The diagram shows a family tree. The grandfather, individual 1, carries a recessive allele on his X chromosome. Which other individuals could have inherited this allele?

Answer:

12) D

Individual 1 has a recessive X allele. Since individual 1 is a male, he must have X and Y chromosomes. Only his daughters can receive his recessive X chromosome. So 4 and 5 will receive it by 6 will not.

Since 5 does not have any offsprings, she cannot pass on the X chromosome.
4 will pass on her X chromosome to her offsprings. So 8 and 9 will obtain the X chromosome.

So 4, 5, 8 and 9 will inherit this allele.