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anka

A family with 1 child with Down syndrom is concerned about the possibility of having additional children with DS, and has asked for genetic studies to define where the error had occurred. As a result, the DNA of both parents and the affected child are all studied. A cloned DNA fragment, when used as a probe, reveals a RFLP in the region adjacent to the centromere of chromosome 21. Four haplotypes exist: A,B,C,D. The father is found to have AC genotype; the mother BD genotype; and the affected child ABC. Nondysjunction most likely occurred in:

A The child during the 1st mitotic division
B. The father during meiosis I
C. The father during meiosis II
D. The mother during meiosis I
E. The mother during meiosis II

Falcon

grzybcia

bbbbb?cos mi swita ze to bylo w mejozie 1 a literki wskazuja na ojca...
Prosimy o odpowiedz!

anka

No więc odp. jest faktycznie B.
During meiosis II sister chromatides, which are two identical copies of the same chromosome, should separate. If a nondysjunction event occurs in this division, two copies of the same chromosome are passed to the progeny. In this case both parents are heterozygous for the probe. The child recieved a B from the mother and A and C from the father, leading us to conclude that the problem occured in the father during meiosis I, when the homologues carry similar but not identical information.
A) If a nondysjunction occures early in developement a child that is mosaic for trisomy is the result.
C) If the father had a failure in meiosis II either AA or CC gamet would be produced and the mother would pass a B or a D gamete. Various combination could produce BAA, DAA, BCC, DCC.
D) If a BD woman had a failure in meiosis I a BD gamet would be produced. When fertilized by the man's A or C sperm, a child that was ABD or CBD would result.
E) If a BD woman's sister chromatids failed to disjoin during meiosis II, BB or DD gametes would result. When fertilized by the A or C male's sperm, a child that was BBA, BBC, DDA or DDC would result.

grzybcia

Super swietne pytanko!!!
Zawsze mi sie to miesza.....

anka

Mi też się mieszało ale po tym wyjaśnieniu chyba przestanie Very Happy