A Reading supplement for Molecular and Cell Biology
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.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.
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’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.
