CeLL reprOduCtIOn 367hoW does meiosis create variation in the
chromosomes gametes receive?- Meiosis moves maternal and paternal chromosomes into
gametes at random. - This random pattern creates new combinations of chromosomes
in sperm and eggs, so children have varied combinations of their
parents’ traits. - Also, crossing over of segments between pairs of homologous
chromosomes creates new combinations of genes on the
chromosomes.
of 2 taKe-hoMe Message
(^23) , or 8,388,608, combinations of maternal and paternal
chromosomes are possible every time meiosis in a germ cell
produces a gamete! This is why striking mixes of traits can
show up even in the same family (Figure 18.13B).
During meiosis II, each homologue normally is sepa-
rated from its partner so that gametes receive only the
required haploid set of chro-
mosomes. This separation is
called disjunction. As Chapter
20 describes, birth defects can
result when this process does
not occur as usual.
disjunction Separation of
homologous chromosomes
during meiosis, so that
gametes receive only a hap-
loid set of chromosomes.
Resulting combina-
tions of maternal and
paternal chromosomes
in the two nuclei that
form at telophase I.
Resulting combinations
of maternal and paternal
chromosomes in the four
nuclei that form at telo-
phase II. Eight different
combinations are possible.
The four possible
alignments of three pairs
of chromosomes in a
nucleus at metaphase I.
1
2
3
A
F i g u r e 18.13 Animated! Maternal and paternal chromosomes are assorted randomly into gametes. A Possible outcomes of the random
alignment of three pairs of homologous chromosomes at metaphase I of meiosis. For simplicity, no crossing over occurs in this example.
B Four generations of a family showing numerous variations in traits that shape physical appearance. (© Cengage Learning)
© iStockphoto.com/stphillips
B
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