1098
UNIT 5
Continuity
29
means that they are distributed to different gametes. Errors in
this important process have been linked to cancer progression,
infertility, and Down syndrome. (2) Alleles on different pairs
of homologous chromosomes are distributed independently of
each other. Te net result is that each gamete has a single allele
for each trait, and that allele represents only one of the four pos-
sible parental alleles.
Te number of different gamete types resulting from this
inde-
pendent assortment
of homologues during meiosis I can be calcu-
lated for any genome from the formula 2
n
, where
n
is the number
of homologous pairs. In our example, 2
n
5
2
3
(2
3
2
3
2), for a
total of 8 different gamete types.
Note that the number of gamete types increases dramatically
as the chromosome number increases. A cell with six pairs of
homologues would produce 2
6
, or 64, kinds of gametes. In a
man’s testes, the number of gamete types that can be produced
on the basis of independent assortment alone is 2
23
, or about 8.5
million—an incredible variety. Te number of different gamete
types produced simultaneously in a woman’s ovaries is substan-
tially less because her ovaries complete at most 500 reduction
divisions in her lifetime. Still, each ovulated oocyte will most
likely be novel genetically because of independent assortment.
Crossover of Homologues
and Gene Recombination
Additional gamete variation results from the crossing over and
exchange of chromosomal parts during meiosis I. Genes are ar-
ranged linearly along a chromosome’s length, and genes on the
same chromosome are said to be
linked
because they are trans-
mitted as a unit to daughter cells during mitosis. However, as we
described in Chapter 27, chromosomes can break and precisely
exchange gene segments with their homologous counterparts
during meiosis. Tis exchange gives rise to
recombinant chro-
mosomes
that have mixed contributions from each parent.
In the hypothetical example shown in
Figure 29.3
, the genes
for hair and eye color are linked. Te paternal chromosome con-
tains alleles coding for blond hair and blue eyes, and the maternal
alleles code for brown hair and brown eyes. In the
crossover
, or
chiasma
, shown, the break occurs between these linked genes,
resulting in one gamete with alleles for blond hair and brown
eyes and another with alleles for brown hair and blue eyes. As a
result of the crossover, two of the four chromatids present in the
tetrad end up with a mixed set of alleles—some maternal and
some paternal. Tis means that when the chromatids segregate,
each gamete will receive a unique combination of parental genes.
Because humans have 23 tetrads, with crossovers going on in
all of them during meiosis I, the variability resulting from this
factor alone is tremendous.
Random Fertilization
At any point in time, gametogenesis is turning out gametes with
all variations possible from independent assortment and random
crossovers. Fertilization compounds this variety because a single
human egg will be fertilized by a single sperm on a totally hap-
hazard, or random, basis. If we consider variation resulting only
H
Allele for brown hair
h
h
e
e
H
H
E
E
h
h
e
E
H
H
e
E
h
h
e
E
H
H
e
E
h
h
e
E
H
H
e
E
h
Allele for blond hair
E
Allele for brown eyes
e
Allele for blue eyes
Paternal chromosome
Maternal chromosome
Homologous pair
Hair color
genes
Eye color
genes
Homologous chromosomes synapse during prophase of
meiosis I. Each chromosome consists of two sister chromatids.
One chromatid segment exchanges positions with a
homologous chromatid segment—in other words, crossing
over occurs, forming a chiasma.
The chromatids forming the chiasma break, and the broken-off
ends join their corresponding homologues.
At the conclusion of meiosis, each haploid gamete has one
of the four chromosomes shown. Two of the chromosomes
are recombinant (they carry new combinations of genes).
Gamete 1
Gamete 2
Gamete 3
Gamete 4
Chiasma
Figure 29.3
Crossover and genetic recombination.
These
meiosis I events increase genetic variability in the gametes formed.
For simplicity, only two chromatids are shown taking part in
crossover. Multiple crossovers result in more complex patterns.
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