Essential Cell Biology 5th edition

662 CHAPTER 19 Sexual Reproduction and Genetics
8.4 × 10 6 —genetically distinct gametes. The actual number of different
gametes each person can produce, however, is much greater than that,
because the crossing-over that takes place during meiosis provides a second
source of randomized genetic reassortment. Between two and three
crossovers occur on average between each pair of human homologs,
generating new chromosomes with novel assortments of maternal and
paternal alleles. Because crossing-over occurs at more or less random
sites along the length of a chromosome, each meiosis will produce four
sets of entirely novel chromosomes (Figure 19−15B).
Taken together, the random reassortment of maternal and paternal chromosomes,
coupled with the genetic mixing of crossing-over, provides a
nearly limitless source of genetic variation in the gametes produced by a
single individual. Considering that every person is formed by the fusion
of such gametes, produced by two completely different individuals, the
richness of human variation that we see around us, even within a single
family, should not be very surprising.
QUESTION 19–2
Ignoring the effects of chromosome
crossovers, an individual human can
in principle produce 2 23 = 8.4 × 10 6
genetically different gametes. How
many of these possibilities can be
“sampled” in the average life of
(A) a female and (B) a male, given
that women produce one egg a
month during their fertile years,
whereas men can make hundreds
of millions of sperm each day?
Meiosis Is Not Flawless
The sorting of chromosomes that takes place during meiosis is a remarkable
feat of molecular bookkeeping: in humans, each meiosis requires
that the starting cell keep track of 92 chromosomes (23 pairs, each of
which has duplicated), handing out one complete set to each gamete.
Not surprisingly, mistakes can occur in the distribution of chromosomes
during this elaborate process.
Occasionally, homologs fail to separate properly—a phenomenon known
as nondisjunction. As a result, some of the haploid cells that are produced
lack a particular chromosome, while others have more than one copy.
Upon fertilization, such gametes form abnormal embryos, most of which
die. Some, however, survive. Down syndrome, for example—a disorder
associated with cognitive disability and characteristic physical abnormalities—is
caused by an extra copy of Chromosome 21. This error results
from nondisjunction of a Chromosome 21 pair during meiosis I, giving
rise to a gamete that contains two copies of that chromosome instead
of one (Figure 19−16). When this abnormal gamete fuses with a normal
diploid
germ-cell
precursor
paternal homolog
of Chromosome 21
maternal homolog
of Chromosome 21
CHROMOSOME DUPLICATION
Figure 19−16 Errors in chromosome
segregation during meiosis can result
in gametes with incorrect numbers
of chromosomes. In this example, the
duplicated maternal and paternal copies
of Chromosome 21 fail to separate
normally during the first meiotic division.
As a result, two of the gametes receive
no copy of the chromosome, while the
other two gametes receive two copies. For
simplicity, only Chromosome 21 is shown.
Gametes that receive an incorrect number
of chromosomes are called aneuploid
gametes. If one of them participates in the
fertilization process, the resulting zygote
will also have an abnormal number of
chromosomes. A child that receives three
copies of Chromosome 21 will have Down
syndrome.
aneuploid gametes with
2 copies of Chromosome 21
MEIOTIC DIVISION II
NONDISJUNCTION DURING
MEIOTIC DIVISION I
aneuploid gametes with no
copies of Chromosome 21