8e Biology, (Campbell) Chapter 15 The Chromosomal Basis of Inheritance Multiple-Choice Questions 1) Why did the improvement of microscopy techniques in the late 1800s set the stage for the emergence of modern genetics? A) It revealed new and unanticipated features of Mendel's pea plant varieties. B) It allowed the study of meiosis and mitosis, revealing parallels between behaviors of genes and chromosomes. C) It allowed scientists to see the DNA present within chromosomes. D) It led to the discovery of mitochondria. E) It showed genes functioning to direct the formation of enzymes. Answer: B Topic: Concept 15.1 Skill: Knowledge/Comprehension 2) When Thomas Hunt Morgan crossed his red-eyed F1 generation flies to each other, the F2 generation included both red- and white-eyed flies. Remarkably, all the white-eyed flies were male. What was the explanation for this result? A) The gene involved is on the X chromosome. B) The gene involved is on the Y chromosome. C) The gene involved is on an autosome. D) Other male-specific factors influence eye color in flies. E) Other female-specific factors influence eye color in flies. Answer: A Topic: Concept 15.1 Skill: Knowledge/Comprehension 3) Morgan and his colleagues worked out a set of symbols to represent fly genotypes. Which of the following are representative? A) AaBb AaBb B) 46, XY or 46, XX C) vg+vgse+se vgvgsese D) +2 +3 Answer: C Topic: Concept 15.1 Skill: Knowledge/Comprehension 4) Sturtevant provided genetic evidence for the existence of four pairs of chromosomes in Drosophila in which of these ways? A) There are four major functional classes of genes in Drosophila. B) Drosophila genes cluster into four distinct groups of linked genes.
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C) The overall number of genes in Drosophila is a multiple of four. D) The entire Drosophila genome has approximately 400 map units. E) Drosophila genes have, on average, four different alleles. Answer: B Topic: Concept 15.1 Skill: Knowledge/Comprehension 5) A man with Klinefelter syndrome (47, XXY) is expected to have any of the following EXCEPT A) lower sperm count. B) possible breast enlargement. C) increased testosterone. D) long limbs. E) female body characteristics. Answer: C Topic: Concept 15.2 Skill: Knowledge/Comprehension 6) A woman is found to have 47 chromosomes, including 3 X chromosomes. Which of the following describes her expected phenotype? A) Masculine characteristics such as facial hair B) Enlarged genital structures C) Excessive emotional instability D) Normal female E) Sterile female Answer: D Topic: Concept 15.2 Skill: Application/Analysis 7) Males are more often affected by sex-linked traits than females because A) males are hemizygous for the X chromosome. B) male hormones such as testosterone often alter the effects of mutations on the X chromosome. C) female hormones such as estrogen often compensate for the effects of mutations on the X. D) X chromosomes in males generally have more mutations than X chromosomes in females. E) mutations on the Y chromosome often worsen the effects of X-linked mutations. Answer: A Topic: Concept 15.2 Skill: Knowledge/Comprehension 8) What is the chromosomal system for determining sex in mammals? A) Haploid-diploid B) X-0 C) X-X D) X-Y
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E) Z-W Answer: D Topic: Concept 15.2 Skill: Knowledge/Comprehension 9) What is the chromosomal system for sex determination in birds? A) Haploid-diploid B) X-0 C) X-X D) X-Y E) Z-W Answer: E Topic: Concept 15.2 Skill: Knowledge/Comprehension 10) What is the chromosomal system of sex determination in most species of ants and bees? A) Haploid-diploid B) X-0 C) X-X D) X-Y E) Z-W Answer: A Topic: Concept 15.2 Skill: Knowledge/Comprehension 11) SRY is best described in which of the following ways? A) A gene region present on the Y chromosome that triggers male development B) A gene present on the X chromosome that triggers female development C) An autosomal gene that is required for the expression of genes on the Y chromosome D) An autosomal gene that is required for the expression of genes on the X chromosome E) Required for development, and males or females lacking the gene do not survive past early childhood Answer: A Topic: Concept 15.2 Skill: Knowledge/Comprehension 12) In cats, black fur color is caused by an X-linked allele; the other allele at this locus causes orange color. The heterozygote is tortoiseshell. What kinds of offspring would you expect from the cross of a black female and an orange male? A) Tortoiseshell females; tortoiseshell males B) Black females; orange males C) Orange females; orange males D) Tortoiseshell females; black males E) Orange females; black males
