(a) What is the mode of inheritance of traits causing haemophilia?
(b) What would be the possibility of a female and male child having
haemophilia in the following scenarios?
(i) the mother is a unaffected and the father is affected
(ii) the mother is a carrier and the father is unaffected
(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
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).
(ii) The mother is a carrier and the father is unaffected
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).
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