Cell & Molec Exam 4 Ch 19 Text Bank

19-10 Why is sexual reproduction more beneficial to a species living in an unpredictable environment than to one living in a constant environment?

19-10 The real benefit in sexual reproduction seems to be that parents produce children that are genetically unlike either parent and that are not genetically identical to each other. Sexual reproduction provides more variation in the population than asexual reproduction could provide, and is an advantage if the environment is variable, because any one combination of the parents' characteristics, however well adapted to the prevailing conditions, may or may not be the best in a new situation.

19-11 Is the following statement true or false? Explain. Somatic cells leave no progeny and thus, in an evolutionary sense, exist only to help create, sustain, and propagate the germ cells.

19-11 *True*. Only germ cells contribute genetic material to the next generation of organisms, and thus only germ cells leave an evolutionary legacy in the gene pool of the species. The only contribution of somatic cells to subsequent generations arises through the assistance they provide to dissemination of the genetic material in the germ cells. In accordance with this, mutations that arise in somatic cells are not passed along to offspring.

19-4 Somatic cells ___________________________. (a) are not necessary for sexual reproduction in all eukaryotic organisms. (b) are used to produce germ-line cells when organisms reach sexual maturity (c) leave no progeny. (d) do not contain sex chromosomes.

19-4 Choice (c) is correct. Somatic cells are used to form the rest of the animal's body, which is required to support sexual reproduction even if these cells are not used to generate the germ cells [choice (a)]. Germ-line cells are generally specified early in development, before organisms reach sexual maturity [choice (b)]. Somatic cells do contain sex chromosomes [choice (d)].

19-25 Which of the following statements about meiosis is *true*? (a) During meiosis, the paternal chromosomes pair with the maternal chromosomes before lining up at the metaphase plate. (b) Unicellular organisms that have a haploid state undergo meiosis instead of mitosis during cell division. (c) Meiosis produces four genetically identical cells. (d) In general, meiosis is faster than mitosis.

(a) During meiosis, the paternal chromosomes pair with the maternal chromosomes before lining up at the metaphase plate.

19-21 A diploid cell containing 32 chromosomes will make a haploid cell containing ___ chromosomes. (a) 8 (b) 16 (c) 30 (d) 64

(b) 16

19-3 Which of the following statements is *false*? (a) Asexual reproduction typically gives rise to offspring that are genetically identical. (b) Mutations in somatic cells are passed on to individuals of the next generation. (c) Sexual reproduction allows for a wide variety of gene combinations. (d) Gametes are specialized sex cells.

(b) Mutations in somatic cells are passed on to individuals of the next generation.

19-28 After the first meiotic cell division ________________________. (a) two haploid gametes are produced. (b) cells are produced that contain the same number of chromosomes as somatic cells. (c) the number of chromosomes will vary depending on how the paternal and maternal chromosomes align at the metaphase plate. (d) DNA replication occurs.

(b) cells are produced that contain the same number of chromosomes as somatic cells.

19-62 Finding co-inheritance of a SNP variant and a disease tells scientists that ____________________. (a) everybody who carries this SNP will get the disease. (b) sequences within the SNP cause the disease. (c) a gene important for causing the disease is linked to the SNP. (d) SNPs on other chromosomes will not be co-inherited with the disease.

(c) a gene important for causing the disease is linked to the SNP.

19-53 Loss-of-function mutations ________________________. (a) cause the production of proteins that are active in inappropriate circumstances. (b) will usually show a phenotype when heterozygous. (c) are only present in a population at barely detectable levels. (d) are usually recessive.

(d) are usually recessive.

19-1 Organisms that reproduce sexually ________________________. (a) must be haploid, unlike organisms that reproduce asexually. (b) can reproduce only with a partner that carries the same alleles. (c) create zygotes that are genetically identical to each other. (d) undergo a sexual reproductive cycle that involves an alternation of haploid cells with the generation of diploid cells.

(d) undergo a sexual reproductive cycle that involves an alternation of haploid cells with the generation of diploid cells.

19-12 Sexual reproduction is a large drain on the limited resources of an individual. Nonetheless, sexual reproduction is common. In fact, to allow sexual reproduction, organisms have evolved many elaborate anatomical structures, cellular processes, and chemical signals. For example, flowers exist entirely to further the goal of sexual reproduction, and many plants have enlisted the help of bees and birds to aid in the dissemination of their germ cells. Describe one reason why most multicellular organisms have evolved to reproduce sexually instead of relying solely on asexual reproduction.