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Answer: D Topic: Concept 15.2 Skill: Application/Analysis 13) Red-green color blindness is a sex-linked recessive trait in humans. Two people with normal color vision have a color-blind son. What are the genotypes of the parents? A) XcXc and XcY B) XcXc and XCY C) XCXC and XcY D) XCXC and XCY E) XCXc and XCY Answer: E Topic: Concept 15.2 Skill: Application/Analysis 14) Cinnabar eyes is a sex-linked recessive characteristic in fruit flies. If a female having cinnabar eyes is crossed with a wild-type male, what percentage of the F1 males will have cinnabar eyes? A) 0% B) 25% C) 50% D) 75% E) 100% Answer: E Topic: Concept 15.2 Skill: Application/Analysis 15) Calico cats are female because A) a male inherits only one of the two X-linked genes controlling hair color. B) the males die during embryonic development. C) the Y chromosome has a gene blocking orange coloration. D) only females can have Barr bodies. E) multiple crossovers on the Y chromosome prevent orange pigment production. Answer: A Topic: Concept 15.2 Skill: Application/Analysis 16) In birds, sex is determined by a ZW chromosome scheme. Males are ZZ and females are ZW. A recessive lethal allele that causes death of the embryo is sometimes present on the Z chromosome in pigeons. What would be the sex ratio in the offspring of a cross between a male that is heterozygous for the lethal allele and a normal female? A) 2:1 male to female B) 1:2 male to female
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C) 1:1 male to female D) 4:3 male to female E) 3:1 male to female Answer: A Topic: Concept 15.2 Skill: Application/Analysis Refer to the following information to answer the questions below. A man who is an achondroplastic dwarf with normal vision marries a color-blind woman of normal height. The man's father was six feet tall, and both the woman's parents were of average height. Achondroplastic dwarfism is autosomal dominant, and red-green color blindness is Xlinked recessive. 17) How many of their daughters might be expected to be color-blind dwarfs? A) All B) None C) Half D) One out of four E) Three out of four Answer: B Topic: Concept 15.2 Skill: Application/Analysis 18) What proportion of their sons would be color-blind and of normal height? A) All B) None C) Half D) One out of four E) Three out of four Answer: C Topic: Concept 15.2 Skill: Application/Analysis 19) They have a daughter who is a dwarf with normal color vision. What is the probability that she is heterozygous for both genes? A) 0 B) 0.25 C) 0.50 D) 0.75 E) 1.00 Answer: E Topic: Concept 15.2 Skill: Application/Analysis
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20) A Barr body is normally found in the nucleus of which kind of human cell? A) Unfertilized egg cells only B) Sperm cells only C) Somatic cells of a female only D) Somatic cells of a male only E) Both male and female somatic cells Answer: C Topic: Concept 15.2 Skill: Knowledge/Comprehension 21) Sex determination in mammals is due to the SRY region of the Y chromosome. An abnormality could allow which of the following to have a male phenotype? A) Turner syndrome, 45, X B) Translocation of SRY to an autosome of a 46, XX individual C) A person with too many X chromosomes D) A person with one normal and one shortened (deleted) X E) Down syndrome, 46, XX Answer: B Topic: Concept 15.2 Skill: Application/Analysis 22) Which of the following statements is true? A) The closer two genes are on a chromosome, the lower the probability that a crossover will occur between them. B) The observed frequency of recombination of two genes that are far apart from each other has a maximum value of 100%. C) All of the traits that Mendel studied seed color, pod shape, flower color, and others are due to genes linked on the same chromosome. D) Linked genes are found on different chromosomes. E) Crossing over occurs during prophase II of meiosis. Answer: A Topic: Concept 15.3 Skill: Knowledge/Comprehension 23) How would one explain a testcross involving F1 dihybrid flies in which more parental-type offspring than recombinant-type offspring are produced? A) The two genes are linked. B) The two genes are linked but on different chromosomes. C) Recombination did not occur in the cell during meiosis. D) The testcross was improperly performed. E) Both of the characters are controlled by more than one gene. Answer: A Topic: Concept 15.3
