Trisomy, << TRY soh mee, >> is the presence of three copies of a particular chromosome in cells that normally have two copies of each chromosome. Chromosomes are the carriers of inheritance. Most kinds of trisomies in human beings result in miscarriage. Among people born with trisomies, the condition is often associated with intellectual disabilities, birth defects, and physical abnormalities.
Each human body cell, except for sperm and egg cells, typically has 46 chromosomes. These chromosomes consist of 22 pairs of autosomes (non-sex chromosomes) and 1 pair of sex chromosomes. Sex chromosomes help determine an individual’s sex. A person with a trisomy has 47 chromosomes in all or some of their cells, with 1 extra autosome or sex chromosome.
Trisomy results from a defect called nondisjunction, in which chromosome pairs fail to separate during cell division (see Cell (Cell division) ). During the normal process of cell division that produces sperm or egg cells in human beings, each chromosome pair is divided between cells. This process results in each sperm or egg cell having one chromosome from each pair. Sperm and egg unite during fertilization, producing an embryo (developing young) with the full complement of pairs. Nondisjunction can result in an egg or sperm cell receiving two copies of a particular chromosome. If that sperm or egg undergoes fertilization with a normal sperm or egg cell, the resulting embryo will have a trisomy.
Genetic experts detect trisomies by organizing images of an organism’s chromosomes into a profile called a karyotype. A karyotype can be prepared before birth through a procedure called amniocentesis (see Amniocentesis ).
Many trisomies occur in autosomes. The most common human trisomy occurs when the embryo has an extra autosome of the pair designated by the number 16. However, this trisomy always results in miscarriage. Down syndrome ranks among the most common trisomies seen in children. Down syndrome is a trisomy of autosome number 21. Children with Down syndrome have varying degrees of intellectual disability, characteristic facial features, and often have heart defects. Edwards syndrome, a trisomy of autosome 18, and Patau syndrome, a trisomy of autosome 13, also occur among liveborn children. Infants born with these trisomies usually have severe birth defects and intellectual disability. Most of them die within their first year of life.
Other trisomies occur in sex chromosomes. Klinefelter syndrome is a common sex chromosome trisomy in which males have an extra X chromosome. Normal females have two X sex chromosomes, whereas normal males have one X and one Y sex chromosome. Klinefelter syndrome typically results in incomplete sexual development.
Scientists know that the extra copies of genes found on the extra chromosome cause the mental and physical abnormalities associated with trisomies. Genes determine which characteristics living things inherit from their parents, influencing chemical and physical processes that control growth, development, and function. However, scientists do not know exactly how having an extra copy of certain genes upsets normal development.
