GCSE biology Sex-linked genetic disorder of haemophilia hemophilia Punnett square diagrams explained Doc Brown's GCSE biology exam revision study notes

GCSE level biology exam revision notes on basic genetics Part 6

More complex genetics: 6.3

The sex-linked genetic disorder of haemophilia (hemophilia)

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INDEX of biology notes on more complex genetics - inherited sex/non-sex linked examples

(6.3) Haemophilia (hemophilia) - example of a genetic inheritance sex link

Again, as with colour blindness, haemophilia is due to a faulty allele on the X chromosome.

Haemophilia a medical condition in which the ability of the blood to clot is severely reduced, causing the sufferer to bleed severely from even a slight injury.

The condition is typically caused by a hereditary lack of a coagulation factor.

A person who suffers from this disorder is called a haemophiliac.

Because haemophilia disorder is sex-linked, both the chromosome and the allele must included in the genetic diagram showing the possible offspring genotypes and phenotypes.

Example 1 of inheriting haemophilia (hemophilia)

In the diagram below crossing an unaffected female carrier of the faulty allele with an unaffected non-carrier male, superscripts H represents the normal allele and h the recessive faulty allele.

From the genetic diagram, in this particular cross the ratio of unaffected to haemophilic is 3 : 1 (25% chance of the offspring being a haemophiliac and male).

BUT, the ratio in more details is:

unaffected non-carrier (male + female) : unaffected female carrier : haemophilic male is 2 : 1 : 1

Genetic Punnett square table for crossing an unaffected female carrier of the faulty recessive gene for haemophilia and an unaffected male (non-carrier)

Parent genotypes cross: X^HX^h x X^HY

Gametes: X^H, X^h , X^H and Y

Genotypes of parents - gametes - alleles X^H X^h

X^H X^HX^H X^HX^h

Y X^HY X^hY

If a boy, there is a 50% chance he will be a haemophiliac.

Example 2 of inheriting haemophilia (hemophilia)

Suppose a woman is unaffected by haemophilia and not a carrier of the defective allele.

BUT, suppose she has a son by a male haemophiliac.

What is the chance that their son will be a haemophiliac?

Genetic Punnett square table for crossing an unaffected female non-carrier of the faulty recessive gene for haemophilia and an affected male (carrier)

Parent genotypes cross: X^HX^H x X^hY

Gametes: X^H, X^H , X^h and Y

Genotypes of parents - alleles X^H X^H

X^h X^HX^h X^HX^h

Y X^HY X^HY

The answer is zero, their son will have a 100% chance of not suffering haemophilia or being a carrier.

If the child is a girl, there is a 100% chance she will be a carrier of the recessive allele.

family tree

Key points Source of information is based on textbooks & syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10).

Haemophilia (hemophilia): A Sex-Linked Genetic Disorder

Understanding haemophilia is essential for demonstrating how sex-linked genetic disorders are inherited.

1. What is Haemophilia (hemophilia)?

Haemophilia is a genetic disorder where blood does not clot properly, leading to excessive bleeding from injuries.

It is caused by a faulty gene on the X chromosome, making it a sex-linked disorder.

2. Why is Haemophilia (hemophilia) Sex-Linked?

The gene responsible for blood clotting is located on the X chromosome.
Males (XY) only have one X chromosome, so if they inherit the faulty gene, they will have haemophilia.
Females (XX) have two X chromosomes, meaning they need two faulty copies to be affected. If they inherit only one, they become carriers.

Since males lack a second X chromosome to compensate, haemophilia is far more common in males than females.

3. Inheritance of Haemophilia (hemophilia)

Using a Punnett square, we can predict inheritance patterns:

Parents	XᴺXᴺ (Normal Female)	XᴺXʰ (Carrier Female)	XʰXʰ (Haemophilic Female)
XᴺY (Normal Male)	Normal Female	Carrier Female	Haemophilic Female
XʰY (Haemophilic Male)	Carrier Female	Haemophilic Female	Haemophilic Female

Carrier females (XᴺXʰ) can pass the faulty gene to their children.
Males (XʰY) inherit haemophilia from their carrier or affected mothers.

4. Importance of haemophilia (hemophilia) in Human Genetics

Studying haemophilia helps scientists:

Understand sex-linked inheritance.
Develop genetic screening methods.
Improve treatment options, such as clotting factor therapy.

Summary of learning objectives and key words or phrases about haemophilia (hemophilia)

Understand the genetics of haemophilia as an example of an inherited sex-linked genetic disorder.

Be able to construct genetic diagrams and Punnett square tables to explain the inheritance of hemophilia.

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