Question

Refer to the figure showing a pedigree of a family affected by an x-linked recessive disorder. A female in generation 3 isn't a carrier of the disorder. How can this be?
A. Females receive X chromosomes from this fathers only, which are unaffected in this case.
B. Females don't have X chromosomes so aren't affected by gene disorders passed on the X chromosome.
C. Females receive an X chromosomes from each point parent, but in cases of recessive disorders, the muted chromosome isn't passed on to daughters.
D. Females receive an X chromosome from each parent. If a mother is a carrier of an X-linked recessive disorder, her daughters have 50 percent chance of receiving her normal X chromosome and a 50 percent chance of receiving her mutated X chromosome.

Answer 1

Answer:

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

Explanation:

Females have XX chromosomes as allosomes. They receive one X chromosome from mother and one from father. The female in the III generation received both the healthy X chromosomes from each parent that is why she is not even carrier for the disease.

The modified diagram clearly indicating the genotyope of all the members of the pedigree tree is attached. In the diagram, healthy X chromosome is indicated by X while mutated X chromosome is indicated by doted X. It is clear from the diagram that this female of III generation has XX chromosomes.  

The cross for that particular female is shown as under:

                               Mother               Father

                                   ẊX      x             XY

                                  /     \                  /     \

                               ẊX     ẊY          XX    XY

XX are the allosomes of this female child.

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Answer 2

Answer: D.

Explanation: Females have XX chromosomes while males have XY