7. Chromosomal Disorders
Some children are born with genetic defects that are not carried by a single gene. Instead, an error in a larger part of the chromosome or even in an entire chromosome causes the disorder. Usually the error happens when the egg or sperm is forming. One common example is Down syndrome (Figure below). Children with Down syndrome are mentally disabled and have collection of recognizable physical deformities. Down syndrome occurs when a baby receives an extra chromosome from one of his or her parents. Usually a child would have one chromosome 21 from its mother and one chromosome 21 from its father. But in an individual with Down syndrome, there are three copies of chromosome 21. Down syndrome is therefore also known as Trisomy 21.
A child with Down syndrome.
Another example of a chromosomal disorder is Klinefelter syndrome, in which a male inherits an extra “X” chromosome. These individuals have underdeveloped sex organs and elongated limbs, and have difficulty learning new things. Outside of chromosome 21 and the sex chromosomes, most embryos with extra chromosomes do not even make it to the fetal stage. Because chromosomes carry many, many genes, a disruption of a chromosome potentially causes severe problems with development of the fetus.
Besides diseases caused by duplicated chromosomes, other chromosomal disorders occur when the structure of a chromosome is disrupted. For example, if a tiny portion of chromosome 5 is missing, the individual will have cri du chat (cat’s cry) syndrome. These individuals have misshapen facial features and the infant’s cry resembles a cat’s cry.