Unit A Reproduction

4.5
Atypical Meiosis
Although meiosis occurs correctly in every male and female thousands of
times throughout adult life, this mechanism, which separates homologous
chromosomes into gametes, can occasionally go wrong. Once in a while, errors
do occur. The most common error is known as nondisjunction. Nondisjunction
occurs when the homologous chromosomes do not separate during meiosis.
This produces gametes with the wrong number of chromosomes.
If one of the gametes that come together during fertilization has the wrong
number of chromosomes, the resulting zygote can either have too much genetic
material or too little. For example, in humans, a gamete with 22 chromosomes
that fuses with a normal gamete with 23 chromosomes will produce a zygote
with only 45 chromosomes (Figure 1). In most cases, the zygote does not
divide and dies.
46
nondisjunction
22 24
46
normal meiosis
23 23
LEARNING TIP
To read Figure 1, begin with the
number at the top of the diagram.
What does this tell you? Now read the
label. What does this tell you? Next,
look at the arrows and numbers. What
do they tell you? Do the same for the
rest of the labels, numbers, and arrows.
normal meiosis
normal meiosis
nondisjunction
22
22 24 24 23 23 22 24
fertilization
45
abnormal chromosome number in the zygote
Figure 1 Nondisjunction can occur either during meiosis I or meiosis II.
Down Syndrome
Sometimes a zygote with the wrong number of chromosomes continues to divide
into an embryo, becomes a fetus, and develops into a baby. Down syndrome
is a common disorder that results from a zygote with 47 chromosomes. The
zygote undergoes cell division with each new daughter cell, also containing
47 chromosomes. People with Down syndrome have three copies of
chromosome 21, instead of the normal pair (Figure 2(a)). They have similar
facial features (Figure 2(b)), mild to severe mental disabilities, as well as a
variety of physical disorders.
Did You
KNOW?
Down Syndrome Facts
Down syndrome occurs in one of
approximately 600 births. It often
occurs in babies born to women
over age 40 and is thought to be
caused by nondisjunction during
egg production. People with Down
syndrome tend to be smaller and
have a shorter lifespan.
NEL
4.5 Atypical Meiosis 127