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Skill: Knowledge/Comprehension 24) New combinations of linked genes are due to which of the following? A) Nondisjunction B) Crossing over C) Independent assortment D) Mixing of sperm and egg E) Deletions Answer: B Topic: Concept 15.3 Skill: Knowledge/Comprehension 25) What does a frequency of recombination of 50% indicate? A) The two genes are likely to be located on different chromosomes. B) All of the offspring have combinations of traits that match one of the two parents. C) The genes are located on sex chromosomes. D) Abnormal meiosis has occurred. E) Independent assortment is hindered. Answer: A Topic: Concept 15.3 Skill: Knowledge/Comprehension 26) A 0.1% frequency of recombination is observed A) only in sex chromosomes. B) only on genetic maps of viral chromosomes. C) on unlinked chromosomes. D) in any two genes on different chromosomes. E) in genes located very close to one another on the same chromosome. Answer: E Topic: Concept 15.3 Skill: Application/Analysis 27) The following is a map of four genes on a chromosome:
Figure 15.1 Between which two genes would you expect the highest frequency of recombination? A) A and W B) W and E C) E and G D) A and E
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E) A and G Answer: E Topic: Concept 15.3 Skill: Application/Analysis 28) What is the reason that linked genes are inherited together? A) They are located close together on the same chromosome. B) The number of genes in a cell is greater than the number of chromosomes. C) Chromosomes are unbreakable. D) Alleles are paired together during meiosis. E) Genes align that way during metaphase I of meiosis. Answer: A Topic: Concept 15.3 Skill: Knowledge/Comprehension 29) What is the mechanism for the production of genetic recombinants? A) X inactivation B) Methylation of cytosine C) Crossing over and independent assortment D) Nondisjunction E) Deletions and duplications during meiosis Answer: C Topic: Concept 15.3 Skill: Knowledge/Comprehension Refer to Figure 15.2 to answer the following questions.
Figure 15.2 30) In a series of mapping experiments, the recombination frequencies for four different linked genes of Drosophila were determined as shown in the figure. What is the order of these genes on a chromosome map? A) rb-cn-vg-b B) vg-b-rb-cn
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C) cn-rb-b-vg D) b-rb-cn-vg E) vg-cn-b-rb Answer: D Topic: Concept 15.3 Skill: Application/Analysis 31) Which of the following two genes are closest on a genetic map of Drosophila? A) b and vg B) vg and cn C) rb and cn D) cn and b E) b and rb Answer: E Topic: Concept 15.3 Skill: Application/Analysis D, F, and J are three genes in Drosophila. The recombination frequencies for two of the three genes are shown in Figure 15.3.
Figure 15.3 32) Genes D and F could be A) located on different chromosomes. B) located very near to each other on the same chromosome. C) located far from each other on the same chromosome. D) Both A and B E) Both A and C Answer: E Topic: Concept 15.3 Skill: Application/Analysis 33) The frequency of crossing over between any two linked genes will be which of the following? A) Higher if they are recessive B) Dependent on how many alleles there are C) Determined by their relative dominance D) The same as if they were not linked E) Proportional to the distance between them
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Answer: E Topic: Concept 15.3 Skill: Knowledge/Comprehension 34) Map units on a linkage map cannot be relied upon to calculate physical distances on a chromosome for which of the following reasons? A) The frequency of crossing over varies along the length of the chromosome. B) The relationship between recombination frequency and map units is different in every individual. C) Physical distances between genes change during the course of the cell cycle. D) The gene order on the chromosomes is slightly different in every individual. E) Linkage map distances are identical between males and females. Answer: A Topic: Concept 15.3 Skill: Knowledge/Comprehension 35) Which of the following is a map of a chromosome that includes the positions of genes relative to visible chromosomal features, such as stained bands? A) Linkage map B) Physical map C) Recombination map D) Cytogenetic map E) Banded map Answer: D Topic: Concept 15.3 Skill: Knowledge/Comprehension 36) If a human interphase nucleus contains three Barr bodies, it can be assumed that the person A) has hemophilia. B) is a male. C) has four X chromosomes. D) has Turner syndrome. E) has Down syndrome. Answer: C Topic: Concept 15.4 Skill: Knowledge/Comprehension 37) If nondisjunction occurs in meiosis II during gametogenesis, what will be the result at the completion of meiosis? A) All the gametes will be diploid. B) Half of the gametes will be n + 1, and half will be n - 1. C) 1/4 of the gametes will be n + 1, one will be n - 1, and two will be n. D) There will be three extra gametes. E) Two of the four gametes will be haploid, and two will be diploid.
