Sex Determination and Inherited Disorders [H]

This lesson covers how biological sex is determined by the sex chromosomes and how some disorders are sex-linked, meaning the gene responsible is located on the X chromosome. The sex-linked inheritance section is Higher Tier only content for AQA GCSE Biology.

Sex Determination

In humans, biological sex is determined by the 23rd pair of chromosomes, known as the sex chromosomes.

Sex	Chromosome combination	Notation
Female	Two X chromosomes	XX
Male	One X and one Y chromosome	XY

The Y chromosome carries a gene called SRY (Sex-determining Region Y) that triggers the development of male characteristics. Without this gene, the embryo develops as female.

Inheritance of Sex

A mother is always XX, so she can only pass on an X chromosome
A father is XY, so he can pass on either an X or a Y chromosome
The sex of the offspring depends on which chromosome the father contributes

	X (from mother)	X (from mother)
X (from father)	XX (girl)	XX (girl)
Y (from father)	XY (boy)	XY (boy)

Result: There is a 50% chance of the offspring being male and a 50% chance of being female.

graph TD
    A[Mother XX] -->|X| C[Offspring]
    A -->|X| C
    B[Father XY] -->|X| C
    B -->|Y| C
    C --> D[XX - Female 50%]
    C --> E[XY - Male 50%]

Exam Tip: You will almost certainly be asked to complete a genetic diagram showing how sex is inherited. Remember: the mother always donates an X, and the father donates either an X (producing a girl) or a Y (producing a boy). This gives a 1:1 ratio.

Sex-Linked Inheritance [H]

Some genes are located on the X chromosome but not on the Y chromosome (because the Y chromosome is much smaller and carries fewer genes). These genes are described as sex-linked.

Because males have only one X chromosome, they only have one copy of any gene located on the X chromosome. This means that if a male inherits a recessive allele on his X chromosome, he will always show the recessive phenotype — he has no second X chromosome to "mask" it.

Females, with two X chromosomes, have two copies of sex-linked genes, so they can be carriers of recessive conditions without being affected.

	Females (XX)	Males (XY)
Number of copies of X-linked genes	Two	One
Can be carriers?	Yes (one normal, one faulty allele)	No — they either have it or they do not
Chance of showing recessive X-linked condition	Must have two recessive alleles	Only needs one recessive allele

Exam Tip: Sex-linked conditions are much more common in males than females. This is because males only need ONE copy of the recessive allele (on their single X), while females need TWO copies. This is a critical point for Higher Tier.

Example: Colour Blindness [H]

Red-green colour blindness is a sex-linked condition. The gene for colour vision is located on the X chromosome.

Let X^N = normal vision (dominant) and X^n = colour blindness (recessive):

Genotype	Phenotype
X^N X^N	Normal vision female
X^N X^n	Carrier female (normal vision but carries the allele)
X^n X^n	Colour-blind female (rare — needs two copies)
X^N Y	Normal vision male
X^n Y	Colour-blind male (only needs one copy)

Cross: Carrier female (X^N X^n) x Normal male (X^N Y)

	X^N (from father)	Y (from father)
X^N (from mother)	X^N X^N (normal female)	X^N Y (normal male)
X^n (from mother)	X^N X^n (carrier female)	X^n Y (colour-blind male)

Result:

25% normal female
25% carrier female
25% normal male
25% colour-blind male

Notice that 50% of the sons are colour-blind, but 0% of the daughters are colour-blind (though 50% are carriers).

Example: Haemophilia [H]

Haemophilia is another sex-linked disorder. It is caused by a recessive allele on the X chromosome that affects the production of clotting factors in the blood. People with haemophilia bleed for much longer than normal after injury because their blood does not clot properly.

Let X^H = normal clotting (dominant) and X^h = haemophilia (recessive):

Genotype	Phenotype
X^H X^H	Normal female
X^H X^h	Carrier female
X^h X^h	Haemophiliac female (very rare)
X^H Y	Normal male
X^h Y	Haemophiliac male

Cross: Carrier female (X^H X^h) x Normal male (X^H Y)

	X^H	Y
X^H	X^H X^H	X^H Y
X^h	X^H X^h	X^h Y

Result:

Girls: 50% normal, 50% carrier (none have haemophilia)
Boys: 50% normal, 50% haemophiliac

Historical Example: The Royal Family

Haemophilia was famously carried by Queen Victoria of the United Kingdom, who was a carrier. She passed the allele to several of her children, who then married into other European royal families, spreading the condition across European royalty. This is sometimes called "the Royal Disease."

Exam Tip: In sex-linked inheritance questions, always write the sex chromosomes as part of the allele notation (e.g. X^H, X^h, Y). This shows the examiner you understand that the gene is on the X chromosome and not on the Y.

Why Are Sex-Linked Conditions More Common in Males?

This is a key exam question. The answer follows this logic:

Sex Determination and Inherited Disorders [H]

Sex Determination and Inherited Disorders [H]