19-12 A definitive explanation of the evolutionary advantages of sexual reproduction over asexual reproduction is elusive, but several benefits seem clear. ---1. The reshuffling of genes that occurs during sexual reproduction generates most of the diversity between organisms within a species. Having a large variety of different genetic combinations in a population may help guarantee that at least a few individuals will survive after a sudden and unpredictable change in the environment. This is why some ecologists are concerned about the global trend toward monoculture, namely limiting cultivation to only a small number of varieties of each species of plant, each variety being inbred and genetically uniform. ---2. The competition between males for the available females may increase the frequency of advantageous alleles in the gene pool while efficiently eliminating deleterious combinations of alleles. This is because, in many species, only the strongest and healthiest males succeed in mating, whereas other males, carrying less successful combinations of alleles, leave no progeny. ---3. Sexual reproduction may help to eliminate deleterious recessive genes from a population in another way also: a recessive deleterious mutation in a gene may be "unmasked" in a haploid germ cell because, in contrast with a heterozygous diploid cell, the haploid cell carrying the mutation contains no normal allele to provide the gene function. Because there is competition between germ cells for fertilizing partner cells, gametes with recessive deleterious mutations are less likely to form zygotes.

19-13 Indicate whether each of the following is true for meiosis, mitosis, both, or neither. A. formation of a bivalent B. genetically identical products C. condensation of chromosomes D. segregation of all paternal chromosomes to one cell E. involvement of DNA replication

19-13 A. formation of a bivalent - *meiosis* B. genetically identical products - *mitosis* C. condensation of chromosomes - *both* D. segregation of all paternal chromosomes to one cell - *neither* E. involvement of DNA replication - *both*

19-14 The formation of a bivalent during meiosis ensures that _______. (a) one chromatid from the mother and one chromatid from the father will segregate together during meiosis I. (b) all four sister chromatids remain together until the cell is ready to divide. (c) recombination will occur between identical sister chromatids. (d) the sex chromosomes, which are not identical, will line up separately at the metaphase plate during meiosis I.

19-14 Choice (b) is correct. Both chromatids from a single parent will segregate together during meiosis I [choice (a)]. Recombination occurs between nonidentical sister chromatids [choice (c)]. The sex chromosomes will come together to form a bivalent, despite not being identical [choice (d)].

19-15 Imagine meiosis in a diploid organism that only has a single chromosome. Like most diploid organisms, it received one copy of this chromosome from each of its parents and the two homologs are genetically distinct. If only a single homologous recombination event occurs during meiosis, which of the following choices below correctly describes the four gametes formed. (a) None of the gametes will contain chromosomes identical to the chromosomes found in the original diploid cell. (b) All four of the gametes will have chromosomes identical to the chromosomes found in the original diploid cell. (c) Three of the gametes will have chromosomes identical to the chromosomes found in the original diploid cell, while one of the gametes will have chromosomes that are different. (d) Two of the gametes will have chromosomes identical to the chromosomes found in the original diploid cell, while two of the gametes will have chromosomes that are different.

19-15 (d) A single recombination event will lead to an exchange of genetic information between two of the chromatids—one from each parent

19-16 There are organisms that go through meiosis but do not undergo recombination when forming haploid gametes. Which of the following statements correctly describes the gametes produced by such an organism. (Assume that these organisms are diploid, that each of the two homologous chromosomes are genetically distinct as typically found in the wild, and that these organisms have more than one chromosome.) (a) All gametes formed during a single meiosis will be identical. (b) Due to the random assortment of homologs, each of the gametes formed during a single meiosis will be different. (c) This organism could potentially produce 2n genetically distinct gametes, where n is its haploid number of chromosomes. (d) The fusion of any two gametes produced by such an organism that does not undergo recombination during meiosis will create a cell that is genetically identical to that individual.

19-16 Choice (c) is correct. Although it is true that the homologs will randomly assort in meiosis I [choice (b)], this will only create two different types of genetic combinations in a single meiosis. Thus, four gametes will be produced, and the two products of meiosis II meiosis will be genetically identical to each other. Because the two homologous chromosomes are genetically distinct, the four gametes formed during meiosis cannot be identical [choice (a)]. Because the two homologs are genetically distinct and segregate randomly in meiosis I, the fusion of any two gametes produced by an individual will lead to unique combinations of maternal and paternal chromosomes, and thus the resulting cell will not be genetically identical to the individual [choice (d)].

19-17 Meiosis is a highly specialized cell division in which several events occur in a precisely defined order. Please order the meiotic events listed below. 1. loss of cohesins near centromeres 2. chromatid pairing 3. chromosome condensation 4. chromosome replication 5. degradation of cohesins bound to chromosome arms 6. formation of chiasmata (chiasmata = plural of chiasma) 7. homolog pairing 8. alignment of chromosomes at the metaphase plate

19-17 4, 2, 7, 6, 3, 8, 5, 1. *chromosome replication *chromatid pairing *homolog pairing *formation of chiasmata (chiasmata = plural of chiasma) *chromosome condensation *alignment of chromosomes at the metaphase plate *degradation of cohesins bound to chromosome arms *loss of cohesins near centromeres Chromosome replication (4) occurs during meiotic S phase, and immediately after replication the resultant sister chromatids are paired tightly by cohesins (2). During meiotic G2, or prophase, the two homologous chromosomes pair (7) and undergo recombination to produce reciprocal exchanges of genetic material that are visible as chiasmata (6). Meiotic division I begins with chromosome condensation (3) and proceeds to metaphase, when chromosomes align at a central plate (8). Anaphase of meiotic division I is triggered by loss of the cohesin glue on the chromosome arms (5), which allows the homologs to be segregated. Anaphase of meiotic division II is triggered by degradation of cohesins near the centromere (1), which allows sister chromatids to be segregated.

