21. Chromosomes

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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.

Karyotype

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

Mitotic nondisjunction

Credit: Wpeissner [CC BY-SA 3.0]

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.

45,X

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]

Klinefelter's syndrome

Klinefelter syndrome anatomy Credit: smithperiod6.wikispaces.com/Klinefelter’s+Syndrome [CC BY-SA 3.0]

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