Congenital Anomalies

The most anxious moment for many new parents is awaiting reassurance that the child has no visible birth defects. Any abnormality present at birth is called a congenital anomaly.12 Here we consider some anomalies that result from infectious diseases, teratogens, mutagens, and genetic defects.

Infectious Diseases

Infectious diseases are largely beyond the scope of this book, but it must be noted at least briefly that several microorganisms can cross the placenta and cause serious congenital anomalies, stillbirth, or neonatal death. Common viral infections of the fetus and newborn include herpes simplex, rubella, cytomegalovirus, and human immunodeficiency virus (HIV). Congenital bacterial infections include gonorrhea and syphilis. Toxoplasma, a protozoan contracted from meat, unpasteurized milk, and housecats, is another common cause of fetal deformity. Some of these pathogens have relatively mild effects on adults, but because of its immature immune system, the con = with + gen = born + a = without + nom = rule, regularity fetus is vulnerable to devastating effects such as blindness, hydrocephalus, cerebral palsy, seizures, and profound physical and mental retardation. These diseases are treated in greater detail in microbiology textbooks.


Teratogens13 are viruses, chemicals, and other agents that cause anatomical deformities in the fetus. Perhaps the most notorious teratogenic drug is thalidomide, a sedative first marketed in 1957. Thalidomide was taken by women in early pregnancy, often before they knew they were pregnant, and caused over 5,000 babies to be born with unformed arms or legs (fig. 29.13) and often with defects of the ears, heart, and intestines. It was taken off the market in 1961 but has recently been reintroduced for limited purposes. Many teratogens produce less obvious effects, including physical or mental retardation, hyperirritability, inattention, strokes, seizures, respiratory arrest, crib death, and cancer. A general lesson to be learned from the thalidomide tragedy and other cases is that pregnant women should avoid all sedatives, barbiturates, and opiates. Even the acne medicine Acutane has caused severe birth defects.

Alcohol causes more birth defects than any other drug. Even one drink a day has noticeable effects on fetal and postpartum development, some of which are not noticed until a child begins school. Alcohol abuse during pregnancy can cause fetal alcohol syndrome (FAS), characterized by a small head, malformed facial features, cardiac and central nervous system defects, stunted growth, and behavioral signs such as hyperactivity, nervousness, and a poor attention span. Cigarette smoking also contributes to fetal and infant mortality, ectopic pregnancy, anencephaly (failure of the cerebrum to develop), cleft lip and palate, and cardiac abnormalities. Diagnostic X rays should be avoided during pregnancy because radiation can have teratogenic effects.

terato = monster + gen = producing

Figure 29.13 Effects of Thalidomide. Taken as a sedative in early pregnancy, thalidomide proved to be a teratogen with severe effects on embryonic limb development.

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Mutagens and Genetic Anomalies

A mutagen is any agent that alters DNA or chromosome structure. Ionizing radiation and some chemicals have mutagenic, teratogenic, and carcinogenic effects, with extremely diverse results. Prenatal exposure to mutagens may result, for example, in stillbirths or in increased risk of childhood cancer.

Some of the most common genetic disorders result not from mutagens, however, but from the failure of homologous chromosomes to separate during meiosis. Recall that homologous chromosomes pair up during prophase I and normally separate from each other at anaphase I (see p. 1034). This separation, called disjunction, produces daughter cells with 23 chromosomes each.

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In nondisjunction, a pair of chromosomes fails to separate. Both chromosomes then go to the same daughter cell, which receives 24 chromosomes while the other daughter cell receives 22. Aneuploidy14 (AN-you-PLOY-dee), the presence of an extra chromosome or lack of one, accounts for about 50% of spontaneous abortions. It can be detected prior to birth by amniocentesis, the examination of cells in a sample of amniotic fluid, or by chorionic vil-lus sampling, the removal and examination of cells from the chorion.

Figure 29.14 compares normal disjunction of the X chromosomes with some effects of nondisjunction. In l4an = not, without + eu = true, normal + ploid, from diplo = double, paired


Figure 29.14 Disjunction and Nondisjunction. (a) The outcome of normal disjunction and fertilization by X- or Y-bearing spermatozoa. (b) Two of the possible outcomes of nondisjunction followed by fertilization with an X-bearing spermatozoon.

