Mary is Aa for the X-linked gene A. In one of her primary oocytes, a crossing ov
ID: 190773 • Letter: M
Question
Mary is Aa for the X-linked gene A. In one of her primary oocytes, a crossing over occurred between the two X chromosomes, and as a result, the two alleles were exchanged. The final egg cell of this primary oocyte fused with a Mark’s sperm, and a female zygote with genotype Aa was produced. Mark’s mother is AA. What alleles did the three second polar bodies carry in that menstrual cycle of Mary?
A- Two of them carried ‘a’, and one of them carried ‘A’
B- Two of them carried ‘A’, and one of them carried ‘a’
C- All of them carried ‘A’
D- All of them carried ‘a’
E- All of them carried ‘Aa’
F- Two of them carried ‘AA’, and one of them carried ‘aa’.
Explanation / Answer
Answer: B- Two of them carried ‘A’, and one of them carried ‘a’
The sperm which fertilized the ovum carried the A allele as Mark's mother only carried those alleles, and those are the ones he could pass. Despite the crossing over, we still end up with X chromosomes with A and a alleles. The X chromosome with the a allele goes to the secodary oocyte and forms the ovum and a second polar body. The X chromosome with the A allele goes to the first polar body and forms two second polar bodies.