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 21st 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. Other less common chromosomal abnormalities of live-born infants occur on chromosome 13 and chromosome 18. Refer to Table 2.2 on the prevalence of these chromosomal disorders in our home state of Illinois.
|Chromosomal Disorder||Age<35 Prevalence Rate||Age>35 Prevalence Rate||Total Prevalence|
|Trisomy 13||1.0 per 10,000||4.3 per 10,000||1.6 per 10,000|
|Trisomy 18||0.9 per 10,000||8.3 per 10,000||2.0 per 10,000|
|Trisomy 21||6.9 per 10,000||43.7 per 10,000||12.7 per 10,000|
When the abnormality is on 23rd pair the result is a sex-linked chromosomal abnormality. A person might have 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’s 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 700 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 has some breast development, infertility (this is the most common cause of infertility in males), and has low levels of testosterone. See Table 2.3 for Chromosomal Disorders descriptions.
|Autosomal Chromosome Disorders: The individual inherits too many or two few chromosomes||Cases per Birth|
1 in 691; 1 in 300 births at age 35
||1 in 7,906|
||1 in 3,762|
Sex-Linked Chromosomal Disorders: The disorder occurs on chromosome pair #23 or the sex chromosomes.
Cases per Birth
||1 in 2500 females|
||1 in 700 males|
Genetic Counseling: Genetic counseling refers to a service that assists individuals identify, test for, and explain potential genetic conditions that could adversely affect themselves or their offspring (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 ultra sounds
- Learning about the chances of having a baby with a genetic condition if the mother is older, has had several miscarriages, has offspring with birth defects, experiences infertility, or has a medical condition