Question

Female parent Child #1 (2n +1) Child 2 (2n) Child 83 (2) Child 14 12n. 1) Male parent

Zylins are a diploid, 2n = 6, species and a common mutation found in baby Zylins is trisomy 3 (2n +1).  You are trying to determine if this trisomy is more commonly caused by nondisjunction events during egg formation or sperm formation.  A Southern blot, labeled with a probe for RFLPs on chromosome 3, for one of your test families is shown below (a thicker, darker line indicates twice the amount of DNA; a text description of the gel follows):

Six lanes are shown on the gel labelled from left to right: Male parent, female parent, Child #1 (2n +1), Child #2 (2n), Child #3 (2n) Child #4 (2n +1). If I label the RFLP alleles from top to bottom, there are four: A, B, C, D.

Male parent is A, D; female parent is B, C; Child #1 is A, C, D; Child #2 is C, D; Child #3 is B, D; and Child #4 is A [this band is thicker and darker than the others], B.

Based on your Southern blot results which statement is correct?

Select one:

a. Child #1 is 2n+1 due to a nondisjunction event in Meiosis I during gamete formation by the male parent

b. Child #1 is 2n+1 due to a nondisjunction event in Meiosis I during gamete formation by the female parent

c. Children #1 and #4 are 2n+1 due to a nondisjunction event in Meiosis I during gamete formation by the male parent

d. Child #4 is 2n+1 due to a nondisjunction event in Meiosis I during gamete formation by the male parent

e. Child #4 is 2n+1 due to a nondisjunction event in Meiosis I during gamete formation by the female parent

Which hypothesis does the data from your Southern blot support?

Select one:

a. The incidence of Trisomy 3 will increase as the age of the parents increase

b. The parent in which the nondisjunction events occurred cannot be determined

c. Trisomy 3 can arise from nondisjunction events in meiosis during sperm and egg formation

d. Trisomy 3 can arise from nondisjunction events in meiosis during sperm formation

Answer 1

1.(A) Ans is child #1 is 2n + 1 due to a now disjunction event in meiosis 1 during gamete formation by the male parent.

Reason : u know in diploid state chromsome occurs in pair therefore Chr 3 has 2 copies. In trisomy 3 , chr 3 has 3 copies.

Male parent has 2 copies of chr 3 ,; one copy has allele A and another has allele D, in female B & C.

In the gel, u can see child#1 has received one extra copy of chr 3 from it's father as it received both A & D hence, there is non-disjuction during meiosis 1.

Child#4 is also 2n+1 and this time also extra copy is from male parent but it is not due to non-disjuction meiosis 1,As the child hadn't relieved both alleles A & D but it receives double A allele hence it is due to non-disjuction during stage 2 of meiosis.

2.Ans is (d) trisomy is arised due to non-disjuction events in meiosis during sperm formation.

Reason: As only a single copy of female allele either b or c is recieved in both child 1 & 4 . Hence it can't be due to female parent.