Unit A Reproduction

Figure 2 The karyotype of
Down syndrome shows three copies
of chromosome 21 (a). People with
Down syndrome may lead active,
productive lives (b).
2000X
(a)
(b)
Sex Chromosome Disorders
Nondisjunction can also occur with the X and Y chromosomes. When this
happens, gametes can be produced with either an extra sex chromosome or
none at all. Turner syndrome occurs when a female is born with only one X
chromosome (Figure 3). One of the gametes is missing the sex chromosome.
The reproductive organs of females with Turner syndrome do not develop at
puberty. Therefore, females with Turner syndrome do not menstruate and cannot
reproduce. Trisomic females have an extra X chromosome. Unlike Turner
syndrome females, trisomic females’ reproductive organs develop at puberty
and they can reproduce. They may, however, be taller and thinner than average.
A nondisjunction disorder that occurs in males is Klinefelter syndrome.
In Klinefelter syndrome, males are born with two X chromosomes and a
Y chromosome. One of the gametes had an extra X chromosome (Figure 3).
Males with Klinefelter syndrome produce less of the sex hormone testosterone
than normal, and they cannot father children.
female
male
female
male
XX XY XX XY
normal
meiosis
nondisjunction
nondisjunction
normal
meiosis
X
X
XY
XX
X
Y
Figure 3 Turner and Klinefelter
syndromes result from nondisjunction
of the sex chromosomes.
XXY X0 XXX X0
Klinefelter
Syndrome
Turner
Syndrome
trisomic
female
Turner
Syndrome
128 Unit A Reproduction
NEL