21. Chromosomes

Abnormal Karyotypes

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.


Standard Human Karyotype with 46 chromosomes. Both XX and XY also shown here.

The appearance of extra or missing chromosomes arises during meiosis in an event called nondisjunction. After fertilization, a zygote with an improper chromosome complement occurs.

Nondisjunction Diagrams

From left to right, the ploidy of the resultant zygotes: 2N, 2N, 2N+1, 2N-1; 2N+1, 2N+1, 2N-1, 2N-1 Credit: Tweety207 [CC BY-SA 3.0]

Nondisjunction can occur during Meiosis I or Meiosis II to yield aneuploid states of 2N+1 or 2N-1.

Down Syndrome Karyotype

Down’s syndrome karytope showing the traditional trisomy 21

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) Credit: The cat~commonswiki [CC BY-SA 3.0]

Klinefelter syndrome

Klinefelter's Syndrome XXY DNA

Karyogram of Klinefelter syndrome (XXY) Credit: The cat~commonswiki [CC BY-SA 3.0]