CK-12 Biology for High School FlexBook® 2.0 Answer Key Chapter 3: Genetics 3.1 Pea Plants Review Questions 1. 2. 3. 4. What is the blending theory of inheritance? Why did Mendel question this theory? List the seven characteristics that Mendel investigated in pea plants. How did Mendel control pollination in pea plants? What are hybrids? Answers 1. The blending theory of inheritance states that offspring have a blend, or mix, of the characteristics of their parents. Mendel noticed plants in his own garden that weren’t a blend of the parents, so he questioned this theory. 2. Mendel studied seed form and color, flower color, pod form and color, and stem place and size. 3. Mendel was interested in the offspring of two different parent plants, so he had to prevent self-pollination. To do so, he removed the anthers from the flowers of some of the plants in his experiments. Then he pollinated them by hand with pollen from other parent plants of his choice. 4. Hybrids are the offspring of a cross-pollination experiment. 3.2 Mendel's First Experiment Review Questions 1. Describe in general terms Mendel’s first set of experiments. 2. State Mendel's first law. 3. Assume you are investigating the inheritance of stem length in pea plants. You cross-pollinate a short-stemmed plant with a long-stemmed plant. All of the offspring have long stems. Then, you let the offspring self-pollinate. Describe the stem lengths you would expect to find in the second generation of offspring. Answers CK-12 Biology Answer Keys - updated July 2019 1 1. Mendel first experimented with just one characteristic of a pea plant at a time. He started with color and crossed two different color plants. The F1 generation results from cross-pollination of two parent (P) plants, and contained all purple flowers. The F2 generation results from self-pollination of F1 plants, and contained 75% purple flowers and 25% white flowers. 2. The law of segregation states that there are two factors controlling a given characteristic, one of which dominates the other, and these factors separate and go to different gametes when a parent reproduces. 3. The F2 would have 75% long-stemmed plants and 25% short-stemmed plants. 3.3 Mendel's Second Experiment Review Questions 1. What was Mendel investigating with his second set of experiments? What was the outcome? 2. State Mendel’s second law. 3. If a purple-flowered, short-stemmed plant is crossed with a white-flowered, long-stemmed plant, would all of the purple-flowered offspring also have short stems? Why or why not? Answers 1. With his second set of experiments, Mendel wondered whether different characteristics are inherited together. He found that different characteristics were not inherited together. 2. The law of independent assortment states that factors controlling different characteristics are inherited independently of each other. 3. No, all of the purple-flowered offspring would not have short stems. From Mendel’s second set of experiments, the independent assortment of factors would result in purple-flowered offspring having either short stems or long stems. 3.4 Mendel's Laws Review Questions 1. If Darwin knew of Mendel’s work, how might it have influenced his theory of evolution? Do you think this would have affected how well Darwin’s work was accepted? CK-12 Biology Answer Keys - updated July 2019 2 2. Explain Mendel’s laws in genetic terms, that is, in terms of chromosomes, genes, and alleles. 3. Explain the relationship between genotype and phenotype. How can one phenotype result from more than one genotype? Answers 1. Darwin may have been able to link genetics and evolution, demonstrating how traits are inherited over time and how this can influence evolution. This may have made for a much stronger argument, and help the acceptance of his work, to some. Answers can vary, but must be supported. 2. Mendel’s first law states that there are two alleles controlling a given characteristic, one of which dominates the other, and these alleles separate and go to different gametes when a parent reproduces. His second law states that genes controlling different characteristics are inherited independently of each other. Genes are located on chromosomes, and the genes encode the traits Mendel studied. Each gene has multiple alleles, and these alleles are Mendel’s factors. As chromosomes segregate during meiosis, alleles and genes end up in unique combinations within gametes. 