Question

A woman, S.R., had a maternal grandfather with hemophilia A (OMIM 306700), an X-linked recessive condition that reduces blood clotting. S.R.’s maternal grandmother and paternal grandparents are free of the condition, as is her partner, his parents, and his grandparents. S.R. has no siblings. She wants to know the chance that a son of hers will have the condition.
What is that probability?

Answer 1

Answer:

Hemophilia is a x-linked recessive inherited bleeding  disorder. in this disease blood does not clot properly due to the decreased level of blood clotting factor VIII (8) or factor IX (9).

In x-linked recessive condition, the disease is transmitted from father (affected male) to his daughter and mother (affected female) to her son and skip of generation getting affected is also occurs.

• there will be 50% chance of her (S.R.) each son to be affected with hemophilia.

Explanation:

Hemophilia is caused due to mutation of one gene which is responsible for the making of proteins of  blood coagulation factor viii or ix. the affected person bleeds continuously from any injury and that can cause serious issues.

here,

grandfather of s.r is affected (xₐy) and grandmother is normal (xx) will produce

                                                   ↓

 s.r's mother who is a carrier (xₐx) [as xₐy and xx will produce 100% carrier xₐx daughter]

                                                    ↓

s.r's mother is a carrier (xₐx) but her father is normal (xy) will produce,

                                                   ↓

s.r who has 50% chance of being carrier (xₐx) and 50% chance of being normal (xx) [as she is the only child] and she has a normal (xy) partner, they will give,

                                                     ↓

1. if s.r is a carrier (xₐx) then one of her son will be affected (xₐy)   [as xₐx and xy =xₐy]
2. if s.r is normal (xx) then her son will be normal (xy) [as xx and xy will give xy]

here, 50% chance of her son to be affected with hemophilia [as S.R of being affected is more considerable].