19-18 For each of the following sentences, choose one of the options enclosed in square brackets to make a correct statement. Starting with a single diploid cell, mitosis produces *[two/four]* *[identical/different]* *[haploid/diploid]* cells, whereas meiosis yields *[two/four]* *[identical/different]* *[haploid/diploid]* cells. This is accomplished in meiosis because a single round of chromosome *[replication/segregation]* is followed by two sequential rounds of *[replication/segregation]*. Mitosis is more like meiosis *[I/II]* than meiosis *[I/II]*. In meiosis I, the kinetochores on sister chromatids behave *[independently/coordinately]* and thus attach to microtubules from the *[same/opposite]* spindle. The cohesin-mediated glue between *[chromatids/homologs]* is regulated differently near the centromeres than along the chromosome arms. Cohesion is lost first at the *[centromeres/arms]* to allow segregation of *[chromatids/homologs]* and is lost later at the *[centromeres/arms]* to trigger segregation of *[chromatids/homologs]*.

19-18 Starting with a single diploid cell, mitosis produces *two* *identical* *diploid* cells, whereas meiosis yields *four* *different* *haploid* cells. This is accomplished in meiosis because a single round of chromosome *replication* is followed by two sequential rounds of *segregation*. Mitosis is more like meiosis *II* than meiosis *I*. In meiosis I, the kinetochores on sister chromatids behave *coordinately* and thus attach to microtubules from the *same* spindle. The cohesin-mediated glue between *chromatids* is regulated differently near the centromeres than along the chromosome *arms*. Cohesion is lost first at the arms to allow segregation of *homologs* and is lost later at the *centromeres* to trigger segregation of *chromatids*.

19-19 In the absence of recombination, how many genetically different types of gamete can an organism with five homologous chromosome pairs produce? (a) 5 (b) 10 (c) 32 (d) 64

19-19 (c) Because homologous chromosomes assort randomly at meiosis and a gamete has two choices for each chromosome (because sexual organisms are diploid), there are 25, or 32, possible genetically different gametes.

19-2 Which of the following statements is *true*? (a) Another name for the fertilized egg cell is the zygote. (b) Diploid organisms reproduce only sexually. (c) All sexually reproducing organisms must have two copies of every chromosome. (d) Gametes have only one chromosome.

19-2 Choice (a) is correct. Some diploid organisms (for example, many plants) are capable of asexual reproduction [choice (b)]. Many organisms have sex chromosomes that are only present in one copy in the diploid organism [choice (c)]. Gametes have only one member of each pair of homologous chromosomes, but because most organisms have more than one pair of homologous chromosomes, most gametes have more than one chromosome [choice (d)].

19-20 Which of the following statements most correctly describes meiosis? (a) Meiosis involves two rounds of DNA replication followed by a single cell division. (b) Meiosis involves a single round of DNA replication followed by four successive cell divisions. (c) Meiosis involves four rounds of DNA replication followed by two successive cell divisions. (d) Meiosis involves a single round of DNA replication followed by two successive cell divisions.

19-20 (d) Meiosis involves a single round of DNA replication followed by two successive cell divisions.

19-22 Imagine a diploid sexually reproducing organism, Diploidus sexualis, that contains three pairs of chromosomes. This organism is unusual in that no recombination between homologous chromosomes occurs during meiosis. A. Assuming that the chromosomes are distributed independently during meiosis, how many different types of sperm or egg cells can a single individual of this species produce? B. What is the likelihood that two siblings of this species will be genetically identical? You can assume that the homologous chromosomes of each parent are different from one another and from their counterparts in the other parent.

19-22 A. Eight different types. With respect to each of the three chromosomes, an individual can produce two kinds of gamete. The gamete can receive the copy that the individual received from "mom" or the copy from "dad." Thus, with three chromosomes, there are 2 × 2 × 2 = 8 possible gametes. B. 1/64. The mother and father together can produce 8 × 8 = 64 different genetic combinations.

19-23 You have received exactly half of your genetic material from your mother, who received exactly half of her genetic material from her mother (your grandmother). A. Explain why it is unlikely that you share exactly one-quarter of your genetic material with your grandmother, and instead it is more accurate to say that in general people receive an average of one-quarter of their genetic endowment from each grandparent. B. Consider a gene on Chromosome 3 that you received from your grandmother. Is it likely you received an entire Chromosome 3 from your grandmother? Why or why not? C. What portion of your genetic material do you share with your sibling? Your aunt? Your cousin?

