Essential Cell Biology 5th edition

Mendel and the Laws of Inheritance
671
duplicated
homologous
chromosomes
r
R
parent plant
genotype:Y Rr
diploid germ-line cell in parent plant
Y
Figure 19−28 The separation of
duplicated homologous chromosomes
during meiosis explains Mendel’s
laws of segregation and independent
assortment. Here we show independent
assortment of the alleles for seed color,
yellow (Y) and green (y), and for seed shape,
round (R) and wrinkled (r), as an example of
how two genes on different chromosomes
segregate independently. Although
crossovers are not shown, they would not
affect the independent assortment of these
traits, as the two genes lie on different
chromosomes.
PAIRED DUPLICATED HOMOLOGS
ALIGN RANDOMLY ON SPINDLE
AT METAPHASE OF MEIOSIS I
OR
meiotic
spindle
MEIOSIS I
MEIOSIS II
RY RY r r R R rY rY
gametes
is true for the homolog pair carrying the alleles for seed shape. Thus,
whether the final gamete receives the YR, Yr, yR, or yr allele combination
depends entirely on which way the two homolog pairs were facing when
they were captured by the meiotic spindle; each outcome has the same
degree of randomness as the tossing of a coin.
Genes That Lie on the Same Chromosome Can
Segregate Independently by Crossing-Over
ECB5 e19.28/19.28
Mendel studied seven traits, each of which is controlled by a separate
gene. It turns out that most of these genes are located on different chromosomes,
which readily explains the independent segregation he observed.
But the independent segregation of different traits does not necessarily
require that the responsible genes lie on different chromosomes. If two
genes are far enough away from each other on the same chromosome,
they will also sort independently, because of the crossing-over that occurs
during meiosis. As we discussed earlier, when duplicated homologs pair
to form bivalents, the maternal and paternal homologs always undergo
crossing-over. This genetic exchange can separate alleles that were formerly
together on the same chromosome, causing them to segregate into