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Answer: C Topic: Concept 15.4 Skill: Application/Analysis 38) If a pair of homologous chromosomes fails to separate during anaphase of meiosis I, what will be the chromosome number of the four resulting gametes with respect to the normal haploid number (n)? A) n + 1; n + 1; n - 1; n - 1 B) n + 1; n - 1; n; n C) n + 1; n - 1; n - 1; n - 1 D) n + 1; n + 1; n; n E) n - 1; n - 1; n; n Answer: A Topic: Concept 15.4 Skill: Application/Analysis 39) A cell that has 2n + 1 chromosomes is A) trisomic. B) monosomic. C) euploid. D) polyploid. E) triploid. Answer: A Topic: Concept 15.4 Skill: Knowledge/Comprehension 40) One possible result of chromosomal breakage is for a fragment to join a nonhomologous chromosome. What is this alteration called? A) Deletion B) Disjunction C) Inversion D) Translocation E) Duplication Answer: D Topic: Concept 15.4 Skill: Knowledge/Comprehension 41) A nonreciprocal crossover causes which of the following products? A) Deletion only B) Duplication only C) Nondisjunction D) Deletion and duplication E) Duplication and nondisjunction
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Answer: D Topic: Concept 15.4 Skill: Knowledge/Comprehension 42) In humans, male-pattern baldness is controlled by an autosomal gene that occurs in two allelic forms. Allele Hn determines nonbaldness, and allele Hb determines pattern baldness. In males, because of the presence of testosterone, allele Hb is dominant over Hn. If a man and woman both with genotype HnHb have a son, what is the chance that he will eventually be bald? A) 0% B) 25% C) 33% D) 50% E) 75% Answer: Topic: E Concept 15.4 Skill: Application/Analysis 43) Of the following human aneuploidies, which is the one that generally has the most severe impact on the health of the individual? A) 47, +21 B) 47, XXY C) 47, XXX D) 47, XYY E) 45, X Answer: A Topic: Concept 15.4 Skill: Knowledge/Comprehension 44) A phenotypically normal prospective couple seeks genetic counseling because the man knows that he has a translocation of a portion of his chromosome 4 that has been exchanged with a portion of his chromosome 12. Although he is normal because his translocation is balanced, he and his wife want to know the probability that his sperm will be abnormal. What is your prognosis regarding his sperm? A) 1/4 will be normal, 1/4 with the translocation, 1/2 with duplications and deletions. B) All will carry the same translocation as the father. C) None will carry the translocation since abnormal sperm will die. D) His sperm will be sterile and the couple might consider adoption. E) 1/2 will be normal and the rest with the father's translocation. Answer: A Topic: Concept 15.4 Skill: Synthesis/Evaluation 45) Abnormal chromosomes are frequent in malignant tumors. Errors such as translocations may place a gene in close proximity to different control regions. Which of the following might then occur to make the cancer worse?
