Problem 4. Nail-patella syndrome is an autosomal dominant disorder in humans tha

Question

Problem 4. Nail-patella syndrome is an autosomal dominant disorder in humans that results in abnormal nail growth and absent or severely reduced patella. The mutation occurs in the NPSI gene located on chromosome 9. In 1955, Renwick and Lawler (Ann. Hum. Genet. 19: 312-331) were the first to demonstrate linkage between two human genes using pedigree analysis when they reported linkage between NPSI and the ABO gene, which controls the ABO blood type. The pedigree to the right shows three generations of a family with members who have Nail-patella syndrome. Shaded symbols show individuals with the syndrome and the blood type of each person in the family is given inside of their respective symbol. Which individual in generation III is the result of a crossover between the NPSI and ABO genes? A. 1 B. 2 C. 3 D. 4 E. 5

Explanation / Answer

Hi,
This problem relies on the concept of cross over. In any genetic cross over, new recombinants are formed. If there is no cross over, then parental types prevail. In the given case, the parents of generation III are diseased B and normal A. Now let us draw the genetic order.:
Let's say, B blood group = iBi ; A = iAi; O = ii
NPS1 gene = N- dominant disease causing; n - normal allele

Parents are: iB n//i N   x    iA n//i n
When you make combinations we get,
iB n//i n(B normal)
iBn//iAn (AB normal)
iA n//i n (A normal)
iA n //i N (A diseased)
i N//i n (O diseased)

so we cannot get O normal in this case. So cross over must take place between i and n genes to get diseased O

Cross over parents are: iB N//i n     iA n//i n
iB N// i n (B diseased)
iB N// iA n (AB diseased)
i n // iA n ( A normal)
i n // i n ( O normal)

So, the option C.3 is the result of cross over.

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