genetics - pongly

[ArchivalUser]

A family with one child with downs is concerned of having another child with downs, so they approach to a specialist.DNA of both parents and the affected child are studied.A cloned DNA fragment when used as probe reveals RFLP in a region adjacent to centromere of ch21.Four haplotypes exist A,B,C,D.fATHER has AC genotype,mother has BD genotype ,while the affected child has ABC genotype.When did the nondisjunction occured??
A Meiosis I of father
B Meiosis II of father
C Meisis I of mother
D meiosis II of mother.

please explain how to arrive for the solution

[ArchivalUser]

this is a very good Q...A. Meiosis I of father...?

[ArchivalUser]

Meiosis 1 of father
During meiosis 1, the homologues that carry similar but non identical,information separate.Since failure occured in meiosis 1 the man has to pass AC and the woman to pass B(either B or D but in this case B)producing ABC genotype

[ArchivalUser]

tona can u also give some explanation as to why u chose A

[ArchivalUser]

Father for sure
Meiosis 1 as thats the only step alleles on 2 homologues come together