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A) An increase in non-disjunction B) Expression of inappropriate gene products C) A decrease in mitotic frequency D) Death of the cancer cells in the tumor E) Sensitivity of the immune system Answer: B Topic: Concept 15.4 Skill: Synthesis/Evaluation 46) Women with Turner syndrome have a genotype characterized as which of the following? A) aabb B) Mental retardation and short arms C) A karyotype of 45, X D) A karyotype of 47, XXX E) A deletion of the Y chromosome Answer: C Topic: Concept 15.4 Skill: Knowledge/Comprehension 47) The frequency of Down syndrome in the human population is most closely correlated with which of the following? A) Frequency of new meiosis B) Average of the ages of mother and father C) Age of the mother D) Age of the father E) Exposure of pregnant women to environmental pollutants Answer: C Topic: Concept 15.4 Skill: Knowledge/Comprehension 48) An inversion in a human chromosome often results in no demonstrable phenotypic effect in the individual. What else may occur? A) There may be deletions later in life. B) Some abnormal gametes may be formed. C) There is an increased frequency of mutation. D) All inverted chromosomes are deleted. E) The individual is more likely to get cancer. Answer: B Topic: Concept 15.4 Skill: Synthesis/Evaluation 49) What is the source of the extra chromosome 21 in an individual with Down syndrome? A) Nondisjunction in the mother only B) Nondisjunction in the father only
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C) Duplication of the chromosome D) Nondisjunction or translocation in either parent E) It is impossible to detect with current technology Answer: D Topic: Concept 15.4 Skill: Knowledge/Comprehension 50) Down syndrome has a frequency in the U.S. population of ~ 1/700 live births. In which of the following groups would you expect this to be significantly higher? A) People in Latin or South America B) The Inuit and other peoples in very cold habitats C) People living in equatorial areas of the world D) Very small population groups E) No groups have such higher frequency Answer: E Topic: Concept: 15.4 Skill: Knowledge/Comprehension 51) A couple has a child with Down syndrome when the mother is 39 years old at the time of delivery. Which is the most probable cause? A) The woman inherited this tendency from her parents. B) One member of the couple carried a translocation. C) One member of the couple underwent nondisjunction in somatic cell production. D) One member of the couple underwent nondisjunction in gamete production. Answer: D Topic: Concept 15.4 Skill: Application/Analysis 52) In 1956 Tijo and Levan first successfully counted human chromosomes. The reason it would have taken so many years to have done so would have included all but which of the following? A) Watson and Crick's structure of DNA was not done until 1953. B) Chromosomes were piled up on top of one another in the nucleus. C) Chromosomes were not distinguishable during interphase. D) A method had not yet been devised to halt mitosis at metaphase. Answer: A Topic: Concept 15.4 Skill: Synthesis/Evaluation 53) At which phase(s) is it preferable to obtain chromosomes to prepare a karyotype? A) Early prophase B) Late telophase C) Anaphase D) Late anaphase or early telophase E) Late prophase or metaphase
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Answer: E Topic: Concept 15.4 Skill: Knowledge/Comprehension 54) In order for chromosomes to undergo inversion or translocation, which of the following is required? A) Point mutation B) Immunological insufficiency C) Advanced maternal age D) Chromosome breakage and rejoining E) Meiosis Answer: D Topic: Concept 15.4 Skill: Application/Analysis 55) Which of the following statements describes genomic imprinting? A) It explains cases in which the gender of the parent from whom an allele is inherited affects the expression of that allele. B) It is greatest in females because of the larger maternal contribution of cytoplasm. C) It may explain the transmission of Duchenne muscular dystrophy. D) It involves an irreversible alteration in the DNA sequence of imprinted genes. Answer: A Topic: Concept 15.5 Skill: Knowledge/Comprehension
Figure 15.4 56) The pedigree in Figure 15.4 shows the transmission of a trait in a particular family. Based on this pattern of transmission, the trait is most likely A) mitochondrial. B) autosomal recessive.
