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NEL Cell Division 583

Nondisjunction Disorders

Nondisjunction is associated with many different human genetic disorders. For example,

Down syndrome is a trisomic condition. Down syndrome is also called trisomy 21

because it usually results from three copies of chromosome 21. People with Down syn-

drome (Figure 2) can be identified by several common traits, regardless of race: a round,

full face; enlarged and creased tongue; short height; and a large forehead. Down syn-

drome is generally associated with mental retardation, although people with this condi-

tion retain a wide range of mental abilities. The risk of having a baby with Down syndrome

increases with the age of the mother. About 1 in 600 babies is born with Down syn-

drome.

Turner syndrome occurs when sex chromosomes undergo nondisjunction. This mono-

somic disorder produces a female with a single X chromosome. In the egg cell, both

homologous X chromosomes move to the same pole during meiosis I (Figure 3). When

the egg with no X chromosome is fertilized by a normal sperm cell with an X chromo-

some, a zygote with 45 chromosomes is produced. Individuals with Turner syndrome

appear female, but do not usually develop sexually and tend to be short and have thick,

widened necks. About 1 in every 3000 female babies is a Turner syndrome baby. Most

Turner syndrome fetuses are miscarried before the 20th week of pregnancy.

Klinefelter syndrome is caused by nondisjunction in either the sperm or egg

(Figure 3). The child inherits two X chromosomes—characteristic of females—and a single

Y chromosome—characteristic of males. The child appears to be a male at birth; how-

ever, as he enters sexual maturity, he begins producing high levels of female sex hor-

mones. Males with Klinefelter syndrome are sterile. It has been estimated that Klinefelter

syndrome occurs, on average, in 1 of every 500 male babies.

Section17.4

Figure 2

People with Down syndrome have a

wide range of abilities.

normal female normal male normal female normal male

normal meiosis nondisjunction nondisjunction normal meiosis

Klinefelter

syndrome

Turner

syndrome

trisomic

female

Turner

syndrome

Figure 3
Nondisjunction disorders in humans

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CAREER CONNECTION

Geneticist

Geneticists are professionals with

specialized education, training,

and experience in genetics. Those

with expertise in medical genetics

may help families understand birth

defects and how diseases are

inherited. They may counsel

people who carry genes that

increase their risk of developing

disease, such as some forms of

cancer.