602
Chapter 17
pair, corresponding segments of homologous chromosomes
align closely. This allows two nonsister chromatids to undergo
an exchange of sites of breakage in a process called
crossing-
over
(e). Thus, crossing-over results in the recombination of
genes on homologous chromosomes.
Following crossing-over, the homologous chromosomes
line up in the center of the cell (f). The orientation of each
pair on the equator is random, meaning that sometimes the
maternal portion points to a particular pole of the cell, and
sometimes the paternal portion does so. The cell then divides
(the fi rst division of meiosis), with the maternal chromatids
of any particular pair going to one of the two cells resulting
from the division, and the paternal chromatids going to the
other (g). Because of the random orientation of the homolo-
gous pairs at the equator, it is extremely unlikely that all 23
maternal chromatids will end up in one cell and all 23 paternal
chromatids in the other. Over 8 million (2
23
) different combi-
nations of maternal and paternal chromosomes can result dur-
ing this fi rst meiotic division.
The second division of meiosis occurs without any fur-
ther replication of DNA. The sister chromatids—both of
which were originally either maternal or paternal—of each
chromosome separate and move apart into the new daugh-
ter cells (h to i). The daughter cells resulting from the second
meiotic division, therefore, contain 23 one-chromatid chro-
mosomes (i).
To summarize, meiosis produces daughter cells having
only 23 chromosomes, and two events during the fi rst meiotic
division contribute to the enormous genetic variability of the
daughter cells: (1) crossing-over and (2) the random distribu-
tion of maternal and paternal chromatid pairs between the
two daughter cells.
Sex Determination
Genetic inheritance sets the gender of the individual, or
sex
determination,
which is established at the moment of fertil-
ization. Gender is determined by genetic inheritance of two
chromosomes called the
sex chromosomes.
The larger of
the sex chromosomes is called the
X chromosome
and the
smaller, the
Y chromosome.
Males possess one X and one Y,
whereas females have two X chromosomes. Thus, the genetic
difference between male and female (
genotype
) is simply the
difference in one chromosome.
The ovum can contribute only an X chromosome,
whereas half of the sperm produced during meiosis are X and
half are Y. When the sperm and the egg join, 50 percent should
have XX and 50 percent XY. Interestingly, however, sex ratios
at birth are not exactly 1:1; rather, there tends to be a slight
preponderance of male births, possibly due to functional dif-
ferences in sperm carrying the X versus Y chromosome.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
Homologous
H
chromosomes
Figure 17–2
Stages of meiosis in a generalized germ cell. For simplicity, the initial cell (a), which is in interphase, is given only four chromosomes rather
than 46, the number in a human cell. Also, cytoplasm is shown only in (a), (h), and (i). Chromosomes from one parent are purple, and those
from the other parent are blue. The letters are keyed to descriptions in the text. From (h) to (i), the size of the cells can vary quite dramatically
in ova development.
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