One of the most common chromosomal abnormalities occurs on pair 21. Trisomy 21 or Down syndrome occurs when there are three rather than two twenty-first 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.
When the abnormality is on twenty-third 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 syndrome also results in short stature and other physical characteristics. Klinefelter syndrome 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 descriptions of 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, and often has heart defects and other health problems. The severity varies greatly among affected individuals. | 1 in 781; 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 6,000 |
| Trisomy 18 is caused by an extra chromosome 18 and the affected individual also has multiple birth defects and early death. | 1 in 2,600 |
| Sex-Linked Chromosomal Disorders: The disorder occurs on chromosome 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 2,500 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 500 to 1 in 1,000 males |
Genetic counselling: This refers to a service that assists individuals to identify, test for, and explain potential genetic conditions that could adversely affect themselves or their offspring (CDC, 2015b). The common reasons for genetic counselling 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 mother is older, has had several miscarriages, has offspring with birth defects, experiences infertility, or has a medical condition.