3. The alleles an individual inherits make up the individual’s genotype. The two alleles may be the same or different. The expression of an organism’s genotype produces its phenotype. As the dominant allele is expressed over the recessive allele, a heterozygote (Bb for example) will express the dominant allele and have the dominant phenotype, just like a homozygote (for the dominant allele, BB). 3.5 Probability Review Questions 1. Define probability. Apply the term to a coin toss. 2. How is gamete formation like tossing a coin? 3. With a BB homozygote, what is the chance of a gamete having the B allele? The b allele? Answers 1. Probability is the likelihood, or chance, that a certain event will occur. When you toss a coin, the chance of a head turning up is 50 percent. This is because a coin has only two sides, so there is an equal chance of a head or tail turning up on any given toss. 2. During meiosis, homologous chromosomes, and the alleles they carry, segregate and go to different gametes. Therefore, using Hh for example, the H and h alleles CK-12 Biology Answer Keys - updated July 2019 3 segregate and go to different gametes. As a result, half the gametes produced by the Hh parent will have the H allele and half will have the h allele. Based on the rules of probability, any given gamete of this parent has a 50% chance of having the H allele and a 50% chance of having the h allele, just like getting heads or tails when tossing a coin. 3. 100% B allele, 0% b allele. 3.6 Punnett Squares Review Questions 1. What is a Punnett square? How is it used? 2. Draw a Punnett square of an Ss x ss cross. The S allele codes for long stems in pea plants and the s allele codes for short stems. If S is dominant to s, what percentage of the offspring would you expect to have each phenotype? 3. What letter should replace the question marks (?) in this Punnett square? Explain how you know. 4. How do the Punnett squares for a monohybrid cross and a dihybrid cross differ? 5. What are the genotypes of gametes of a AaBb self-pollination? 6. Mendel carried out a dihybrid cross to examine the inheritance of the characteristics for seed color and seed shape. The dominant allele for yellow seed color is Y, and the recessive allele for green color is y. The dominant allele for round seeds is R, and the recessive allele for a wrinkled shape is r. The two plants that were crossed were F1 dihybrids RrYy. Identify the ratios of traits that Mendel observed in the F2 generation. Create a Punnett square to help you answer the question. Answers 1. A Punnett square is a chart that allows you to easily determine the expected percentage of different genotypes in the offspring of two parents. 2. 50% long stems, 50% short stems 3. The ? should all be replace by "a." As one parent only contributes an "A," and as offspring in each column and row have an "a," the "a" must come from the second parent, which must be homozygous aa. 4. A monohybrid cross Punnett square is a very simple 4 box square. A dihybrid cross Punnett square is more complex at 16 boxes. 5. The genotypes of the gametes of a AaBb self-pollination cross are AB, Ab, aB, ab. 6. 9 yellow, round seeds; 3 yellow, wrinkled seeds; 3 green, round seeds; 1 green, wrinkled seeds. CK-12 Biology Answer Keys - updated July 2019 4 3.7 Non-Mendelian Inheritance Review Questions 1. A classmate tells you that a person can have type AO blood. Do you agree? Explain. 2. Mendelian inheritance does not apply to the inheritance of alleles that result in incomplete dominance and codominance. Explain why this is so. 3. Describe the relationship between environment and phenotype. 4. Mendel investigated stem length, or height, in pea plants. What if he had investigated human height instead? Why would his results have been harder to interpret? Answers 1. No, a person cannot have type AO blood. There is no such blood type. Type O blood only exists with a recessive genotype. 2. Incomplete dominance shows an offspring phenotype between that of the two parents, which is unlike Mendel’s observations. Also, codominance occurs when both alleles are expressed equally in the phenotype of the heterozygote. Two dominant alleles is also unlike Mendel’s observations. 3. For some characteristics, such as human height, the environment interacts with alleles to determine the phenotype. 