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C) sex-linked dominant. D) sex-linked recessive. E) autosomal dominant. Answer: A Topic: Concept 15.5 Skill: Application/Analysis 57) A gene is considered to be non-Mendelian in its inheritance pattern if it seems to "violate" Mendel's laws. Which of the following would then NOT be considered non-Mendelian? A) A gene whose expression varies depending on the gender of the transmitting parent B) A gene derived solely from maternal inheritance C) A gene transmitted via the cytoplasm or cytoplasmic structures D) A gene transmitted to males from the maternal line and from fathers to daughters E) A gene transmitted by a virus to egg-producing cells Answer: D Topic: Concept 15.5 Skill: Synthesis/Evaluation 58) Genomic imprinting is generally due to the addition of methyl (CH3) groups to C nucleotides in order to silence a given gene. If this depends on the sex of the parent who transmits the gene, which of the following must be true? A) Methylation of C is permanent in a gene. B) Genes required for early development stages must not be imprinted. C) Methylation of this kind must occur more in males than in females. D) Methylation must be reversible in ovarian and testicular cells. E) The imprints are transmitted only to gamete-producing cells. Answer: D Topic: Concept 15.5 Skill: Synthesis/Evaluation 59) Correns described that the inheritance of variegated color on the leaves of certain plants was determined by the maternal parent only. What phenomenon does this describe? A) Mitochondrial inheritance B) Chloroplast inheritance C) Genomic imprinting D) Infectious inheritance E) Sex-linkage Answer: B Topic: Concept 15.5 Skill: Knowledge/Comprehension 60) Mitochondrial DNA is primarily involved in coding for proteins needed for electron transport. Therefore in which body systems would you expect most mitochondrial gene mutations to be exhibited?
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A) The immune system and the blood B) Excretory and respiratory systems C) The skin and senses D) Nervous and muscular systems E) Circulation Answer: D Topic: Concept 15.5 Skill: Synthesis/Evaluation 61) A certain kind of snail can have a right-handed direction of shell coiling (D) or left handed coiling (d). If direction of coiling is due to a protein deposited by the mother in the egg cytoplasm, then a Dd egg-producing snail and a dd sperm-producing snail will have offspring of which genotype(s) and phenotype(s)? A) 1/2 Dd : 1/2 dd; all right coiling B) All Dd; all right coiling C) 1/2 Dd : 1/2 dd; half right and half left coiling D) All Dd; all left coiling E) All Dd; half right and half left coiling Answer: A Topic: Concept 15.5 Skill: Application/Analysis Self-Quiz Questions 1) A man with hemophilia (a recessive, sex-linked condition) has a daughter of normal phenotype. She marries a man who is normal for the trait. What is the probability that a daughter of this mating will be a hemophiliac? That a son will be a hemophiliac? If the couple has four sons, what is the probability that all four will be born with hemophilia? Answer: 0; 12, 1/16 2) Pseudohypertrophic muscular dystrophy is an inherited disorder that causes gradual deterioration of the muscles. It is seen almost exclusively in boys born to apparently normal parents and usually results in death in the early teens. Is this disorder caused by a dominant or a recessive allele? Is its inheritance sex-linked or autosomal? How do you know? Explain why this disorder is almost never seen in girls. Answer: Recessive; if the disorder were dominant, it would affect at least one parent of a child born with the disorder. The disorder's inheritance is sex-linked because it is seen only in boys. For a girl to have the disorder, she would have to inherit recessive alleles from both parents. This would be very rare, since males with the recessive allele on their X chromosome die in their early teens. 3) Red-green color blindness is caused by a sex-linked recessive allele. A color-blind man marries a woman with normal vision whose father was color-blind. What is the probability that
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they will have a color-blind daughter? What is the probability that their first son will be colorblind? (Note the different wording in the two questions.) Answer: 14 for each daughter (12 chance that child will be female x 12 chance of a homozygous recessive genotype); 12 for first son. 4) A wild-type fruit fly (heterozygous for gray body color and normal wings) is mated with a black fly with vestigial wings. The offspring have the following phenotypic distribution: wild type, 778; black-vestigial, 785; black-normal, 158; gray-vestigial, 162. What is the recombination frequency between these genes for body color and wing size? Answer: 17% 5) In another cross, a wild-type fruit fly (heterozygous for gray body color and red eyes) is mated with a black fruit fly with purple eyes. The offspring are as