Question

question 19 of 20 : select the best answer for the question.
generation
i (pedigree diagram)
ii (pedigree diagram)
iii (pedigree diagram)

1. refer to the figure showing a pedigree of a family affected by an x - linked recessive disorder. a female in generation 3 isnt a carrier of the disorder. how can this be?

a. females receive x chromosomes from their fathers only, which are unaffected in this case.
b. females receive an x chromosome from each parent, but in cases of recessive disorders, the mutated chromosome isnt passed on to daughters.
c. females receive an x chromosome from each parent. if a mother is a carrier of an x - linked recessive disorder, her daughters have a 50 percent chance of receiving her normal x chromosome and a 50 percent chance of receiving her mutated x chromosome.
d. females dont have x chromosomes so arent affected by gene disorders passed on the x chromosome.
mark for review (will be highlighted on the review page)

Explanation:

• Option A is wrong because females get one X from each parent (father and mother), not just father.
• Option B is wrong as X - linked recessive mutated chromosomes can be passed to daughters (they can be carriers).
• Option C is correct. For X - linked recessive, a female (XX) gets one X from mom (carrier, so has one normal and one mutated X) and one X from dad (unaffected, so normal X). So daughters have a 50% chance to get mom's normal X (and thus not be a carrier) and 50% chance to get mom's mutated X (and be a carrier).
• Option D is wrong because females have two X chromosomes, so they can be affected or carriers of X - linked disorders.

Answer:

C. Females receive an X chromosome from each parent. If a mother is a carrier of an X - linked recessive disorder, her daughters have a 50 percent chance of receiving her normal X chromosome and a 50 percent chance of receiving her mutated X chromosome.