Trisomy 21
        Caused by an error in cell division called “non- disjunction”, which results in an embryo with three of 21chromosomes instead of two.
        Most occurs in meiosis I in the mother with advancing age.
        Recurrence risk is about 1%

The picture shows non-disjunction in meiosis I.
When disomic gamete fertilizes with a normal monosomic gamete will result in trisomic embryos.

Down syndrome
trisomy 21, male
Karyotype 47, XY, +21

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