2.4: Chromosomal Abnormalities

A chromosomal abnormality occurs when a child inherits too many or two few chromosomes. The most common cause of chromosomal abnormalities is the age of the mother. As the mother ages, the ovum is more likely to suffer abnormalities due to longer term exposure to environmental factors. Consequently, some gametes do not divide evenly when they are forming. Therefore, some cells have more than 46 chromosomes. In fact, it is believed that close to half of all zygotes have an odd number of chromosomes. Most of these zygotes fail to develop and are spontaneously aborted by the mother's body.

One of the most common chromosomal abnormalities is on pair 21. Trisomy 21 or Down syndrome occurs when there are three rather than two 21^st chromosomes. A person with Down syndrome typically exhibits an intellectual disability and possesses certain physical features, such as short fingers and toes, folds of skin over the eyes, and a protruding tongue. There is as much variation in people with Down syndrome as in most populations, and those differences need to be recognized and appreciated. Refer to Figure \(\PageIndex{1}\) on the prevalence of Down syndrome in our home state of Illinois. Other less common chromosomal abnormalities of live-born infants occur on chromosome 13 and chromosome 18.

Figure \(\PageIndex{1}\). Illinois prevalence rates (2002-2014) for Down Syndrome (Trisomy 21) based on maternal age. Source.

When the abnormality is on the 23^rd pair the result is a sex-linked chromosomal abnormality. A person might have genetic combinations such as XXY, XYY, XXX, XO. Two of the more common sex-linked chromosomal disorders are Turner syndrome and Klinefelter syndrome. Turner syndrome occurs when part or all of one of the X chromosomes is lost and the resulting zygote has an XO composition. This occurs in 1 of every 2,500 live female births (Carroll, 2007) and affects the individual's cognitive functioning and sexual maturation. The external genitalia appear normal, but breasts and ovaries do not develop fully and the woman does not menstruate. Turner syndrome also results in short stature and other physical characteristics. Klinefelter syndrome (XXY) results when an extra X chromosome is present in the cells of a male and occurs in 1 out of 650 live male births. The Y chromosome stimulates the growth of male genitalia, but the additional X chromosome inhibits this development. An individual with Klinefelter syndrome typically has small testes, some breast development, infertility, and low levels of testosterone (National Institutes of Health, 2019). See Table \(\PageIndex{1}\) for descriptions of chromosomal disorders.

Table \(\PageIndex{1}\). Chromosomal Disorders.

Autosomal Chromosome Disorders: The individual inherits too many or two few chromosomes.	Cases per Birth
Down Syndrome/Trisomy 21 is caused by an extra chromosome 21 and includes a combination of birth defects. Affected individuals have some degree of intellectual disability, characteristic facial features, often heart defects, and other health problems. The severity varies greatly among affected individuals.	1 in 691 1 in 300 births at age 35
Trisomy 13 is caused by an extra chromosome 13. Affected individuals have multiple birth defects and generally die in the first weeks or months of life.	1 in 7,906
Trisomy 18 is caused by an extra chromosome 18 and the affected individual also has multiple birth defects and early death.	1 in 3,762
Sex-Linked Chromosomal Disorders: The disorder occurs on chromosome pair #23 or the sex chromosomes.	Cases per Birth
Turner Syndrome is caused when all or part of one of the X chromosomes is lost before or soon after conception due to a random event. The resulting zygote has an XO composition. Turner Syndrome affects cognitive functioning and sexual maturation in girls. Infertility and a short stature may be noted.	1 in 2500 females
Klinefelter Syndrome is caused when an extra X chromosome is present in the cells of a male due to a random event. The Y chromosome stimulates the growth of male genitalia, but the additional X chromosome inhibits this development. The male can have some breast development, infertility, and low levels of testosterone.	1 in 650 males

Genetic Counseling:

A service that assists individuals identify, test for, and explain potential genetic conditions that could adversely affect themselves or their offspring is referred to as genetic counseling (CDC, 2015b). The common reasons for genetic counseling include:

• Family history of a genetic condition
• Membership in a certain ethnic group with a higher risk of a genetic condition
• Information regarding the results of genetic testing, including blood tests, amniocentesis, or ultrasounds
• Learning about the chances of having a baby with a genetic condition if the parents are older, have had several miscarriages, have offspring with birth defects, experience infertility, or have a medical condition