What is haemophilia and how is it inherited? Work out the results of F₁ and F₂ mating (with reasons) between :

What is Haemophilia?

Haemophilia is a genetic disorder characterized by a deficiency in one of the several proteins (clotting factors) needed for normal blood coagulation. There are several types of haemophilia, the most common being Haemophilia A (Factor VIII deficiency) and Haemophilia B (Factor IX deficiency). The severity of haemophilia varies depending on the level of the deficient clotting factor. Individuals with severe haemophilia experience frequent spontaneous bleeding episodes, while those with milder forms may only bleed excessively after injury or surgery.

Inheritance Pattern: X-linked Recessive

Haemophilia is inherited in an X-linked recessive pattern. This means:

• The gene responsible for the disorder is located on the X chromosome.
• Males (XY) have only one X chromosome, so if they inherit an affected X chromosome, they will express the disorder.
• Females (XX) have two X chromosomes. To express the disorder, a female must inherit two copies of the affected gene – one from each parent. If a female inherits only one affected gene, she becomes a carrier; she doesn't exhibit the disorder herself but can pass the gene on to her children.

F₁ Generation Mating

Let's consider a mating between a normal male (XY) and a carrier female (X^HX^h), where X^H represents the normal allele and X^h represents the haemophilia allele.

Punnett Square for F₁ Generation

	XH	Y
XH	XHXH	XHY
Xh	XHXh	XhY

Results of F₁ Generation

• Genotypes: X^HX^H, X^HX^h, X^HY, X^hY
• Phenotypes:
  • Females: 50% normal (X^HX^H or X^HX^h), 50% carriers (X^HX^h)
  • Males: 50% normal (X^HY), 50% affected (X^hY)

Reasoning: The Punnett square demonstrates that the carrier mother (X^HX^h) has a 50% chance of passing on the normal allele (X^H) and a 50% chance of passing on the haemophilia allele (X^h) to her daughters. The father (XY) contributes either an X chromosome (to daughters) or a Y chromosome (to sons). Since males only have one X chromosome, inheriting X^h results in haemophilia.

F₂ Generation Mating

Now, let's consider a mating between a male from the F₁ generation who is affected (X^hY) and a female from the F₁ generation who is a carrier (X^HX^h).

Punnett Square for F₂ Generation

	XH	Xh
Xh	XhXH	XhXh
Y	XhY	XY

Results of F₂ Generation

• Genotypes: X^hX^H, X^hX^h, X^hY, XY
• Phenotypes:
  • Females: 50% carriers (X^hX^H), 50% affected (X^hX^h)
  • Males: 50% affected (X^hY), 50% normal (XY)

Reasoning: The affected male (X^hY) will always pass on the haemophilia allele (X^h) to his daughters and the Y chromosome to his sons. The carrier female (X^HX^h) has a 50% chance of passing on the normal allele (X^H) and a 50% chance of passing on the haemophilia allele (X^h) to her children. This results in a higher proportion of affected individuals in the F₂ generation compared to the F₁ generation.