Aneuploidy is a condition in which the number of chromosomes in a cell is either higher or lower than the normal number. Cells with extra or fewer chromosomes do not function normally due to the imbalance of chromosomes as well as the extra genetic expression or lack of. In the vast majority of cases, cells with an aneuploid chromosome count do not survive to continue to pass on their defects. The vast majority of aneuploid embryos do not live and are spontaneously miscarried early in the pregnancy. Those that do survive to a full term pregnancy and birth are the resulting individuals with genetic disorders and syndromes caused by their aneuploid chromosome count.
In humans, the most common types of aneuploid disorders are known as trisomies and monsomies. All of these disorders have either one extra chromosome, an extra piece of a chromosome, or a lack of a chromosome. A normal human cell contains 23 pairs of chromosomes or 46 total. An individual with a trisomy will have 47 total chromosomes in every cell while an individual with a monosomy will have 45 in each cell of their body.
Disorders associated with trisomies include Down syndrome, trisomy 13 (Patau syndrome), trisomy 18 (Edwards syndrome), Klinefelter’s syndrome, Triple X syndrome, and 47, XYY syndrome.
Down syndrome is caused by the presence of an extra copy of chromosome 21 and is the most widely recognized trisomy disorder. Trisomy disorders 13 and 18 are much less commonly seen as most babies born with these disorders die within a month after birth. All three of these disorders are trisomies of autosomal chromosomes or, in other words, non-sex chromosomes, and can effect both males and females.
Klinefelter’s syndrome, Triple X syndrome and 47, XYY syndrome are trisomies of the sex chromosomes. Klinefelter’s syndrome results in a male with an XXY configuration rather than the normal XY. Triple X syndrome, as the name implies, causes a trisomy of the X chromosome and effects only females. 47, XYY syndrome is characterized by the presence of an extra Y chromosome and only effects males.
Trisomies of the autosomal chromosomes effect an individual the most severely. Most pregnancies of individuals with these types of trisomies are spontaneously miscarried or aborted. Individuals that are born and survive past their first month of life will have physical and developmental disorders that can effect their mental cognitive abilities and predispose them to mild to severe chronic health issues.
Trisomies of the sex chromosomes do not impact the individual as severely as the autosomal trisomies do. The frequency of babies born with an extra X or Y chromosome has been found to be 1 in every 1,000 live births. Males born with Klinefelter’s syndrome will have decreased testosterone production and will have exhibit feminine characteristics such as breast tissue development, reduced testicle size, and a reduced amount of body and facial hair. These individuals are also infertile due to their smaller testicles and reduced amount of testosterone production. Males with 47, XYY syndrome will have normal testosterone levels and normal male development and fertility. However, these individuals may have an increased rate of learning and cognitive issues. Females with Triple X syndrome will also have a slightly higher rate of learning and cognitive issues but will develop normally physically and will also have normal fertility.
Turner syndrome is a chromosomal disorder that is caused by a monosomy of chromosome X. This condition results in an individual with only one X chromosome and is found to effect 1 in every 2,500 live births. Because the individual only has one X chromosome, they are always female. Characteristics of this disorder include shorter than normal stature, loss of fertility, decreased development of breast tissue, extra folds of skin at the neck (neck webbing), and some cognitive problems involving spatial and non-verbal memory. Most individuals have normal intelligence and do well in school. Females with Turner syndrome are usually placed on hormone therapy at puberty to assist with female development of the breasts and body.
In some situations, an extra piece of a chromosome will end up attached to another chromosome. This also results in a trisomy. Emanuel syndrome is a trisomy disorder caused by the movement of a piece of chromosome 11 that attaches to a separate piece of chromosome 22 which results in a “third” chromosome made up of these two pieces. This movement of the genetic material is known as a translocation. Emanuel syndrome is very rare and has only been reported in 100 living individuals worldwide. Individuals with this disorder have many physical, mental, and visual characteristics that effect cognitive and overall health. Most babies that make it full term die shortly after birth.
All of the aneuploidy disorders listed previously are the results of aneuploidy of the sex cells (egg and sperm) and embryonic cells. Aneuploidy will also effect somatic cells (tissue and blood cells) as well. It has been found that cancer cells taken from tumors are aneuploid. The degree of aneuploidy in these cancer cells is much more dramatic that what is found in the sex and embryonic cells. Aneuploid cancers cells have been observed to contain 60-90 chromosomes. Aneuploidy is theorized to be a major cause of cancer as the aneuploid cells have lost the specific enzyme controlled checkpoints during mitotic division that help regulate the number of chromosomes that go into each subsequent daughter cell.
Aneuploidy can effect an individual in more than one way. Syndromes caused by aneuploidy of the embryonic cells during the beginning stages of life will result in the trisomy and monosomy disorders such as Down syndrome and Turner syndrome. Aneuploidy of cells of the body can result in the formation of cancer which is still being heavily researched as to find the underlying causes of how and why this happens in the first place. Overall, any type of aneuploidy cell that was allowed to survive to continue its existence will bring upon some sort of abnormality to the affected individual.