Cromosomal Variations

A karyotype is a photographic image that depicts all of the chromosomes in an individual cell. Laboratory workers use computers to rearrange the images so that the chromosomes are lined up in pairs, typically beginning with the autosomes—chromosomes 1 through 22—and ending with the sex chromosomes—normally XX or XY. A complete karyotype helps doctors determine if a person has extra chromosomes, missing chromosomes, or chromosomes that have attached to one another in unusual ways.

Human Male Karyotype

 

This karyotype of a human male shows the 23 pairs of chromosomes that are typically present in human cells. The chromosome pairs labeled 1 through 22 are called autosomes, and have a similar appearance in males and females. The 23rd pair, shown on the bottom right, represents the sex chromosomes. Females have two identical-looking sex chromosomes that are both labeled X, whereas males have a single X chromosome and a smaller chromosome labeled Y.

Down Syndrome

  

Down syndrome is often called Trisomy 21 because most people with this condition have three copies of the number 21 chromosome—one of the smallest of the human autosomes. In this karyotype, the sex chromosomes—marked with letters instead of numbers—are XX rather than XY, showing that these chromosomes belong to a female. Down syndrome almost always results in mental retardation, though the severity of the retardation varies.

Klinefelter’s Syndrome

 

This karyotype is indicative of Klinefelter’s syndrome because it has three sex chromosomes—a single Y chromosome and two X chromosomes—instead of the usual two. People with Klinefelter’s syndrome are always male. They are typically tall, and they may have slight breast development and small testes.