19-23 A. It is unlikely that you share exactly one-quarter of your genetic material with your grandmother, but it is true that organisms receive an average of one-quarter of their genes from each grandparent. When cells in your mother's germ line were undergoing meiosis, the chromosomes that she received from your grandmother and your grandfather were shuffled by recombination and then randomly assorted into the gametes. Each of these gametes probably received slightly less or slightly more than half of its genetic material from your grandmother. Thus, fusion of your mother's gamete with your father's created a zygote that shared approximately, but not exactly, one-quarter of its genes with your grandmother; this zygote divided repeatedly to form all the cells in your body. B. It is unlikely that you received the entire Chromosome 3 from your grandmother, because each chromosome undergoes at least one recombinational crossover with its homolog to ensure proper chromosome segregation in meiotic division I. C. You share an average of one-half of your genetic material with your sibling, one- quarter with your aunt, and one-eighth with your cousin. Because you and your sibling each get half of each parent's DNA, you have approximately half of your genetic material in common with your sibling. As your mother and her sister share approximately half of their genetic material, you and your aunt share 1/2 × 1/2 = 1/4. As your cousin has half of your aunt's genetic material, you share 1/2 × 1/4 = 1/8.

19-24 You examine a worm that has two genders: males that produce sperm and hermaphrodites that produce both sperm and eggs. The diploid adult has four homologous pairs of chromosomes that undergo very little recombination. Given a choice, the hermaphrodites prefer to mate with males, but just to annoy the worm, you pluck a hermaphrodite out of the wild and fertilize its eggs with its own sperm. Assuming that all the resulting offspring are viable, what fraction do you expect to be genetically identical to the parent worm? Assume that each chromosome in the original hermaphrodite is genetically distinct from its homolog. (a) all (b) none (c) 1/16 (d) 1/256

19-24 (c) 1/16 Because each chromosome is genetically distinct from its homolog, the parent is heterozygous for each chromosome and thus can produce 24 = 16 types of egg and 24 = 16 types of sperm. Any of the eggs produced will be able to give rise to an adult that is identical to the parent, but to do so it must be fertilized by the right type of sperm. For each type of egg, only one of the 16 possible sperm will produce a diploid that is identical to the parent. Therefore, one out of 16 of the offspring should be identical to the parent. In other words, a sexually reproducing organism with several heterozygous chromosomes has a relatively high probability of producing genetically distinct offspring even when the parent mates to itself.

19-27 During recombination ________________________. (a) sister chromatids undergo crossing-over with each other. (b) chiasmata hold chromosomes together. (c) one crossover event occurs for each pair of human chromosomes. (d) the synaptonemal complex keeps the sister chromatids together until anaphase II.

19-27 Choice (b) is correct. Non-sister chromatids undergo crossing-over; because sister chromatids are identical, there would be no exchange of genetic material if sister chromatids underwent recombination [choice (a)]. The number of crossover events can vary for each meiosis; on average, two to three crossover events occur between each pair of human chromosomes [choice (c)]. Although the sister chromatids do not separate until anaphase II, their arms become unglued because the cohesins holding them together are degraded during anaphase I. The synaptonemal complex is important for holding together and aligning the duplicated homologs and is not involved directly in sister-chromatid cohesion [choice (d)].

19-29 Meiosis includes a recombination checkpoint that is analogous to the checkpoints in cell-cycle progression. Double-strand breaks in the DNA initiate recombination in meiosis. The broken end of a DNA molecule finds the corresponding sequence on a homologous chromosome and exchanges a chromosomal segment with its homolog, thereby repairing the break. Ongoing recombination sends a negative regulatory signal that prevents cells from entering meiotic division I. A. Mutations in several genes inactivate the recombination checkpoint. What do you predict will happen if a cell proceeds through meiotic division I before completing recombination? B. What will happen if a cell fails to initiate recombination and proceeds through meiotic division I? Meiotic division II?

19-29 A. If a cell proceeds through meiotic division I when its chromosomes are broken and incompletely repaired, segregation will be disastrous. Some recombination intermediates will be pulled to opposite spindle poles, thus breaking the DNA. Other chromosome fragments lacking centromeres will not be attached to microtubules and thus will segregate randomly, causing some meiotic products to have too little DNA and others to have too much. B. In the absence of recombination, the homologs will not segregate from each other properly in meiotic division I but the sister chromatids will segregate normally in meiotic division II. In meiotic division I, the unrecombined homologous chromosomes will not be held together by chiasmata. Thus, the homologous chromosomes will line up independently on the metaphase plate and segregate randomly, causing chromosome nondisjunction. Some products will have both homologs of a given chromosome and others will have none. In meiotic division II, the events will proceed normally and sister chromatids will be properly segregated to opposite poles. Nonetheless, because the chromosome sets at the start of meiotic division II were distributed unevenly, the gametes produced after meiotic division II will be aneuploid (that is, they will have an incorrect number of chromosomes).

