Why are sex-linked disorders such as colour blindness more common in males?

Males have only one X chromosome, so a recessive allele on it is always expressed

Blood groups, codominance and sex-linked disorders

Some genes have more than two alleles, and some alleles are equally dominant. Genes on the X chromosome are inherited differently in males and females.

GCSE · 1BI0 · 3.16, 3.17B, 3.18B (Biology only)
About 16 minutes

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ABO blood groups and codominance

The ABO blood group gene has three alleles: I^A, I^B and I^O.
I^A and I^B are codominant: if both are present, both are expressed. I^O is recessive to both.

Genotype	Blood group
I^AI^A or I^AI^O	A
I^BI^B or I^BI^O	B
I^AI^B	AB (codominance)
I^OI^O	O

Sex-linked inheritance

A sex-linked gene is carried on a sex chromosome, usually the X. The Y chromosome is smaller and carries fewer genes.
Females (XX) have two copies, so one normal allele can hide a faulty recessive allele: they are carriers.
Males (XY) have only one X, so a recessive allele on it is always expressed. This is why sex-linked disorders such as red-green colour blindness and haemophilia are more common in males.

A sex-linked cross

Use X^H for the normal allele and X^h for the haemophilia allele.
Carrier mother X^HX^h × unaffected father X^HY gives: X^HX^H, X^HX^h, X^HY, X^hY.
So: no daughters are affected (half are carriers); half the sons are affected. Overall, 1 in 4 children (25%) has haemophilia.

Worked example

A man with blood group A (I^AI^O) and a woman with blood group B (I^BI^O) have a child. What is the probability the child has blood group O?

Man's gametes: I^A or I^O. Woman's gametes: I^B or I^O.
Offspring: I^AI^B (AB), I^AI^O (A), I^BI^O (B), I^OI^O (O).
One out of four is I^OI^O.

Answer: 1 in 4 (25%)

Key idea

I^A and I^B are codominant and I^O is recessive. Sex-linked genes are on the X chromosome, so males (one X) show recessive sex-linked disorders more often. Carrier females have one faulty allele.

Key words

Codominance: When both alleles in a heterozygote are expressed in the phenotype, such as the IA and IB alleles in blood group AB.
Chromosome: A long coiled molecule of DNA carrying many genes, found in the nucleus.
Allele: A different form of the same gene.
Gene: A section of DNA that codes for a specific protein.
Recessive allele: An allele that is only expressed when two copies are present.

All GCSE and International GCSE Biology key terms and definitions →

Quick check

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Extra practice

2 more questions

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Which genotype gives blood group O?
1. I^BI^O
2. I^OI^O
3. I^AI^O
4. I^AI^B
Show the answer
Answer: I^OI^O
I^O is recessive.
What blood group does a person with genotype I^BI^O have?
1. Group A
2. Group O
3. Group B
4. Group AB
Show the answer
Answer: Group B
I^B is dominant to I^O.

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Sources and references

Pearson Edexcel GCSE Biology (1BI0) specification, topic 3.16, 3.17B, 3.18B (Biology only)

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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