Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution

Question 1

changes in an organism’s DNA create the different versions of genes, known as

Answer 1

alleles.

Question 2

Three mechanisms contribute to the genetic variation arising from sexual reproduction:

Answer 2

independent assortment of chromosomes, crossing over, and random fertilization.

Question 3

One aspect of sexual reproduction that generates genetic variation is the random orientation of pairs of homologous chromosomes at metaphase of

Answer 3

meiosis I.

Question 4

At metaphase I, the homologous pairs, each consisting of one

Answer 4

maternal and one paternal chromosome, are situated at the metaphase plate

Question 5

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

Answer 5

maternal chromosome of a certain homologous pair and a 50% chance that it will get the paternal chromosome.

Question 6

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

Answer 6

independent assortment.

Question 7

Each daughter cell represents one outcome of all possible combinations of maternal and paternal

Answer 7

chromosomes.

Question 8

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

Answer 8

four:

Question 9

two possible arrangements for the first pair times two possible arrangements for the

Answer 9

second pair

Question 10

Note that only two of the four combinations of daughter cells shown in the figure would result from meiosis of a

Answer 10

single diploid cell

Question 11

a single parent cell would have one or the other possible chromosomal arrangement at metaphase I, but not

Answer 11

both.

Question 12

the population of daughter cells resulting from meiosis of a large number of diploid cells contains all

Answer 12

four types in approximately equal numbers.

Question 13

In the case of n=3, ________________________ of chromosomes are possible for daughter cells.

Answer 13

eight combinations (2x2x2=2ּּּּ³)

Question 14

More generally, the number of possible combinations when chromosomes sort independently during meiosis is 2ּּᶯ, where n is the

Answer 14

haploid number of the organism.

Question 15

figure 13.11, animation genetic variation from independent assortment of chromosomes

Question 16

In the case of humans (n=23), the number of possible combinations of maternal and paternal chromosomes in the resulting gametes is

Answer 16

2ּּ²³, or about 8.4 million

Question 17

individual chromosomes that carry genes (DNA) from two different parents

Answer 17

recombinant chromosomes

Question 18

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.

Answer 18

one to three

Question 19

figure 13.12, animation genetic variation from crossing over

Question 20

As you learned in Figure 13.9, crossing over produces chromosomes with new combinations of maternal and paternal

Answer 20

alleles

Question 21

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

Answer 21

identical

Question 22

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

Answer 22

meiosis.

Question 23

The fusion of a male gamete with a female gamete during fertilization will produce a zygote with any of about

Answer 23

70 trillion (2ּּ²³x2ּּ²³) diploid combinations.

Question 24

animation genetic variation from random fertilization

Question 25

natural selection results in the accumulation of genetic variations favored by the

Answer 25

environment.

Question 26

Mutations are the original source of different alleles, which are then

Answer 26

mixed and matched during meiosis.

Question 27

sexual reproduction is more expensive energetically than

Answer 27

asexual reproduction.

Question 28

In spite of these apparent disadvantages, sexual reproduction is almost universal among

Answer 28

animals

Question 29

bdelloid rotifers are an exception to the “rule” that

Answer 29

sex alone generates genetic diversity

Question 30

figure 13.13 a bdelloid rotifer, an animal that reproduces only asexually

Question 31

Although Darwin realized that heritable variation is what makes evolution possible, he could not explain why

Answer 31

offspring resemble—but are not identical to—their parents.

Question 32

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

Answer 32

1900, more than 15 years after Darwin (1809–1882) and Mendel (1822–1884) had died