Examples of Multifactorial and Polygenic Effects

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Cleft lip with or without cleft palate (CL/P) is a heterogeneous disorder (those children affected may have somewhat different abnormalities) occurring in 1 out of 1,000 births. Some CL/P cases occur as isolated birth defects, while others occur as part of a larger syndrome. The majority of CL/P cases are associated with multifactorial inheritance. The risk to relatives of affected individuals can be anywhere from 0.5 to 15 percent, depending on the severity of the clefting and the degree of relationship to the affected individual, with risks highest for first-degree relatives. Some unique cases of CL/P may be associated with genetic syndromes that are due to single-gene mutations or chromosomal abnormalities.

Clubfoot is another primarily multifactorial defect and occurs in 1 out of 10,000 Caucasian newborns. The estimated risk to relatives of inheriting this defect is between 2 to 20 percent, depending upon the family history. Clubfoot can also have genetic causes such as chromosomal abnormalities or single-gene disorders, or it may have an environmental origin, such as problems caused by amniotic fluid or structural abnormalities of the uterus that restrict fetal growth and mobility. Clubfoot can also be due to autosomal recessive as well as autosomal dominant inheritance, and it may also occur as part of a larger syndrome.

Another class of multifactorial disorders is known as neural tube defects (NTDs). The neural tube is the embryonic structure that develops into the

Figure 1. The threshold model for multifactorial traits. The vertical axis represents the number of people with a given level of overall liability. On the horizontal axis, individuals below the threshold do not have the disease, while those above it do.

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Thawiphop Phimthep waits for his surgery to fix his cleft lip and palate in Si Saket Hospital in Sisaket Province, Thailand. Thawiphop's surgery will be performed by a group of plastic surgeons from Bangkok.

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Thawiphop Phimthep waits for his surgery to fix his cleft lip and palate in Si Saket Hospital in Sisaket Province, Thailand. Thawiphop's surgery will be performed by a group of plastic surgeons from Bangkok.

brain and spinal cord. Failure of the neural tube to close, which normally occurs during in the fourth week of gestation, results in an NTD, usually spina bifida or anencephaly. Spina bifida ("open spine") is a defect of the spine. The most common form of spina bifida causes some degree of leg paralysis, impaired bladder and bowel control, and sometimes mental retardation. Anencephaly is a rapidly fatal condition in which a baby is born with a severely underdeveloped brain and skull.

While most NTDs are inherited as multifactorial disorders, a few result from single-gene disorders, chromosomal abnormalities, or terato-gens. NTDs currently have an incidence of 1 per 2,000 births. This rate has fallen dramatically over the past thirty years, due to the remarkable effects of NTD-prevention efforts. Maternal deficiency in folate (a B vitamin) greatly increases the risk of NTDs, but taking multivitamins containing folic acid before conception and early in pregnancy is highly effective in preventing these disorders. High doses of folic acid are needed to help protect the fetuses of women with pregnancies previously affected by NTDs, and for those who need to take certain medications that interfere with folate metabolism.

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