19-32 Which of the following would *not* lead to aneuploidy during meiosis? (a) sister chromatids segregating inappropriately (b) non-sister chromatids segregating inappropriately (c) a reciprocal rearrangement of parts between nonhomologous chromosomes (for example, the left arm of Chromosome 2 exchanging places with the right arm of Chromosome 3) (d) an extra set of chromosomes produced during S phase (for example, if paternal Chromosome 3 were replicated twice)

19-32 (c) Aneuploidy describes cells with an incorrect chromosome number. A reciprocal rearrangement of parts between nonhomologous chromosomes will not lead to aneuploidy—it will simply change the assortment of genes attached to each centromere, while leaving the total amount of DNA and the total number of chromosomes unchanged.

19-33 A single nondisjunction event during meiosis ___________________. (a) will block recombination. (b) will occur only during meiosis II. (c) cannot occur with sex chromosomes. (d) will involve the production of two normal gametes if it occurs during meiosis II.

19-33 (d) A single nondisjunction event will lead to the segregation of one homologous pair of chromosomes, resulting in two aneuploid gametes and two normal gametes.

19-34 When a reciprocal translocation occurs, part of one chromosome is exchanged with a part of another chromosome. For example, one half of Chromosome 3 may now be found fused to Chromosome 10, and part of Chromosome 10 is now found fused to Chromosome 3. In a balanced reciprocal translocation, an even exchange of material occurs such that no genetic information is extra or missing. Individuals can carry balanced reciprocal translocations and be quite healthy. Consider the case where a gamete containing a balanced reciprocal translocation of a single chromosome is used to fertilize a genetically normal egg. Explain why individuals carrying a single balanced reciprocal translocation might have problems with chromosome segregation during meiosis but not in mitosis.

19-34 During meiosis, homologous chromosomes pair to form bivalents. The chromosomes with the translocations will have difficulty pairing with the homologous chromosomes of normal structure during meiosis. However, during mitosis, there is no need for the homologous chromosomes to interact.

19-35 During fertilization in humans, _______________________. (a) a wave of Ca2+ ions is released in the fertilized egg's cytoplasm. (b) only one sperm binds to the unfertilized egg. (c) a sperm moves in a random fashion until it encounters an egg. (d) several sperm pronuclei compete in the cytoplasm to fuse with the egg nucleus.

19-35 Choice (a) is correct. Many sperm can bind to an egg [choice (b)]. Cells surrounding the egg release a chemoattractant signal, attracting the sperm to the correct place; sperm are moving toward the chemoattractant signal and not moving randomly [choice (c)]. Mechanisms exist to ensure that only one sperm fertilizes each egg, and thus only one sperm pronucleus will reach the cytoplasm of the egg [choice (d)].

19-36 Do you agree or disagree with the following statement? Explain your answer. If a diploid organism has 16 chromosomes (and thus 8 pairs of homologous chromosomes), that organism can produce only 28 genetically different gametes.

19-36 Disagree. There are two distinct mechanisms for genetic variation in gametes. One method, the reassortment of chromosomes during meiosis, would result in a diploid organism with 16 chromosomes producing 28 genetically different gametes. However, a much greater number of genetically different gametes can be produced as a result of the recombination between chromosomes that occurs during every meiosis.

19-37 Which of the following statements about Mendel's experiments is *false*? (a) The pea plants could undergo both cross-fertilization and self-fertilization. (b) The true-breeding strains were homozygous for the traits that Mendel examined. (c) The egg can carry either the allele from the maternal or the paternal chromosome. (d) All traits that Mendel studied were recessive.

19-37 (d) The traits that Mendel studied were inherited in a discrete fashion. For this to occur, for each pair of alleles, one allele is necessarily dominant and the other is recessive.

19-43 Which of the following reasons was essential for Mendel to disprove the theory of blended inheritance? (a) The traits that Mendel examined all involved genes that did not display linkage. (b) The traits that Mendel examined all involved the reproductive structures of the pea plant. (c) Mendel pioneered techniques permitting the fusion of male and female gametes from the same plant to produce a zygote. (d) The traits that Mendel examined involved an allele that was dominant and an allele that was recessive.

19-43 (d) To see the traits disappear in the F1 generation and reappear in the F2 generation, one of the alleles for each trait needed to be dominant while the other was recessive. Although choices (a) and (b) are true, they were not necessary for Mendel to disprove the theory of blended inheritance. Choice (c) is untrue; the plants that Mendel worked with had both male and female reproductive structures and could self-fertilize naturally.

