot be carriers of X-linked traits. They are hemizygous.
• Saying "the father passed the allele to his son". Fathers pass Y to sons, not X. Sons inherit X-linked conditions from their mother, not their father.
• Confusing X-linked dominant and recessive patterns. Dominant = affects both sexes roughly equally and does not skip generations. Recessive = mostly males, skips generations via carriers.
• Assuming an affected female means an autosomal trait. Not always — she could be homozygous for an X-linked recessive (rare but possible).
• Not distinguishing genotype from phenotype in carriers. XᴴXʰ looks normal (phenotype) but carries the allele (genotype).

---

Quick Recap

• Sex linkage means a gene is on the X or Y chromosome.
• Females are XX and can be carriers; males are XY and hemizygous for X-linked genes.
• X-linked recessive conditions affect many more males than females and often skip generations.
• Named examples: haemophilia A, red-green colour blindness, Duchenne muscular dystrophy.
• Carrier mother × unaffected father: half of sons affected, half of daughters carriers.
• Affected father × unaffected non-carrier mother: all daughters carriers, all sons unaffected.
• Always write sex-linked genotypes as XᴬXᵃ, XᴬY etc., not just Aa.

---

X-Linked Recessive Pedigree — Inline SVG

Generation I: X^H Y unaffected X^H X^h carrier

Gen II: X^H X^H unaffected F

X^H X^h carrier F X^H Y unaffected M X^h Y affected M X^H Y

Gen III: X^H X^h carrier F

X^h Y affected M