Chapter 20, Sexual Reproduction, Meiosis, and Genetic Recombination Video Solutions, Becker's World of the Cell

Problem 1

The Truth About Sex. For each of the following statements, indicate with an $\mathrm{S}$ if it is true of sexual reproduction, with an $\mathrm{A}$ if it is true of asexual reproduction, with a $\mathrm{B}$ if it is true of both, or with an $\mathrm{N}$ if it is true of neither.
(a) Traits from two different parents can be combined in a single offspring.
(b) Each generation of offspring is virtually identical to the previous generation.
(c) Mutations are propagated to the next generation.
(d) Some offspring in every generation will be less suited for survival than the parents, but others may be better suited.
(e) Mitosis is involved in the life cycle.

Anitha Mary

Numerade Educator

10:46

Problem 2

Ordering the Phases of Meiosis. Drawings of several phases of meiosis in an organism, labeled A through $\mathrm{F}$, are shown in Figure $20-33$
FIGURE $20-33$ Six Phases of Meiosis to Be Ordered and Identified. See Problem $20-2$
(a) What is the diploid chromosome number in this species?
(b) Place the six phases in chronological order, and name each one.
(c) Between which two phases do homologous centromeres separate?
(d) Between which two phases does recombination occur?

Nicole Hewett

Numerade Educator

01:52

Problem 3

Telling Them Apart. Briefly describe how you might distinguish between each of the following pairs of phases in the same organism:
(a) Metaphase of mitosis and metaphase I of meiosis.
(b) Metaphase of mitosis and metaphase II of meiosis.
(c) Metaphase I and metaphase II of meiosis.
(d) Telophase of mitosis and telophase II of meiosis.
(e) Pachytene and diplotene stages of meiotic prophase I.

Rikhil Makwana

Numerade Educator

02:22

Problem 4

Your Centromere Is Showing. Suppose you have a diploid organism in which all the chromosomes contributed by the sperm have cytological markers on their centromeres that allow you to distinguish them visually from the chromosomes contributed by the egg.
(a) Would you expect all the somatic cells (cells other than gametes to have equal numbers of maternal and paternal centromeres in this organism? Explain.
(b) Would you expect equal numbers of maternal and paternal centromeres in each gamete produced by that individual? Explain.

Dennis Howard

Numerade Educator

02:26

Problem 5

How Much DNA? Let $X$ be the amount of DNA present in the gamete of an organism that has a diploid chromosome number of $4 .$ Assuming all chromosomes to be of approximately the same size, how much DNA $(X, 2 X, 1 / 2 X, \text { and so on })$ would you expect in each of the following?
(a) A zygote immediately after fertilization
(b) A single sister chromatid
(c) A daughter cell following mitosis
(d) $\mathrm{A}$ single chromosome following mitosis
(e) A nucleus in mitotic prophase
(f) The cell during metaphase II of meiosis
(g) One bivalent

John Barone

Numerade Educator

01:13

Problem 6

Meiotic Mistakes. Infants born with Patau syndrome have an extra copy of chromosome $13,$ which leads to developmental abnormalities such as cleft lip and palate, small eyes, and extra fingers and toes. Another type of genetic disorder, called Turner syndrome, results from the presence of only one sex chromosome-an X chromosome. Individuals born with one $X$ chromosome are females exhibiting few noticeable defects until puberty, when they fail to develop normal breasts and internal sexual organs. Describe the meiotic events that could lead to the birth of an individual with either Patau syndrome or Turner syndrome.

James Kiss

Numerade Educator

12:25

Problem 7

Punnett Squares as Genetic Tools. A Punnett square is a diagram representing all possible outcomes of a genetic cross. The genotypes of all possible gametes from the male and female parents are arranged along two adjacent sides of a square, and each box in the matrix is then used to represent the genotype resulting from the union of the two gametes at the heads of the intersecting rows. By the law of independent assortment, all possible combinations are equally likely, so the frequency of a given genotype among the boxes represents the frequency of that genotype among the progeny of the genetic cross represented by the Punnett square.
Figure $20-34$ shows the Punnett squares for two crosses of pea plants. The genetic characters involved are seed color
(a) One-factor cross
(b) Two-factor cross
FIGURE $20-34$ Punnett Squares. See Problem $20-7$
(where $Y$ is the allele for yellow seeds and $y$ for green seeds) and seed shape (where $R$ is the allele for round seeds and $r$ for wrinkled seeds). The Punnett square in Figure $20-34$ a represents a one-factor cross between parent plants that are both heterozygous for seed color $(Y y \times Y y) .$ The Punnett square in Figure $20-34 b$ is a two-factor cross between plants heterozygous for both seed color $(Y y)$ and seed shape $(R r)$
(a) Using the Punnett square of Figure $20-34 a$, explain the 3: 1 phenotypic ratio Mendel observed for the offspring of such a cross.
(b) Explain why the Punnett square of Figure $20-34$ b is a $4 \times 4$ matrix with 16 genotypes. In general, what is the mathematical relationship between the number of heterozygous allelic pairs being considered and the number of different kinds of gametes?
(c) How does the Punnett square of Figure $20-34$ b reflect Mendel's law of independent assortment?
(d) Complete the Punnett square of Figure $20-34$ b by writing in each of the possible progeny genotypes. How many different genotypes will be found in the progeny? In what ratios?
(e) For the case of Figure $20-34$ b, how many different phenotypes will be found in the progeny? In what ratios?

Bryan Lynn

Numerade Educator

03:15

Problem 8

Genetic Mapping. The following table provides data concerning the frequency with which four genes $(w, x, y, \text { and } z)$ located on the same chromosome recombine with each other.
(a) Construct a genetic map indicating the order in which these four genes occur and the number of map units that separate the genes from each other.
(b) In constructing this map, you may have noticed that the map distances are not exactly additive. Can you provide an explanation for this apparent discrepancy?

Danielle Ashley

Numerade Educator

03:14

Problem 9

Homologous Recombination. Bacterial cells use at least three different pathways for carrying out genetic recombination. All three pathways require the RecA protein, but in each case, a different set of steps precedes the action of RecA in catalyzing strand invasion. One of these three pathways utilizes an enzyme complex called RecBCD, which binds to double-strand breaks in DNA and exhibits both helicase and single-strand nuclease activities.
(a) Briefly describe a model showing how the RecBCD enzyme complex might set the stage for genetic recombination.
(b) When bacterial cells are co-infected with two different strains of bacteriophage 1 , genes located near certain regions of the phage DNA, called CHI sites, recombine at much higher frequencies than other genes do. However, in mutant bacteria lacking the RecBCD protein, genes located near CHI sites do not recombine any more frequently than do other genes. How can you modify your model to accommodate this additional information?

Khalida Dawar

Numerade Educator

01:03

Problem 10

Gene Cloning and Recombination. Not only are the plasmid vectors used in molecular biology engineered, but the strains of $E .$ coli used in cloning are as well.
(a) Nearly all strains of $E .$ coli used in DNA cloning carry mutations in the recA gene that result in loss of RecA activity. Why would a recA mutation make an $E .$ coli cell a better host for propagating recombinant plasmid DNA?
(b) Recall that restriction endonucleases are normally made by bacteria such as $E .$ coli (Box 18B, page 520). $E$ coli used in molecular biology also carry mutations in restriction endonucleases. Why do you think these mutations would be useful?

Sana Riaz

Numerade Educator