Sex-linked inheritance and pedigrees

Why more men are colour blind

About 8 in 100 men cannot tell red from green, but only about 1 woman in 200. The reason is that the gene lies on the X chromosome. A gene on the X chromosome is X-linked, and a gene on the Y chromosome is Y-linked. Most X-linked genes have no matching gene on the Y chromosome.

• A woman has two X chromosomes. She has two copies of the gene.
• A man has one X and one Y. He has only one copy of an X-linked gene, so the allele that he has decides his trait. He is hemizygous.

So a recessive X-linked allele shows in every man who has it, but in a woman only if she has it twice.

Haemophilia

Haemophilia is a disorder in which the blood does not clot properly. A single protein of the clotting cascade is faulty, so even a small cut can bleed for a very long time. It is an X-linked recessive disorder. We write H for the normal allele and h for the haemophilia allele, both on the X chromosome.

• $X^H X^H$ is a normal woman. $X^H X^h$ is a carrier: she has the allele but does not show the disorder. $X^h X^h$ is a haemophilic woman.
• $X^H Y$ is a normal man. $X^h Y$ is a haemophilic man.

A haemophilic woman is very rare. Her father must be haemophilic and her mother must be at least a carrier. The famous pedigree of Queen Victoria, who was a carrier, shows how the allele passed to several royal families of Europe.

Colour blindness

Red-green colour blindness is also X-linked and recessive. A defect in the red cone or the green cone of the retina makes the red-green difference hard to see. About 8 per cent of men and about 0.5 per cent of women are colour blind. NCERT prints 0.4 per cent for women. The standard figures are close, and the pattern is the same.

The crosses

Make the Punnett squares for the two most common crosses.

• Carrier mother × normal father. Of the daughters, half are normal and half are carriers. None is haemophilic. Of the sons, half are normal and half are haemophilic. Each child has a chance of one quarter of being a haemophilic son.
• Haemophilic father × normal mother. All the daughters are carriers, and all the sons are normal. The son gets his X from his mother and only a Y from his father.

The allele goes from a man through his daughter to his grandson. This is criss-cross inheritance. An X-linked allele cannot pass from father to son, because a father gives his son the Y.

You should see that a carrier mother with a normal father has no haemophilic daughters, but half the sons are haemophilic. A haemophilic father with a carrier mother can have haemophilic daughters.

Reading a pedigree

A pedigree is a family tree that follows a trait. Use these symbols.

• A square is a male, and a circle is a female.
• A line between a square and a circle joins the parents, and a line down from it goes to the children.
• A filled symbol is a person who shows the trait. For an X-linked gene, a dot in a circle marks a carrier.

Look at the pedigree of haemophilia. The mother I-2 is a carrier, so half her sons are affected (II-2) and half her daughters are carriers (II-4). II-4 passes the allele on to her son III-1.

These signs tell you that a trait is X-linked and recessive.

• More men than women are affected.
• An affected man never has an affected son from a normal woman, but his daughters are carriers.
• The trait often skips a generation, passing through a carrier woman.

Try it in Python

The program counts the children of a carrier mother and a normal father, with exact fractions.

Practise with new numbers

Common mistakes

• Writing the allele on the Y chromosome. In haemophilia and colour blindness, the allele is on the X.
• Forgetting that a carrier has no symptoms. A carrier woman is normal, but she can have a haemophilic son.
• Thinking that a father can pass an X-linked allele to his son. He gives his son a Y.
• Saying that haemophilia is a disease of men only. A woman can have it, but only if her father and mother both give her the allele.

Remember these

• X-linked recessive disorders (haemophilia, red-green colour blindness) are more common in men, because a man has only one X.
• A carrier mother and a normal father have a chance of one quarter for a haemophilic son. No daughter is affected.
• A haemophilic father and a normal mother have only carrier daughters and normal sons.
• A pedigree uses a square for a male, a circle for a female and a filled symbol for an affected person.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. A colour-blind man marries a woman who has normal vision and no colour-blind allele. What are their children like?

2. A woman with normal vision has a colour-blind father. What is her genotype?

3. Why is a haemophilic woman rare?

4. A carrier woman and a normal man have four children. What is the expected number of haemophilic sons?

5. In a pedigree, an affected man has an affected son. Can the trait be X-linked recessive if the mother has no allele?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 4, sections on sex-linked inheritance and pedigree analysis.
• Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 13.2.