168 Chapter 5 Answers: Genetics
5.2 Chromosomes and Genes: Review Questions and Answers
- What are chromosomes and genes? How are the two related? Chromosomes are coiled structures made of DNA and proteins that form during cell division and are encoded with genetic instructions for making RNA and proteins. These instructions are organized into units called genes, which are segments of DNA that code for particular pieces of RNA. The RNA molecules can then act as a blueprint for proteins, or directly help regulate various cellular processes. There may be hundreds or even thousands of genes on a single chromosome.
- Describe human chromosomes and genes. Most human cells contain 23 pairs of chromosomes, for a total of 46 chromosomes. One set of chromosomes is inherited from each parent. Of the 23 pairs of chromosomes, 22 pairs are autosomes, which control traits unrelated to sex, and the remaining pair consists of sex chromosomes (XX or XY). Human chromosomes contain a total of 20,000 to 22,000 genes, the majority of which have two more possible versions, called alleles.
- Explain the difference between autosomes and sex chromosomes. Autosomes are chromosomes that contain genes unrelated to sex. They are the same in males and females. Sex chromosomes differ in males and females. Normal males have the chromosomes XY and females the chromosomes XX. Genes on the X chromosome are not related to sex. Only genes on the Y chromosome play a role in determining an individual’s sex.
- What are linked genes, and what does a linkage map show? Linked genes are genes that are located on the same chromosome. A linkage map shows the location of specific genes on a chromosome.
- Explain why females are considered the default sex in humans. Females are considered the default sex in humans because only genes on the Y chromosome determine sex and trigger the development of the embryo into a male. Without a Y chromosome, an embryo will develop as female.
- Explain the relationship between genes and alleles. Alleles are different versions of the same gene.
- Most males and females have two sex chromosomes. Why do only females have Barr bodies? Only females usually have Barr bodies because Barr bodies refer to an inactivated X chromosome. This X chromosome is inactivated because cells should only have one functioning X chromosome. Since females have two X chromosomes, they need a Barr body, but since males are XY and only have one X chromosome, they do not have a Barr body.
5.3 DNA: Review Questions and Answers
- Outline the discoveries that led to the determination that DNA (not protein) is the biochemical molecule that contains genetic information. The first discovery that led to the determination that DNA is the biochemical molecule that contains genetic information was made in the 1920s, when Frederick Griffith showed that something in virulent bacteria could be transferred to nonvirulent bacteria and make them virulent as well. In the early 1940s, Oswald Avery and colleagues showed that the “something” Griffith found in his research was DNA and not protein. This result was confirmed by Alfred Hershey and Martha Chase who demonstrated that viruses insert DNA into bacterial cells so the cells will make copies of the viruses.
- State Chargaff’s rules. Explain how the rules are related to the structure of the DNA molecule. Chargaff’s rules state that, within the DNA of any given species, the concentration of adenine is always the same as the concentration of thymine, and the concentration of guanine is always the same as the concentration of cytosine. Bonds between nitrogen bases hold together the two polynucleotide chains of DNA. Adenine and guanine have a two-ring structure, whereas cytosine and thymine have just one ring. If two-ring adenine, for example, were to bond with two-ring guanine as well as with one-ring thymine, the distance between the two chains would be variable. However, when two-ring adenine bonds only with one-ring thymine, and two-ring guanine bonds only with one-ring cytosine, the distance between the two chains remains constant. This maintains the uniform shape of the DNA double helix and explains how Chargaff’s rules are related to DNA’s structure.
- Explain how the structure of a DNA molecule is like a spiral staircase. Which parts of the staircase represent the various parts of the molecule? The DNA molecule has a double-helix structure, which is similar to the structure of a spiral staircase. The sugar-phosphate backbones of the two polynucleotide chains of DNA are like the two outside edges, or sides, of the spiral staircase. The bonded nitrogen bases are like the steps.
- Why do you think dead S-strain bacteria injected into mice did not harm the mice, but killed them when mixed with living (and normally harmless) R-strain bacteria? Answers may vary. Sample answer. The DNA from the S strain bacteria was what was making the R strain bacteria harmful. It appears that the S strain DNA requires living bacteria (such as the R bacteria) to be harmful to a host organism. Therefore, it could not hurt the mice when injected alone.
- In Griffith’s experiment, do you think the heat treatment that killed the bacteria also inactivated the bacterial DNA? Why or why not? No, because after heat treatment, the DNA from the S strain bacteria was able to make the R strain bacteria, which is normally harmless, deadly. So the DNA was still causing the same effects after the heat treatment, and therefore seemed to be functioning normally.
