HUMAN BIOLOGY

Chromosomes and human GenetiCs 399

wHat causes cHanges in tHe normal number

of cHromosomes?

• Most changes in the chromosome number are due to
  nondisjunction during meiosis. In nondisjunction, one or
  more pairs of chromosomes fail to separate as gametes
  are forming.

taKe-home messaGe

abnormally small testes, sparse body hair, and may develop

enlarged breasts. Testosterone injections can reverse some

aspects of the phenotype.

About 1 in every 1,000 males has one X and two

Y chromosomes, a condition due to nondisjunction of

duplicated Y chromosomes during meiosis. XYY males

tend to be taller than average, but otherwise they have a

normal male phenotype.

missing an X chromosome (in most cases, the one that
would have come from the father), and the condition is
symbolized as XO. Turner syndrome occurs less often than
other sex chromosome abnormalities, probably because
most XO embryos are miscarried early in the pregnancy.
Affected people are female and have a webbed neck and
other phe notypic abnormalities. Their ovaries don’t func-
tion, they are sterile, and secondary sexual traits don’t
develop at puberty. People who have Turner syndrome
often age prematurely and have shortened life expectancies.
Roughly 1 in 1,000 females has three X chromosomes.
Two of these X chromosomes are condensed to Barr bodies,
and most XXX females develop normally.
In Klinefelter syndrome, nondisjunction produces
the genotype XXY. This sex chromosome abnormality
occurs in about 1 in 500 males. XXY males have low fertil-
ity and many have some mental retardation. They have

F i g u r e 20.19 Genetic disorders can come about due to

nondisjunction of X chromosomes followed by fertilization

by normal sperm. (© Cengage Learning)

Nondisjunction Resulting

zygotes

Fertilization by

normal sperm

XXY

XO

XXX

XX

XX

× YO (not viable)

Turner

syndrome

O

O

×

Klinefelter

× syndrome

Triple X

syndrome

Y

Y

X

X

×

XX

XX

O

F i g u r e 20.18 In Down syndrome there are three copies
of chromosome 21. A In this karyotype of a girl with Down
syndrome, note the extra copy of chromosome 21. B A boy
with Down syndrome.

A

CNRI/Science Source

B

Lauren Shear/Science Source

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