follows: wild type, 721; black-purple, 751; gray-purple, 49; black-red, 45. What is the recombination frequency between these genes for body color and eye color? Using information from problem 4, what fruit flies (genotypes and phenotypes) would you mate to determine the sequence of the body-color, wing-size, and eyecolor genes on the chromosome? Answer: 6%. Wild type (heterozygous for normal wings and red eyes) recessive homozygote with vestigial wings and purple eyes 6) A fruit fly that is true-breeding for gray body, vestigial wings (b+ b+ vg vg) is mated with one that is true-breeding for black body, normal wings (b b vg+ vg+). A) Draw the chromosomes for each P generation fly, showing the position of each allele. B) Draw the chromosomes and label the alleles for an F1 fly. C) Suppose an F1 female is testcrossed. Draw the chromosomes of the resulting offspring in a Punnett square like the one at the bottom of Fig. 15.10 in your textbook. D) Knowing that the distance between these two genes is 17 map units, predict the phenotypic ratios of these offspring. Answer: A)
B)
C)
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D) 41.5% gray body, vestigial wings 41.5% black body, normal wings 8.5% gray body, normal wings 8.5% black body, vestigial wings 7) What pattern of inheritance would lead a geneticist to suspect that an inherited disorder of cell metabolism is due to a defective mitochondrial gene? Answer: The disorder would always be inherited from the mother. 8) Women born with an extra X chromosome (XXX) are healthy and phenotypically indistinguishable from normal XX women. What is a likely explanation for this finding? How could you test this explanation? Answer: The inactivation of two X chromosomes in XXX women would leave them with one genetically active X, as in women with the normal number of chromosomes. Microscopy should reveal two Barr bodies in XXX women. 9) Determine the sequence of genes along a chromosome based on the following recombination frequencies: AB, 8 map units; AC, 28 map units; AD, 25 map units; BC, 20 map units; BD, 33 map units. Answer: DABC 10) Assume that genes A and B are linked and are 50 map units apart. An animal heterozygous at both loci is crossed with one that is homozygous recessive at both loci. What percentage of the offspring will show phenotypes resulting from crossovers? If you did not know that genes A and B were linked, how would you interpret the results of this cross? Answer: Fifty percent of the offspring would show phenotypes that resulted from crossovers. These results would be the same as those from a cross where A and B were not linked. Further crosses involving other genes on the same chromosome would reveal the linkage and map distances. 11) A space probe discovers a planet inhabited by creatures that reproduce with the same hereditary patterns seen in humans. Three phenotypic characters are height (T = tall, t = dwarf), head appendages (A = antennae, a = no antennae), and nose morphology (S = upturned snout, s = downturned snout). Since the creatures are not intelligent, Earth scientists are able to do some controlled breeding experiments, using various heterozygotes in testcrosses. For tall heterozygotes with antennae, the offspring are: tall-antennae, 46; dwarf-antennae, 7; dwarf-no antennae, 42; tall-no antennae, 5. For heterozygotes with antennae and an upturned snout, the
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offspring are: antennae-upturned snout, 47; antennae-downturned snout, 2; no antennaedownturned snout, 48; no antennae-upturned snout, 3. Calculate the recombination frequencies for both experiments. Answer: Between T and A, 12%; between A and S, 5% 12) Two genes of a flower, one controlling blue (B) versus white (b) petals and the other controlling round (R) versus oval (r) stamens, are linked and are 10 map units apart. You cross a homozygous blue-oval plant with a homozygous white-round plant. The resulting F1 progeny are crossed with homozygous white-oval plants, and 1,000 F2 progeny are obtained. How many F2 plants of each of the four phenotypes do you expect? Answer: 450 each of blue-oval and white-round (parentals) and 50 each of blue-round and white-oval (recombinants) 13) You design Drosophila crosses to provide recombination data for gene a, which is located on the chromosome shown in Figure 15.12 in the textbook. Gene a has recombination frequencies of 14% with the vestigial-wing locus and 26% with the brown-eye locus. Where is a located on the chromosome? Answer: About one-third of the distance from the vestigial-wing locus to the brown-eye locus 14) Bananas plants, which are triploid, are seedless and therefore sterile. Propose a possible explanation. Answer: Because bananas are triploid, homologous pairs cannot line up during meiosis. Therefore, it is not possible to generate gametes that can fuse to produce a zygote with the triploid number of chromosomes.
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