Questions

(a) What is the mode of inheritance of traits causing haemophilia?

Haemophilia is a X-linked recessive genetic disorder. This means the gene responsible for haemophilia is located on the X chromosome. Since males have only one X chromosome (XY), if they inherit an X chromosome carrying the haemophilia allele, they will express the disorder because they don't have a second X chromosome to potentially carry a normal (non-mutated) allele.

In females (XX), the situation is different. A female would need to inherit two copies of the mutated allele (one from each parent) to express the disorder. If she inherits one normal X chromosome and one X chromosome with the haemophilia allele, she becomes a carrier but usually does not show symptoms of the disease because the normal allele on the other X chromosome compensates for the defective one.

(b) What would be the possibility of a female and male child having haemophilia in the following scenarios?

(i) The mother is unaffected and the father is affected

• Mother (unaffected): Since she is unaffected, her genotype is likely XX, with both X chromosomes carrying the normal allele (X⁰X⁰). However, if she is a carrier, her genotype could be X⁰Xh, where X⁰ is the normal X chromosome and Xh carries the haemophilia allele.
• Father (affected): Since he is affected, his genotype is XhY, where Xh carries the haemophilia allele and Y is the male chromosome.

Now, let’s look at the possible outcomes for the children:

• Male child (XY): The son inherits the Y chromosome from the father (since males inherit the Y chromosome from their fathers). He will inherit one X chromosome from the mother. If the mother is X⁰X⁰ (unaffected), the son will be X⁰Y and unaffected. If the mother is a carrier (X⁰Xh), there is a 50% chance the son will inherit the Xh chromosome and thus have haemophilia (XhY).

• Female child (XX): The daughter will inherit one X chromosome from the father (Xh, since he is affected) and one X chromosome from the mother. If the mother is X⁰X⁰, the daughter will be a carrier (X⁰Xh) but unaffected. If the mother is a carrier (X⁰Xh), there is a 50% chance the daughter will inherit the normal X chromosome (X⁰Xh) and be a carrier, or inherit the Xh chromosome from both parents and be affected (XhXh).

Probabilities:

• Male child: If the mother is unaffected (X⁰X⁰), the male child will not have haemophilia. If the mother is a carrier (X⁰Xh), there is a 50% chance the male child will have haemophilia.
• Female child: If the mother is unaffected (X⁰X⁰), the female child will be a carrier but unaffected. If the mother is a carrier (X⁰Xh), there is a 50% chance the female child will be a carrier and a 50% chance she will be affected.

(ii) The mother is a carrier and the father is unaffected

• Mother (carrier): The mother’s genotype is X⁰Xh, where one X chromosome carries the normal allele (X⁰) and the other carries the haemophilia allele (Xh).
• Father (unaffected): The father’s genotype is X⁰Y, as he does not have the haemophilia allele.

Now, let’s look at the possible outcomes for the children:

• Male child (XY): The son will inherit the Y chromosome from the father and one X chromosome from the mother. If the mother is a carrier (X⁰Xh), there is a 50% chance the son will inherit the X⁰ chromosome and be unaffected (X⁰Y), and a 50% chance he will inherit the Xh chromosome and have haemophilia (XhY).

• Female child (XX): The daughter will inherit one X chromosome from the father (X⁰, since he is unaffected) and one X chromosome from the mother. If the mother is a carrier (X⁰Xh), there is a 50% chance the daughter will inherit the normal X chromosome (X⁰X⁰) and be unaffected, or a 50% chance she will inherit the Xh chromosome and be a carrier (X⁰Xh).

Probabilities:

• Male child: There is a 50% chance the male child will inherit the Xh chromosome from the mother and have haemophilia.
• Female child: There is a 50% chance the female child will be a carrier of haemophilia.