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

More complex genetics: 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

(c) Haemophilia - 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

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

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.

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