4. Human height has a wide range of phenotypes. Firstly, it would be difficult to control the crosses of humans and wait for the phenotype to develop. Secondly, the wide range of phenotypes (as human height is a polygenic characteristic) would have been difficult to interpret genetically with what was known in Mendel’s time. 3.8 Human Genome Review Questions 1. Describe the human genome. 2. What has the Human Genome Project achieved? 3. Describe the makeup of the human genome. Answers CK-12 Biology Answer Keys - updated July 2019 5 1. The human genome is all the DNA of Homo sapiens. Humans have about 3 billion bases of information, divided into roughly 20,000 to 22,000 genes, which are spread among non-coding sequences and distributed among 24 distinct chromosomes (22 autosomes plus the X and Y sex chromosomes). 2. The Human Genome Project has determined the DNA sequence of the entire human genome. 3. The human genome consists of protein-coding exons, associated introns and regulatory sequences, genes that encode other RNA molecules, and other DNA sequences. 3.9 Human Chromosomes Review Questions 1. 2. 3. 4. Describe human chromosomes. Compare and contrast human autosomes and sex chromosomes. What is SRY? Why are females the "default sex" of the human species? Answers 1. Chromosomes are coiled structures made of DNA and proteins called histones. Chromosomes are the form of the genetic material of a cell during cell division. The human genome has 23 pairs of chromosomes located in the nucleus of somatic cells: 22 pairs are autosomes, and 1 pair is the sex chromosomes, which may be two X-chromosomes, or one X and one Y-chromosome. 2. Autosomes are chromosomes that contain genes for characteristics that are unrelated to sex. These chromosomes are the same in males and females. The sex chromosomes are related to sex determination (actually only the Y chromosome contains genes that determine sex). 3. SRY is a gene on the Y-chromosome known as the sex-determining region Y, and triggers an embryo to develop into a male. Without a Y chromosome, an individual develops into a female. 4. Without a Y chromosome, an individual develops into a female. As females have two X-chromosomes, and no Y-chromosome, they lack the SRY gene, so the female sex automatically develops. 3.10 Genetic Linkage Review CK-12 Biology Answer Keys - updated July 2019 6 Questions 1. What are linked genes? 2. Explain how you would construct a linkage map for a human chromosome. What data would you need? 3. People with red hair usually have very light skin. What might be a genetic explanation for this observation? 4. How often does crossing-over occur between non-linked genes? Explain your answer. Answers 1. Linked genes are genes located on the same chromosome. 2. Frequencies of crossing-over are used to construct a linkage map. Linkage is assessed by determining how often crossing-over occurs between two genes on the same chromosome. The lower the frequency of crossing over, the closer together on the same chromosome the genes are presumed to be. 3. The genes for red hair and light skin are usually inherited together, meaning they are linked, and close together on the same chromosome. 4. Never, crossing-over does not occur between non-linked genes, as they are on different chromosomes. 3.11 Mendelian Inheritance Review Questions 1. Describe the inheritance pattern for a single-gene autosomal dominant trait, such as free-hanging earlobes. 2. Draw a pedigree for hitchhiker’s thumb. Your pedigree should cover at least two generations and include both dominant and recessive forms of the trait. Label the pedigree with genotypes, using the letter H to represent the dominant allele for the trait and the letter h to represent the recessive allele. 3. Why is a recessive X-linked allele always expressed in males? 4. What is necessary for a recessive X-linked allele to be expressed in females? 5. What is an example of a recessive X-linked trait? Answers 1. Single-gene autosomal dominant traits are passed from parent to offspring when at least one dominant allele is inherited. Only when two recessive alleles are inherited, which can only occur when both parents are heterozygous, will the trait be absent. CK-12 Biology Answer Keys - updated July 2019 7 2. Answers may vary but should be at least two generations and cover both the dominant and recessive forms of the trait. 3. Because males have just one X chromosome, they have only one allele for any X-linked trait. If that allele is recessive, there is no dominant allele to be expressed, so a recessive X-linked allele will always be expressed in males. 