Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution Flashcards
changes in an organism’s DNA create the different versions of genes, known as
alleles.
Three mechanisms contribute to the genetic variation arising from sexual reproduction:
independent assortment of chromosomes, crossing over, and random fertilization.
One aspect of sexual reproduction that generates genetic variation is the random orientation of pairs of homologous chromosomes at metaphase of
meiosis I.
At metaphase I, the homologous pairs, each consisting of one
maternal and one paternal chromosome, are situated at the metaphase plate
Each pair may orient with either its maternal or paternal homolog closer to a given pole—its orientation is as random as the flip of a coin. Thus, there is a 50% chance that a particular daughter cell of meiosis I will get the
maternal chromosome of a certain homologous pair and a 50% chance that it will get the paternal chromosome.
Because each pair of homologous chromosomes is positioned independently of the other pairs at metaphase I, the first meiotic division results in each pair sorting its maternal and paternal homologs into daughter cells independently of every other pair. This is called
independent assortment.
Each daughter cell represents one outcome of all possible combinations of maternal and paternal
chromosomes.
As shown in Figure 13.11, the number of combinations possible for daughter cells formed by meiosis of a diploid cell with two pairs of homologous chromosomes (n=2) is
four:
two possible arrangements for the first pair times two possible arrangements for the
second pair
Note that only two of the four combinations of daughter cells shown in the figure would result from meiosis of a
single diploid cell
a single parent cell would have one or the other possible chromosomal arrangement at metaphase I, but not
both.
the population of daughter cells resulting from meiosis of a large number of diploid cells contains all
four types in approximately equal numbers.
In the case of n=3, ________________________ of chromosomes are possible for daughter cells.
eight combinations (2x2x2=2ּּּּ³)
More generally, the number of possible combinations when chromosomes sort independently during meiosis is 2ּּᶯ, where n is the
haploid number of the organism.
figure 13.11, animation genetic variation from independent assortment of chromosomes
In the case of humans (n=23), the number of possible combinations of maternal and paternal chromosomes in the resulting gametes is
2ּּ²³, or about 8.4 million
individual chromosomes that carry genes (DNA) from two different parents
recombinant chromosomes
In meiosis in humans, an average of _____________ crossover events occurs per chromosome pair, depending on the size of the chromosomes and the position of their centromeres.
one to three
figure 13.12, animation genetic variation from crossing over
As you learned in Figure 13.9, crossing over produces chromosomes with new combinations of maternal and paternal
alleles
At metaphase II, chromosomes that contain one or more recombinant chromatids can be oriented in two alternative, nonequivalent ways with respect to other chromosomes because their sister chromatids are no longer
identical
The different possible arrangements of nonidentical sister chromatids during meiosis II further increase the number of genetic types of daughter cells that can result from
meiosis.
The fusion of a male gamete with a female gamete during fertilization will produce a zygote with any of about
70 trillion (2ּּ²³x2ּּ²³) diploid combinations.
animation genetic variation from random fertilization
natural selection results in the accumulation of genetic variations favored by the
environment.
Mutations are the original source of different alleles, which are then
mixed and matched during meiosis.
sexual reproduction is more expensive energetically than
asexual reproduction.
In spite of these apparent disadvantages, sexual reproduction is almost universal among
animals
bdelloid rotifers are an exception to the “rule” that
sex alone generates genetic diversity
figure 13.13 a bdelloid rotifer, an animal that reproduces only asexually
Although Darwin realized that heritable variation is what makes evolution possible, he could not explain why
offspring resemble—but are not identical to—their parents.
Gregor Mendel, a contemporary of Darwin, published a theory of inheritance that helps explain genetic variation, but his discoveries had no impact on biologists until
1900, more than 15 years after Darwin (1809–1882) and Mendel (1822–1884) had died
