Essential Cell Biology 5th edition

Mendel and the Laws of Inheritance
669
generation I
generation II
generation III
generation IV
1
1 2 3 4
2 3 4
2 3
1 4
1 2 3 4 5 6 7 8 9 10
crosses, examining the simultaneous inheritance of two or more apparently
unrelated traits.
In the simplest situation, a dihybrid cross, Mendel followed the inheritance
of two traits at once: for example, pea color and pea shape. In the case
ECB5 e19.26/19.26
of pea color, we have already seen that yellow is dominant over green;
for pea shape, round is dominant over wrinkled (see Figure 19−20). What
would happen when plants that differ in both of these characters are
crossed? Again, Mendel started with true-breeding parental strains: the
dominant strain produced yellow round peas (its genotype is YYRR), the
recessive strain produced green wrinkled peas (yyrr). One possibility is
that the two characters, pea color and shape, would be transmitted from
parents to offspring as a linked package. In other words, plants would
always produce either yellow round peas or green wrinkled ones. The
other possibility is that pea color and shape would be inherited independently,
which means that at some point plants bearing a novel mix of
traits—yellow wrinkled peas or green round peas—would arise.
The F 1 generation of plants all showed the expected phenotype: each
produced peas that were yellow and round. But this result would occur
whether or not the parental alleles were linked. When the F 1 plants were
then allowed to self-fertilize, the results were clear: the two alleles for
seed color segregated independently from the two alleles for seed shape,
producing four different pea phenotypes: yellow-round, yellow-wrinkled,
green-round, and green-wrinkled (Figure 19−27). Mendel tried his seven
pea characters in various pairwise combinations and always observed
a characteristic 9:3:3:1 phenotypic ratio in the F 2 generation. The independent
segregation of each pair of alleles during gamete formation is
Mendel’s second law—the law of independent assortment.
Figure 19−26 A pedigree shows the risks
of first-cousin marriages. Shown here is an
actual pedigree for a family that harbors a
rare recessive mutation causing deafness.
According to convention, squares represent
males, circles are females. Here, family
members that show the deaf phenotype are
indicated by a blue symbol, those that do
not by a gray symbol. A black horizontal line
connecting a male and female represents a
mating between unrelated individuals, and
a red horizontal line represents a mating
between blood relatives. The offspring of
each mating are shown underneath, in order
of their birth from left to right.
Individuals within each generation are
labeled sequentially from left to right for
purposes of identification. In the third
generation in this pedigree, for example,
individual 2, a man who is not deaf, marries
his first cousin, individual 3, who is also
not deaf. Three out of their five children
(individuals 7, 8, and 9 in the fourth
generation) are deaf. Meanwhile, individual
1, the brother of 2, also marries a first
cousin, individual 4, the sister of 3. Two out
of their five children are deaf. (Adapted from
Z.M. Ahmed et al., BMC Med. Genet. 5:24,
2004. With permission from BMC Medical
Genetics.)
The Behavior of Chromosomes During Meiosis
Underlies Mendel’s Laws of Inheritance
So far we have discussed alleles and genes as if they are disembodied
entities. We now know that Mendel’s “factors”—the things we call
genes—are carried on chromosomes that are parceled out during the formation
of gametes and then brought together in novel combinations in
the zygote at fertilization. Chromosomes therefore provide the physical
basis for Mendel’s laws, and their behavior during meiosis and fertilization—which
we discussed earlier—explains these laws perfectly.
During meiosis, the maternal and paternal homologs—and the genes that
they contain—pair and then separate from each other as they are parceled
out into gametes. These maternal and paternal chromosome copies will
possess different variants—or alleles—of many of the genes they carry.
Take, for example, a pea plant that is heterozygous for the yellow-pea