19-44 Which of the following reasons was essential for Mendel's law of independent assortment? (a) All the traits that Mendel examined involved genes that did not display linkage. (b) Several of the phenotypes that Mendel examined involved color. (c) Mendel observed chromosomal segregation in pea-plant cells. (d) Mendel carried out his experiments on plants and not on fungi.

19-44 (a) To see the 9:3:3:1 segregation from a dihybrid cross, the two traits cannot be linked. Although choice (b) is true, it was not necessary for Mendel to determine the law of independent assortment. Mendel did not observe chromosomal segregation [choice (c)]. The law of independent assortment holds true in animal, fungal, and plant cells.

19-45 Is the following statement true or false? Explain. The phenotype of an organism reflects all of the alleles carried by that individual.

19-45 False. The phenotype, or the observable traits, of an organism often does not fully reflect its genotype, or the catalog of all alleles in the chromosomes. The reason is that some alleles are dominant (call these A) and other alleles are recessive (a). An individual who is heterozygous (Aa) for a dominant allele will have the same phenotype as one who is homozygous (AA).

19-46 Gregor Mendel studied pea plants and developed some very important ideas about how genes are inherited. These studies used plant strains that were true breeding and always produced progeny that had the same __________________ as the parent. These strains were true breeding because they were __________________ for the gene important for a specific trait. In other words, for these true-breeding strains, both chromosomes in the diploid pea plant carried the same __________________ of the gene. Mendel started out examining the inheritance of a single trait at a time, and then moved on to examining two traits at once in a __________________ cross. His studies examining the inheritance of two traits in one cross allowed him to discover what is now known as Mendel's law of __________________ assortment. Geneticists can study the inheritance of specific traits in humans by analyzing a __________________, which shows the phenotypes of different family members over several generations for a particular trait. allele chromosome dependent dihybrid genotype heterozygous homozygous independent monohybrid pedigree phenotype segregation

19-48 Gregor Mendel studied pea plants and developed some very important ideas about how genes are inherited. These studies used plant strains that were true breeding and always produced progeny that had the same *phenotype* as the parent. These strains were true breeding because they were *homozygous* for the gene important for a specific trait. In other words, for these true-breeding strains, both chromosomes in the diploid pea plant carried the same *allele* of the gene. Mendel started out examining the inheritance of a single trait at a time, and then moved on to examining two traits at once in a *dihybrid* cross. His studies examining the inheritance of two traits in one cross allowed him to discover what is now known as Mendel's law of *independent* assortment. Geneticists can study the inheritance of specific traits in humans by analyzing a *pedigree*, which shows the phenotypes of different family members over several generations for a particular trait.

19-49 Cystic fibrosis results from mutations in a single gene that lies on Chromosome 7. Only homozygous mutant (ff) individuals are sick; homozygous wild-type (FF) and heterozygous (Ff) individuals are healthy. A healthy married couple has one child with cystic fibrosis and the wife is pregnant with a second child. A. What is the genotype of the mother? The father? B. What is the chance that the second child will have cystic fibrosis?

19-49 A. The genotypes of the mother and father are the same: Ff. The only way that a child can have the disease is if both parents are carriers of the mutant cystic fibrosis gene. B. The chance that the second child will have cystic fibrosis is one-quarter. The chance that the mother will transmit her mutant f allele to the offspring is one- half, multiplied by an equal chance that the father will transmit his mutant f allele to the offspring: 1/2 × 1/2 = 1/4. (Having one child with cystic fibrosis does not change the probability of having another child with the disease.)

19-5 Which of the following statements about the benefits of sexual reproduction is false? (a) Sexual reproduction permits enhanced survival because the gametes that carry alleles enhancing survival in harsh environments are used preferentially during fertilization. (b) Unicellular organisms that can undergo sexual reproduction have an increased ability to adapt to harsh environments. (c) Sexual reproduction reshuffles genes, which is thought to help species survive in novel or varying environments. (d) Sexual reproduction can speed the elimination of deleterious alleles.

19-5 (a) Alleles enhancing survival in harsh environments will not be selected for until the organism encounters the harsh environment. - *FALSE*

19-50 Sickle-cell anemia is caused by a mutant allele of a hemoglobin gene. Individuals with two mutant alleles have sickle-cell anemia. Individuals homozygous and heterozygous for the mutant gene are more resistant to malaria than those with two wild-type alleles. Is this mutation dominant, recessive, or co-dominant?

19-50 The classification of all mutations depends on the phenotypic feature under consideration. With regard to the sickle-cell anemia phenotype, the mutant allele is recessive because a heterozygous individual has the same healthy phenotype as a homozygous wild-type individual. With regard to the malaria phenotype, the mutant allele is dominant because a heterozygous individual has the same phenotype as a homozygous mutant individual, namely resistance to malaria. With regard to the phenotype as a whole, the two alleles could be said to be co-dominant.