4. Because females have two X-chromosomes, for a recessive allele to be expressed, the female must be homozygous for that allele. 5. An example of a recessive X-linked trait is red-green color blindness. 3.12 Genetic Disorders Review Questions 1. 2. 3. 4. 5. Describe a genetic disorder caused by a mutation in a single gene. What causes Down syndrome? What is nondisjunction? What is gene therapy? Explain why genetic disorders caused by abnormal numbers of chromosomes most often involve the X chromosome. Answers 1. Marfan syndrome is a genetic disorder caused by a mutation in a single gene. It is an autosomal dominant disorder caused by a defective protein in connective tissue. People with Marfan syndrome have heart and bone defects and unusually long, slender limbs and fingers. (Add: Other disorders may be discussed.) 2. Down syndrome is caused by an extra copy (complete or partial) of chromosome 21. 3. Nondisjunction is the failure of replicated chromosomes to separate properly during meiosis. 4. Gene therapy is a method to cure genetic disorders by inserting normal genes into cells with mutant genes. 5. Most chromosomal disorders involve the X chromosome. This is because the X and Y chromosomes are very different in size, so nondisjunction of the sex chromosomes occurs relatively often. Also, zygotes with abnormal numbers of autosomes usually do not survive. 3.13 Biotechnology Review CK-12 Biology Answer Keys - updated July 2019 8 Questions 1. 2. 3. 4. 5. Define biotechnology. What is recombinant DNA? Identify the steps of gene cloning. What is the purpose of the polymerase chain reaction? Describe the three steps of PCR. Answers 1. Biotechnology is the use of technology to change the genetic makeup of living things for human purposes. 2. Recombinant DNA is the DNA that results when DNA from two organisms is combined. 3. The four main steps of gene cloning are (1) the use of restriction enzymes to cut DNA at specific location, (2) the ligation of the isolated gene/DNA with a plasmid to produce recombinant DNA, (3) transformation of bacteria with the recombinant DNA, and (4) selection of bacteria with the recombinant DNA. 4. The polymerase chain reaction (PCR) makes many copies of a gene or other DNA segment. 5. PCR involves three steps: denaturing, annealing, and extension. a. Denaturing involves heating DNA to break the bonds holding together the two DNA strands. This yields two single strands of DNA. b. Annealing involves cooling the single strands of DNA to allow the primers to bind to the DNA sequence. c. Extension occurs when Taq polymerase adds nucleotides to the primers, doubling the amount of DNA molecules. 3.14 Biotechnology Applications Review Questions 1. What are transgenic crops? 2. Make a flow chart outlining the steps involved in creating a transgenic crop. 3. Explain how bacteria can be genetically engineered to produce a human protein. Answers 1. Transgenic crops are genetically modified with new genes that code for traits useful to humans. 2. (1) The desired gene is ligated into a plasmid. (2) The recombinant DNA is inserted back into the bacteria. (3) The bacteria is used to insert the gene of CK-12 Biology Answer Keys - updated July 2019 9 interest into the chromosome of a plant cell. (4) Plant cells are grown in culture, and (5) a plant is generated from a cell clone. 3. If a gene of interest for a human protein is ligated into a plasmid, and the plasmid is transformed into bacteria, the bacteria will treat the transformed gene as if it belonged in the cell, transcribing the gene and translating the protein. As the DNA is replicated prior to cell division, the plasmid will be replicated and divided among the daughter cells. The bacteria will continue to produce the human protein this way. 3.15 Ethical, Legal, and Social Issues in Biotechnology (ELSI) Review Questions 1. Identify three ethical, legal, or social issues raised by biotechnology. 2. State your view on an ELSI issue, and develop a logical argument to support your view. Answers 1. Answers will vary. ELSI issues include the following: Are genetically modified foods safe to eat? Who controls a person’s genetic information? How far should we go to ensure that children are free of mutations? etc. 2. Answers will vary. CK-12 Biology Answer Keys - updated July 2019 10