Chapter 13

Question 1

heredity

Answer 1

The transmission of traits from one generation
to the next is called inheritance, or heredity (from the Latin heres, heir). However,
sons and daughters are not identical copies of either parent or of their siblings. A

Question 2

variation

Answer 2

Along
with inherited similarity, there is also variation. What are the biological mechanisms leading to the “family resemblance” evident among the family members in
the photo? A detailed answer to this question eluded biologists until the advance of
genetics in the 20th century.

Question 3

genetics

Answer 3

is the scientific study of heredity and inherited variation. I

Question 4

. Inherited information is passed on

in the form of

Answer 4

each gene’s specific sequence of DNA nucleotides, much as printed information is communicated in the
form of meaningful sequences of letters.

Question 5

somatic cells

Answer 5

For example, humans have
46 chromosomes in their somatic cells—all cells of the body
except the gametes and their precursors.

Question 6

locus

Answer 6

gene’s specific location along the length of a chromosome

is called the gene’s locus (plural, loci; from the Latin, meaning “place”).

Question 7

Our genetic endowment (our genome) consists

of

Answer 7

the genes and other DNA that make up the chromosomes

we inherited from our parents.

Question 8

clone

Answer 8

. An individual that reproduces asexually gives

rise to a clone, a group of genetically identical individuals.

Question 9

homologs

Answer 9

The two chromosomes of
a pair have the same length, centromere position, and staining
pattern: These are called homologous chromosomes (or
homologs).

Question 10

diploid cell

Answer 10

The number of chromosomes in
a single set is represented by n. Any cell with two chromosome
sets is called a diploid cell and has a diploid number of chromosomes, abbreviated 2n. For humans, the diploid number is
46 (2n = 46), the number of chromosomes in our somatic cells.

Question 11

meiosis

Answer 11

, gamete formation involves a type of cell division called
meiosis. This type of cell division reduces the number of sets
of chromosomes from two to one in the gametes, counterbalancing the doubling that occurs at fertilization.

Question 12

alternation of generations

Answer 12

Plants and some species of algae exhibit a second type of life
cycle called alternation of generations (Figure 13.6b). This
type includes both diploid and haploid stages that are multicellular. The multicellular diploid stage is called the sporophyte.

Question 13

synaptonemal complex

Answer 13

Next, the formation of a zipper-like structure called the
synaptonemal complex holds one homolog tightly to
the other.

Question 14

synapsis

Answer 14

During this association, called synapsis, the
DNA breaks are closed up so that each broken end is joined to
the corresponding segment of the nonsister chromatid. Thus,
a paternal chromatid is joined to a piece of maternal
chromatid beyond the crossover point, and vice versa.

Question 15

Three events unique to meiosis occur during meiosis I:

1. Synapsis and crossing over.

Answer 15

During prophase I,
duplicated homologs pair up and crossing over occurs,
as described previously and in Figure 13.9. Synapsis and
crossing over do not occur during prophase of mitosis

Question 16

Three events unique to meiosis occur during meiosis I: 2. Alignment of homologous pairs at the metaphase
plate.

Answer 16

At metaphase I of meiosis, pairs of homologs are
positioned at the metaphase plate, rather than individual
chromosomes, as in metaphase of mitosis.

Question 17

Three events unique to meiosis occur during meiosis I: 3. Separation of homologs.

Answer 17

At anaphase I of meiosis, the
duplicated chromosomes of each homologous pair move
toward opposite poles, but the sister chromatids of each
duplicated chromosome remain attached. In anaphase
of mitosis, by contrast, sister chromatids separate.

Question 18

explain the separtion of sister chromatids in mitosis

Answer 18

. In mitosis, this
attachment lasts until the end of metaphase, when enzymes
cleave the cohesins, freeing the sister chromatids to move
to opposite poles of the cell

Question 19

explain the separtion of sister chromatids in meiosis. when does it occur and how?

Answer 19

In meiosis, sister chromatid
cohesion is released in two steps, one at the start of anaphase I and one at anaphase II. In metaphase I, the two
homologs of each pair are held together because there is still
cohesion between sister chromatid arms in regions beyond
points of crossing over, where stretches of sister chromatids
now belong to different chromosomes. The combination
of crossing over and sister chromatid cohesion along the
arms results in the formation of a chiasma. Chiasmata hold
homologs together as the spindle forms for the first meiotic
division. At the onset of anaphase I, the release of cohesion along sister chromatid arms allows homologs to separate.
At anaphase II, the release of sister chromatid cohesion at
the centromeres allows the sister chromatids to separate.
Thus, sister chromatid cohesion and crossing over, acting
together, play an essential role in the lining up of chromosomes by homologous pairs at metaphase I.

Question 20

omosomes during meiosis and fertilization is responsible for most of the

Answer 20

variation that arises in each generation. Three mechanisms contribute to the genetic variation arising from sexual
reproduction: independent assortment of chromosomes,
crossing over, and random fertilization.

Question 21

independent assortment

Answer 21

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.

Question 22

recombinant chromosomes

Answer 22

. Figure 13.11 suggests that each chromosome in a gamete is exclusively maternal
or paternal in origin. In fact, this is not the case, because crossing over produces recombinant chromosomes, individual
chromosomes that carry genes (DNA) from two different parents (Figure 13.12). In meiosis in humans, an average of one
to three crossover events occurs per chromosome pair, depending on the size of the chromosomes and the position of their
centromeres.