19-51 You are given two true-breeding strains of hamster. One strain has white fur color and the other has a dark brown fur color. When you cross the white-fur strain to the dark-brown-fur strain, you obtain F1 progeny that have a light brown fur color. When you cross the F1 progeny with each other, 25% of the F2 generation have white fur, 25% have dark brown fur, and 50% have light brown fur. How many genes crucial for fur coloration differ between the two starting strains? Explain your answer.

19-51 One gene involved in determining fur color differs between these two strains. The white allele is not fully recessive to the dark brown allele. Therefore, any animal that is heterozygous (and has one white allele and one dark brown allele) will have a light brown fur color. Thus, the white allele and the dark brown allele are co-dominant. When two heterozygous animals are crossed together, 25% of the offspring will be homozygous for one allele, 25% will be homozygous for the other allele, and 50% will be heterozygous, exactly what is observed in the cross you performed.

19-52 Gene A is located near gene B on Chromosome 13 in humans. A mutation in the germ line of an individual with the haplotype AB generates gametes with the genotype Ab. Many descendants of this founder individual carry the b mutation, which predisposes carriers to high blood pressure. Initially, all descendants who inherit the b mutation also inherit the neighboring A allele. Through the generations, fewer and fewer descendants with the b mutation carry the A allele, and instead they have the a allele. (Individuals with A and a are equally healthy and fit.) Explain how the b and A alleles are separated.

19-52 Eventually, the b mutation will be separated from the A allele by meiotic recombination. Meiotic recombination exchanges portions of homologous chromosomes, and thereby generates great diversity among the gametes of each individual. The locations of the one to five exchanges per chromosome during meiosis in humans are more or less random. Thus, each passing generation increases the cumulative likelihood of a recombinational crossover between any two neighboring genes (A and b). Such a recombinational crossover in an individual heterozygous for both genes (Ab and aB) will separate two alleles that were originally linked (yielding AB and ab).

19-56 Do you agree or disagree with the following statement? Explain your answer. A trait that is found at a low frequency in the population has to be a recessive trait.

19-56 Disagree. The frequency of a trait in a population has nothing to do with its dominance or recessiveness. To test dominance or recessiveness, the segregation of this trait must be observed. For example, the defective version of the gene involved in Huntington's disease occurs at a relatively low frequency in the population but behaves in a dominant fashion to cause disease.

19-58 Haplotype blocks can be seen in humans because _______. (a) disease genes are found in haplotype blocks. (b) modern humans descended from a relatively small population of about 10,000 individuals that existed about two thousand generations ago. (c) some of our human ancestors interbred with Neanderthals. (d) new mutations cannot be introduced into existing haplotype blocks.

19-58 (b) The relatively small population of human ancestors means that the chromosomes in modern-day humans are a shuffled set of a relatively small number of chromosome sets. Because this small population existed only about two thousand generations ago, there hasn't been enough time for recombination to scramble these haplotype blocks. Disease genes can certainly be found in haplotype blocks [choice (a)], although this is not why haplotype blocks exist. Although genome analysis does show that some ancestors of modern humans did interbreed with Neanderthals, this is not the main reason for the existence of haplotype blocks [choice (c)]. New mutations can be introduced into an existing haplotype block, so choice (d) is untrue.

19-6 To reproduce sexually, an organism must create haploid __________________ cells, or __________________, from diploid cells via a specialized cell division called __________________. During mating, the father's haploid cells, called __________________ in animals, fuse with the mother's haploid cells, called __________________. Cell fusion produces a diploid cell called a __________________, which undergoes many rounds of cell division to create the entire body of the new individual. The cells produced from the initial fusion event include __________________ cells that form most of the tissues of the body as well as the __________________-line cells that give rise to the next generation of progeny. allele bivalent eggs gametes germ pollen meiosis somatic mitosis sperm pedigree zygote

19-6 To reproduce sexually, an organism must create haploid *germ* cells, or *gametes*, from diploid cells via a specialized cell division called *meiosis*. During mating, the father's haploid cells, called *sperm* in animals, fuse with the mother's haploid cells, called *eggs*. Cell fusion produces a diploid cell called a *zygote*, which undergoes many rounds of cell division to create the entire body of the new individual. The cells produced from the initial fusion event include *somatic* cells that form most of the tissues of the body as well as the *germ*-line cells that give rise to the next generation of progeny.

19-60 The single-nucleotide polymorphisms found in the human population __________________. (a) are important for genetic mapping because they represent mutations in genes important for human disease. (b) are rarely found among blood relatives. (c) can be linked into haplotype blocks. (d) arose mainly during the past 10,000 years.

19-60 (c) Haplotype blocks are segments of chromosomes spanning a set of single-nucleotide polymorphisms that are linked and tend to be inherited as a unit.

