Down syndrome is one of the most common genetically linked causes of intellectual disability; it is also characterized by certain physical features (Lewanda & others, 2016). An individual with Down syndrome has a round face, a flattened skull, an extra fold of skin over the eyelids, a thickened tongue, short limbs, and retardation of motor and mental abilities. The syndrome is caused by the presence of an extra copy of chromosome 21. It is not known why the extra chromosome is present, but the health of the male sperm or female ovum may be involved. Down syndrome appears approximately once in every 700 live births. Women between the ages of 16 and 34 are less likely to give birth to a child with Down syndrome than are younger or older women. African American children are rarely born with Down syndrome.
These athletes, several of whom have Down syndrome, are participating in a Special Olympics
competition. Notice the distinctive facial features of the individuals with Down syndrome, such as a
round face and a flattened skull. What causes Down syndrome?© James Shaffer/PhotoEdit
Sex-Linked Chromosome Abnormalities Recall that a newborn normally has either an X and a Y chromosome, or two X chromosomes. Human embryos must possess at least one X chromosome to be viable. The most common sex-linked chromosome abnormalities involve the presence of an extra chromosome (either an X or a Y) or the absence of one X chromosome in females.
How Would You…?
As a social worker, how would you respond to a 33-year-old pregnant woman who is concerned
about the risk of giving birth to a baby with Down syndrome?
Klinefelter syndrome is a chromosomal disorder in which males have an extra X chromosome, making them XXY instead of XY. Males with this disorder have undeveloped testes, and they usually have enlarged breasts and become tall (Lunenfeld & others, 2015). Klinefelter syndrome occurs approximately once in every 1,000 live male births. Only 10 percent of individuals with Klinefelter syndrome are diagnosed before puberty, with the majority not identified until adulthood (Aksglaede & others, 2013). Page 44 Fragile X syndrome is a genetic disorder that results from an abnormality in the X chromosome, which becomes constricted and often breaks (Karmiloff-Smith & others, 2016). The outcome frequently takes the form of an intellectual disability, autism, a learning disability, or a short attention span (Hall & others, 2014). This disorder occurs more frequently in males than in females, possibly because the second X chromosome in females negates the effects of the other, abnormal X chromosome (McDuffie & others, 2015; Rocca & others, 2016). Turner syndrome is a chromosomal disorder in females in which either an X chromosome is missing, making the person XO instead of XX, or part of one X chromosome is deleted. Females with Turner syndrome are short in stature and have a webbed neck (Miguel-Neto & others, 2016; Vlatkovic & others, 2014). In some cases, they are infertile. They have difficulty in mathematics, but their verbal ability is often quite good. Turner syndrome occurs in approximately 1 of every 2,500 live female births. XYY syndrome is a chromosomal disorder in which the male has an extra Y chromosome (Lepage & others, 2014). Early interest in this syndrome focused on the belief that the extra Y chromosome found in some males contributed to aggression and violence. However, researchers subsequently found that XYY males are no more likely to commit crimes than are XY males (Witkin & others, 1976).
Gene-Linked Abnormalities Abnormalities can be produced not only by an abnormal number of chromosomes, but also by defective genes. Figure 5 describes some gene-linked abnormalities and outlines their treatment and incidence.