20- How is a mosaic Down syndrome with the karyotype 50% 47, XX+21 / 50% 46, XX

Question

20- How is a mosaic Down syndrome with the karyotype 50% 47, XX+21 / 50% 46, XX produced? (Hint:

It could have been the result of more than one errors!)

A- A nondisjunction of cs 21 in a zygote, which is already trisomic for cs 21

B- A nondisjunction of cs 21 in one of the cells, and an autosomal lagging in the other cell of a normal two-cell embryo

C- A lagging of cs 21 in a zygote, which is already trisomic for cs 21

D- A nondisjunction of cs 21 in a zygote, which is already monosomic for cs 21 (suppose the zygote lives long enough to divide!)

E- A nondisjunction of cs 21 in one of the daughter cells of mitosis of an already trisomic zygote for cs 21

F- Two of the above

G- Three of the above

Explanation / Answer

A,B,C and G ae the mechanism for errors occured in mosaic down syndrome.

In mosaic down syndrome an error occurred in the chromosome replication and separation process, and the extra copy of the chromosome 21 was not passed on to both cells. In this way, a second cell grouping was created with only 46 chromosomes.

it always inherit total number of 46 chromosomes.

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