Mitotic nondisjunction

Mitotic nondisjunction. Cytokinesis following mitosis results in a daughter cell having one more chromosome (N+1) and the other one having a missing chromosome (N-1).

On the right figure, the two chromatids of the red chromosome are not properly separated during anaphase of mitosis. The ploidy of the daughter cells has changed.

Meiotic nondisjunction

Nondisjunction can also happen during meiosis I or meiosis II. Subsequent fertilization event yields zygotes with aneuploid states.

Left: secondary nondisjunction during meiosis II resulting in 2N, 2N, 2N+1, 2N-1 zygotes. Right: primary nondisjunction during meiosis I resulting in 2N+1, 2N+1, 2N-1, 2N-1 zygotes.

Nondisjunction during meiosis I or meiosis II results in daughter cells with aneuploid states (2N+1 and 2N-1). The presence of diploid cells (left example) indicates secondary nondisjunction.

Down syndrome

Down’s Syndrome is a common genetic abnormality referred to as Trisomy 21. Instead of having the complement of 46 chromosomes of 22 homologous pairs plus 2 sex chromosomes, there are 47 chromosomes consisting of an additional Chromosome 21.

Down syndrome karyotype.

Turner syndrome

The default gender for mammals is female. The absence of second sex chromosome defaults to a female with Turner syndrome. This individual will have a karyotype of 45 chromosomes and is referred to as X0.

Karyogram of Turner syndrome (X0).

Klinefelter syndrome

Karyogram of Klinefelter syndrome (XXY).
Anatomy of a person with Klinefelter syndrome.
Video explaining Klinefelter syndrome.
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