HUMAN BIOLOGY

398 Chapter 20

individual will have three of one type of chromosome

(2n 1 1). If the gamete is missing a chromosome, then the

result is monosomy (2n 2 1). Most changes in the number of

autosomes arise through nondisjunction when meiosis is

forming gametes. About 1 in every 1,000 children is born

with trisomy 21—three copies of chromosome 21.

A person with trisomy 21 will have Down syndrome

(Figure 20.18). Symptoms vary, but most affected people are

mentally retarded. About 40 percent develop heart defects.

Because of abnormal skeletal development, older children

have shortened body parts, loose joints, and poorly aligned

hip, finger, and toe bones. Their muscles and muscle

reflexes are weak, and their motor functions develop

slowly. With special training, though, people with Down

syndrome often engage in normal activities.

For women, the incidence of nondisjunction increases

with age. The probability that a woman will conceive an

embryo with Down syndrome rises steeply after age 35.

Yet 80 percent of trisomic 21 infants are born to younger

mothers. This statistic reflects the fact that between the

ages of 18 and 35 women are the most fertile, so more

babies are born to mothers in this age range.

nondisjunction also can change

the number of sex chromosomes

Most sex chromosome abnormalities come about as a

result of nondisjunction as gametes are forming. Let’s look

at a few of the resulting phenotypes, which are listed in

Fig u re 20.19.

About 1 in every 5,000 newborns has Turner syndrome,

in which a nondisjunction has reduced the chromosome

number to 45. Most people with Turner syndrome are

Changes in Chromosome number

F i g u r e 20.17 Animated! In nondisjunction, chromosomes don’t separate properly during meiosis.

In this example, chromosomes fail to separate during anaphase I of meiosis and so there is a change in

the chromosome number in gametes that form. (© Cengage Learning)

anaphase II

alignments at

metaphase II

chromosome

alignments at

metaphase I

n + 1

n + 1

n − 1

n − 1

Chromosome

number in gametes

Nondisjunction

at anaphase I

20.9

n Several kinds of events can increase or decrease the

number of chromosomes in gametes.

Sometimes gametes—and, later on, embryos—end up with

the wrong chromosome number. The effects range from

minor physical ones to deadly disruption of body function.

More often, an affected fetus is miscarried, or spontane-

ously aborted before birth.

About half of fertilized eggs have a lethal condition

called aneuploidy (an-yoo-ploy-dee). In this situation, the

embryo doesn’t have an exact multiple of the normal hap-

loid set of 23 chromosomes. A polyploid embryo has three,

four, or more sets of the normal haploid set of 23 chromo-

somes. All but 1 percent of human polyploids die before

birth, and the rare newborns die soon afterward.

Chromosome numbers can change during mitosis

or meiosis or even at fertilization. For instance, a cell

cycle might advance through DNA duplication and mito-

sis; then for some reason it stops

before the dividing cell’s cyto-

plasm divides. The cell then is

polyploid—it has four of each type

of chromosome.

nondisjunction is a common cause

of abnormal numbers of autosomes

In nondisjunction, one or more pairs of chromosomes fail

to separate during mitosis or meiosis. Here again, some or

all of the resulting cells end up with too many or too few

chromosomes (Figure 20.17).

If fertilization involves a gamete that has an extra

chromosome (n 1 1), the result will be trisomy: The new

nondisjunction The failure
of pairs of chromosomes to
separate during mitosis or
meiosis.

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