Aneuploidy and Polyploidy
Trisomy 21 is quite common in humans. About 1 in 800 babies born have the condition. Many more pregnancies result in miscarriage when the fetus has trisomy 21. One of the factors that can contribute to the frequency is the age of the mother. For reasons that are not clear, human females over the age of 35 are 500 times more likely to have a child with Down syndrome than those who are younger.
While nondisjunction is the most common way for trisomy to occur, another event called translocation can also produce Down syndrome in offspring. During this event, a piece of one chromosome breaks off and fuses with another. If the extra piece attaches itself to one of the 21st chromosomes, a child with Down syndrome will be produced.
Turner's Syndrome Karyotype
Klinefelter Syndrome Karyotype
Other Types of Chromosomal Disorders
Cri du chat syndrome is one of these disorders and is the result of a deletion of chromosome 5. It is named because of the distinctive cry of children who have this disorder. The name is French for "cry of the cat." It occurs in 1 out of every 35,000 babies born. It involves an abnormal development of the larynx, which is why the cry sounds different. Most of the time this disorder is not inherited, but individuals with the disorder will pass on the defective chromosome to their children if they have any.
Cat eye syndrome, on the other hand, results from duplication of material on chromosome 22. Symptoms of the disorder include mild mental impairment, impaired growth, and malformations of the skull and internal organs.