- Give one example of the specific evidence that helped rule out proteins as genetic material. Answers may vary, but may include evidence from Avery’s or Hershey and Chase’s experiments. Sample answer: When proteins were inactivated, the dead S strain bacteria were still able to cause the normally harmless R strain bacteria to become deadly. Therefore, proteins were not the genetic material being passed to the R strain bacteria.
5.4 DNA Replication: Review Questions and Answers
- Why are Okazaki fragments formed? Because DNA polymerase only replicates DNA in one direction.
5.5 RNA: Review Questions and Answers
- State the central dogma of molecular biology. The central dogma of molecular biology states that the genetic instructions encoded in DNA are first transcribed to RNA and then translated from RNA into a protein.
5.6 Genetic Code: Review Questions and Answers
- Describe the genetic code and explain how it is “read”. The genetic code consists of the sequence of nitrogen bases in a polynucleotide chain of DNA or RNA (A, G, C, and T or U). The four bases make up the “letters” of the code. The letters are combined in groups of three to form “words” called codons. There are 64 possible codons, and each codon codes for one amino acid or for a start or stop signal. The codon AUG is the start codon that establishes the reading frame of the code. After the AUG start codon, the next three bases are read as the second codon. The next three bases after that are read as the third codon, and so on. The sequence of bases is read, codon by codon, until a stop codon is reached. UAG, UGA, and UAA are all stop codons.
- Identify three important characteristics of the genetic code. The genetic code is universal, which means that the same code is found in all living things, providing evidence of common evolutionary origins of all organisms. The genetic code is unambiguous. This means that each codon codes for just one amino acid (or for start or stop). As a result, there is no mistaking which amino acid is encoded by a given codon. The genetic code is also redundant. This means that each amino acid is encoded by more than one codon. This helps prevent errors in protein synthesis because an accidental change in a single base often has no effect on which amino acid the codon encodes.
- Summarize how the genetic code was deciphered. The genetic code was deciphered by a series of ingenious experiments carried out mainly by Marshall Nirenberg, along with his colleague Heinrich Matthaei. These researchers added contents of bacterial cells to 20 test tubes. This was done to provide the “machinery” needed to synthesize proteins. They also added all 20 amino acids to the test tubes, with a different amino acid “tagged” by a radioactive element in each test tube. Then they added synthetic RNA containing just one type of base to each test tube, starting with the base uracil. They discovered that an RNA molecule consisting only of uracil bases produces a polypeptide chain of the amino acid phenylalanine. The researchers used similar experiments to determine that each codon consists of three bases and eventually to discover the codons for all 20 amino acids.
- Use the decoder above to answer the following questions:
- Is the code depicted in the table from DNA or RNA? How do you know? RNA, because RNA has U (uracil) as a base instead of T (thymine) which is found in DNA. This code shows only U, no T, so therefore it represents the code of RNA not DNA.
- Which amino acid does the codon CAA code for? Glutamine.
- What does UGA code for? UGA is a stop codon, so it causes the code to stop being read.
- Look at the codons that code for the amino acid glycine. How many of them are there and how are they similar and different? There are 4 codons for the amino acid glycine. They all start with GG, but what is different in each of them is their final base, which may be U, C, A, or G. (Note: this is a common pattern in the redundancy of the genetic code).
5.7 Protein Synthesis: Review Questions and Answers
- Relate protein synthesis and its two major phases to the central dogma of molecular biology. The way proteins are synthesized in cells is summed up by the central dogma of molecular biology: DNA → RNA → Protein. The first phase of protein synthesis, called transcription, is the DNA → RNA part of the central dogma. The second phase of protein synthesis, called translation, is the RNA → Protein part of the central dogma.
- Explain how mRNA is processed before it leaves the nucleus. Before it leaves the nucleus, mRNA may be processed in several ways, including splicing, editing, and polyadenylation. Splicing removes introns (noncoding regions) from mRNA. Editing changes some of the nucleotides in mRNA, which allows different versions of proteins to be synthesized. Polyadenylation adds adenine bases to the mRNA, which serves several functions, such as helping mRNA leave the nucleus and protecting mRNA from enzymes that might break it down.
- What additional processes might a polypeptide chain undergo after it is synthesized? After a polypeptide chain is synthesized, it may assume a folded shape due to interactions among its amino acids. It may also bind with other polypeptides or with different types of molecules, such as lipids or carbohydrates.
