Chromosomes are long sequences of DNA, containing the genes of an organism. Chromosomal diseases result from errors in the number of chromosomes or the physical structure of the chromosomes. According to the March of Dimes, about 1 in every 150 babies born have some type of chromosomal disease. Normally, humans have 23 pairs of chromosomes--the 22 numbered chromosomes and the sex chromosomes X and Y.
Down Syndrome
Down syndrome, also called trisomy 21, is a genetic defect in which a person is born with three copies of chromosome 21, instead of the usual 2 copies. Down syndrome individuals have distinctive facial features, delayed development and intellectual disabilities. According to the March of Dimes, 1 in every 800 babies is born with Down syndrome.
Other Trisomies
Patau syndrome, also known as trisomy 13, and Edwards syndrome, or trisomy 18, are two other disorders with an extra copy of a particular chromosome. Children born with these two trisomies tend not to survive more than about a year after birth. Most other trisomies cause fetal death and miscarriages.
Monosomies
A monosomy is a disorder in which one of a pair of chromosomes is lost. In most cases, this results in the death of the fetus while still in the womb. One case where a monosomy survives is in a fetus that has a single X chromosome, instead of two X chromosomes, which is the standard for females, or one X and one Y, the genetic complement for males. This condition is known as Turner syndrome, and the girls who have it tend to be short and have heart problems or other internal defects. They also typically do not experience puberty unless supplied with artificial hormones.
Sex Chromosome Trisomies
In some individuals, the sex chromosomes end up as the trisomies XXY, otherwise known as Klinefelter syndrome, or XYY, also called Jacob's syndrome. Klinefelter syndrome produces boys whose bodies make unusually low amounts of testosterone. Boys with Jacob's syndrome often appear perfectly normal, although they sometimes exhibit learning disabilities. Girls with three copies of the X chromosome have a disorder called Triple-X, which is associated with an increased risk of learning disabilities and delayed development of language skills.
Deletion Syndromes
A deletion syndrome, also called a subtraction disorder, involves the loss of a portion of a chromosome instead of a whole chromosome, according to the Merck Manuals Online Medical Library. These disorders are rare and children born with them are frequently severely disabled. Cri du chat syndrome, or cat's cry syndrome, is one such disorder and is named for the cat-like wail that infants with this disease emit. Another deletion syndrome called Prader-Willi syndrome is caused by the loss of a portion of chromosome 15, and results in children with severe mental retardation.
