Question

it's super important that I get this question right, so please only answer if you are absolutely certain! thank you so much for understanding!

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.

Female Male parent parent Child #1 (2n +1) Child #2 (2n) Child #3 (2n) Child #4 (2n + 1)

Based on your Southern blot results which statement is correct?

Select one:

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

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

c. Child #1 is 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 female parent

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

Answer 1

Non- disjunction is a failure of proper seperation of homologous chromosome during meiosis.

It can be seen in figure that the male parent have allele A,D and the female parent have allele B,C and the two normal children that is children #2 and #3 have allele C, D and B, D respectively.

If we see children #1 and #4 , it is already mention that these lanes are showing trisomy having chromosome in the fashion of 2n+1. The observation from both the figure of #1 and #4 showing three lines each composed of allele A, C, D and A, D. The allele A is common in both the trisomy and also A got a thicker line in lane 6 indicating that allele A is extra in lane 6 as mentioned in question that thicker band represent twice the DNA. So by all this observation it can be concluded that RFLP allele A is extra allele which is causing trisomy in children #1 an #4.

In the first two lane it can be observed that allele A is only present in the male parent and the trisomy in both the children occurs through allele A.  So by all this observation it is concluded that trisomy occurs in Children #1 and #4 due to non-disjunction in male gametes as allele A is only present in male so non-disjunction cannot be occur in female gamete.

So by concluding it can be said that trisomy is not due to female gamete so option including non disjunction in female gametes can be excluded and we have seen that trisomy does not occur only in  in child #1 and child #4 but occurs in both of them so option showing disjunction in only one child can be excluded as well. So the correct option is a. Children #1 and #4 are 2n+1 due to a nondisjunction event in Meiosis I during gamete formation by the male parent (after excluding option b,c,d and e).