In the right half of the figure, what would the two outcomes be if the sperm carried a Y chromosome instead of an X?

Figure 29.14 Disjunction and Nondisjunction. (a) The outcome of normal disjunction and fertilization by X- or Y-bearing spermatozoa. (b) Two of the possible outcomes of nondisjunction followed by fertilization with an X-bearing spermatozoon.

In the right half of the figure, what would the two outcomes be if the sperm carried a Y chromosome instead of an X?

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1106 Part Five Reproduction and Development nondisjunction, an egg cell may receive both X chromosomes. If it is fertilized by an X-bearing sperm, the result is an XXX zygote and a set of anomalies called the triplo-X syndrome. Triplo-X females are sometimes infertile and sometimes have mild intellectual impairments. If an XX egg is fertilized by a Y-bearing sperm, the result is an XXY combination and Klinefelter15 syndrome. People with Klinefelter syndrome are sterile males, usually of average intelligence, but with undeveloped testes, sparse body hair, unusually long arms and legs, and enlarged breasts (gynecomastia). This syndrome often goes undetected until puberty, when failure to develop the secondary sex characteristics may prompt genetic testing.

The other possible outcome of X chromosome nondisjunction is that an egg cell may receive no X chromosome (both X chromosomes are discarded in the first polar body). If fertilized by a Y-bearing sperm, it dies for lack of the indispensable genes on the X chromosome. If it is fertilized by an X-bearing sperm, how-

15Harry F. Klinefelter, Jr. (1912-), American physician

15Harry F. Klinefelter, Jr. (1912-), American physician

Webbed Neck
Figure 29.15 Turner Syndrome. Note the short stature (about 145 cm, or 4 ft 9 in.), lack of sexual development, webbed neck, and widely spaced nipples. This person was 22 years old.

ever, the result is Turner16 syndrome, with an XO combination (O represents the absence of one sex chromosome). Only 3% of fetuses with Turner syndrome survive to birth. Girls with Turner syndrome show no serious impairments as children but tend to have a webbed neck and widely spaced nipples. At puberty, the secondary sex characteristics fail to develop (fig. 29.15). The ovaries are nearly absent, the girl remains sterile, and she usually has a short stature.

The other 22 pairs of chromosomes (the autosomes) are also subject to nondisjunction. Nondisjunction of chromosomes 13 and 18 results in Edward syndrome (trisomy-13) and Patau syndrome (trisomy-18), respectively. Affected individuals have three copies of the respective chromosome. Nearly all fetuses with these trisomies die before birth. Infants born with these syndromes are severely deformed, and fewer than 5% survive for one year.

The most common autosomal anomaly is Down17 syndrome (trisomy-21). The signs include retarded physical development; short stature; a relatively flat face with a flat nasal bridge; low-set ears; epicanthal18 folds at the medial corner of the eyes; an enlarged, protruding tongue; stubby fingers; and a short broad hand with only one palmar crease (fig. 29.16). People with Down syndrome tend to have outgoing, affectionate personalities. Mental retardation is common and sometimes severe, but is not inevitable. Down syndrome occurs in about 1 out of 700 to 800 live births in the United States and increases in proportion to the age of the mother. The chance of having a child with Down syndrome is about 1 in 3,000 for a woman under 30, 1 in 365 by age 35, and 1 in 9 by age 48.

About 75% of the victims of trisomy-21 die before birth. About 20% of infants born with it die before the age of 10 from such causes as immune deficiency and abnormalities of the heart or kidneys. For those who survive beyond that age, modern medical care has extended life expectancy to about 60 years. After the age of 40, however, many of these people develop early-onset Alzheimer disease, linked to a gene on chromosome 21.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

10. How does inflation of the lungs at birth affect the route of blood flow through the heart?

11. Why is respiratory distress syndrome common in premature infants?

12. Define nondisjunction and explain how it causes aneuploidy. Name two syndromes resulting from aneuploidy.

16Henry H. Turner (1892-1970), American endocrinologist

17John Langdon H. Down (1828-96), British physician

18epi = upon + canth = commissure of the eyelids

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29. Human Development Text

Chapter 29 Human Development 1107

Down Syndrome Multiple Births

Chapter 29 Human Development 1107

Simian Crease

Incurved finger

Single palmar ("simian") crease

Short broad hands

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