19-61 Which of the following statements about genome-wide association studies (GWAS) is *false*? (a) GWAS use SNPs to compare populations of people with disease and people without disease to look for SNPs more likely to be present in those with disease. (b) GWAS can be used even if more than one gene can cause the disease of interest. (c) Sometimes GWAS will identify SNPs that are associated with a disease but these SNPs do not affect the gene product of the gene that causes the disease. (d) Studies using GWAS only examine SNPs that occur very rarely (<0.001%) in the population, as those SNPs are most likely to cause disease.

19-61 (d) GWAS are best at detecting SNPs that are common in the human population. But because they are so common, these variants are likely to alter disease susceptibility slightly, as otherwise they would have been strongly selected against in the population during evolution (and therefore rare). Significant differences in the inheritance of a very rare SNP will be difficult to determine in a GWAS unless the number of people used in the study is extremely large.

19-64 Any two human beings typically have an estimated 0.1% difference in their nucleotide sequences, which is equivalent to about 3 million nucleotide differences. These differences are the basis of the SNPs used to construct genetic linkage maps. Some of these SNPs actually lie in the region of the DNA that codes for the protein, yet they have no effect on the phenotype of individuals carrying the SNP on both homologous chromosomes. Explain how some SNPs can lie within the portion of the DNA that codes for the protein and yet have no discernible effect on the protein's activity.

19-64 Because of the redundancy of the genetic code, in which more than one codon can code for the same amino acid, some single nucleotide changes may not cause changes in the amino acid coded for by that codon. Furthermore, some amino acid substitutions are neutral (for example, the substitution of a small, uncharged amino acid for another amino acid with similar properties)—that is, they have no perceptible effect on the function of the protein.

19-65 You decide to carry out genetic association studies and identify a SNP variant that is found significantly more often in individuals who have schizophrenia than in those who are not affected. This SNP is found within an intron of the SZP gene. A. Can you deduce that an abnormality of the SZP gene is a cause of increased risk of schizophrenia? B. Can you say whether the SNP variant itself is a cause?

19-65 A. No, you cannot be sure of this. The SZP gene would be a prime suspect, but the abnormality causing the heightened risk of schizophrenia might well lie instead in some other nearby gene. B. No, you cannot say for certain. Most point mutations in introns have no functional effect, but some can be functionally important. For example, the intronic SNP might alter an enhancer element that is lying within the intron and is involved in the regulation of SZP transcription; or it might affect the splicing of the SZP gene transcripts.

19-7 During sexual reproduction, novel mixtures of alleles are generated. This is because ______. (a) in all diploid species, two alleles exist for every gene. (b) a diploid individual has two different alleles for every gene. (c) every gamete produced by a diploid individual has several different alleles of a single gene. (d) during meiosis, the segregation of homologs is random such that different gametes end up with different alleles of each gene.

19-7 Choice (d) is correct. Many alleles can exist for any gene [choice (a)], and a diploid individual can have either two different or two identical alleles of any given gene [choice (b)]. Every gamete only has a single allele of any gene [choice (c)].

19-8 Which of the following does *not* describe a situation of asexual reproduction? (a) A bacterium multiplying by simple cell division. (b) Using a part of a plant to create a new independent plant. (c) Using in vitro fertilization to combine a sperm and an egg to create an embryo. (d) The parthenogenetic development of eggs produced by some species of lizards.

19-8 (c) Asexual reproduction gives rise to offspring that are genetically identical to the parent. The in vitro fertilization process requires gametes (sperm and egg), which when combined, will produce an offspring that is genetically distinct from either parent.

19-9 Both budding yeast and the bacteria E. coli are unicellular life. Which of the following statements explains why budding yeast can undergo sexual reproduction while E. coli cannot. (a) Unlike E. coli, budding yeast can alternate between a diploid state and a haploid state. (b) Unlike E. coli, budding yeast cannot multiply by undergoing cell division. (c) Unlike E. coli, haploid budding yeast cells can undergo meiosis to produce the gametes necessary for sexual reproduction. (d) E. coli DNA is unable to undergo homologous recombination, making it incapable of producing gametes.

19-9 Choice (a) is correct. The budding yeast can also multiply by undergoing cell division [choice (b)], although the diploid state can either reproduce by cell division or undergo meiosis to produce haploid gametes that can either fuse with another haploid gamete to form a diploid cell, or can become a free-living haploid cell that multiplies by cell division. A budding yeast cell must be diploid to undergo meiosis; haploid cells cannot undergo meiosis [choice (c)]. E. coli can undergo homologous recombination and uses this process for DNA repair [choice (d)]. In fact, E. coli can shuffle their genomes through a process of conjugation, where DNA is transferred from one bacterium to another and may get incorporated in the E. coli genetic material by recombination. Although this conjugation process can lead to the transfer of genetic material, it is not typically considered sexual reproduction.