- Where does transcription take place in eukaryotes? Transcription takes place in the nucleus of eukaryotic cells.
- Where does translation take place? Translation takes place at ribosomes, which are in the cytoplasm of a cell.
5.8 Mutations: Review Questions and Answers
- Define mutation. Mutation is a random change in the sequence of bases in DNA or RNA.
- Identify causes of mutation. Mutations may occur spontaneously when errors occur during DNA replication or during the transcription of DNA during protein synthesis. Other mutations are caused by mutagens. Mutagens are environmental factors that cause mutations. They include radiation, certain chemicals, and some infectious agents.
- Compare and contrast germline and somatic mutations. Germline mutations occur in gametes and may be passed on to offspring. Every cell in the body of the offspring will then carry the mutation. Somatic mutations occur in other cells of the body than gametes. They are confined to a single cell and its daughter cells, and they cannot be passed on to offspring. They are likely to have little or no effect on the organism in which they occur.
- Describe chromosomal alterations, point mutations, and frameshift mutations. Identify the potential effects of each type of mutation. Chromosomal alterations are mutations that cause major changes in the structure of chromosomes. They are very serious and often result in the death of the organism in which they occur. If the organism survives, it may be affected in multiple ways. Point mutations are changes in a single nucleotide. Their effects depend on how they change the genetic code and may range from no effects to serious effects. Frameshift mutations change the reading frame of the genetic code. They are likely to have drastic effects on the encoded protein.
- Why do many mutations have neutral effects? Many mutations are neutral in their effects because they do not change the amino acids in the proteins they encode or because they are repaired before protein synthesis occurs.
- Give one example of a beneficial mutation and one example of a harmful mutation. Answers will vary. Sample answer: An example of a beneficial mutation is a mutation that is found in people in a small Italian town that protects from atherosclerosis. An example of a harmful mutation is the mutation that causes the genetic disorder cystic fibrosis.
- Why do you think that exposure to mutagens (such as cigarette smoke) can cause cancer? Mutagens are things in the environment that can cause mutations. Mutations in genes that control the cell cycle can cause cancer. Therefore, mutagens can cause cancer by causing mutations in these genes.
- Explain why the insertion or deletion of a single nucleotide can cause a frameshift mutation. Because the genetic code is read in sets of three nucleotides (a set of three is a codon), adding or removing a single nucleotide throws the whole reading frame off by changing which three nucleotides make up each codon. All of the codons after the insertion or deletion will be changed because of this, resulting in what is known as a frameshift mutation.
- Compare and contrast missense and nonsense mutations. Missense and nonsense mutations are both point mutations, where a single nucleotide is changed. The difference is that missense mutations cause an amino acid to be changed, while nonsense mutations cause a premature stop codon to be produced.
- Explain why mutations are important to evolution. Mutations are important for evolution because they are the source of new genetic variation. This variation can lead to organisms being more or less well adapted to their environments, which, over time, leads to evolutionary changes through natural selection.
5.9 Regulation of Gene Expression: Review Questions and Answers
- Define gene expression. Gene expression means using a gene to make a protein.
- Why must gene expression be regulated? Gene expression must be regulated so that the correct proteins are made where and when they are needed. This is necessary, for example, so that different types of cells have different shapes and other traits that suit them for their particular functions.
- Explain how regulatory proteins may activate or repress transcription. Regulatory proteins regulate the transcription phase of protein synthesis by binding to regions of DNA called regulatory elements, which are located near promoters. Regulatory proteins typically either activate or repress transcription. Activators promote transcription by enhancing the interaction of RNA polymerase with the promoter, thus initiating transcription of DNA to mRNA. Repressors prevent transcription by impeding the progress of RNA polymerase along the DNA strand so the DNA cannot be transcribed to mRNA.
- What is the TATA box, and how does it work? The TATA box is a regulatory element that is part of the promoter of almost every eukaryotic gene. A number of regulatory proteins bind to the TATA box, forming a multi-protein complex. It is only when all of the appropriate proteins are bound to the TATA box that RNA polymerase recognizes the complex and binds to the promoter so transcription can begin.
- Describe homeobox genes and their role in an organism’s development. Homeobox genes are a large group of similar genes that direct the formation of many body structures during the embryonic stage. In humans, homeobox genes code for chains of 60 amino acids called homeodomains. Proteins containing homeodomains are transcription factors that bind to and control the activities of other genes. They turn on certain genes in particular cells at just the right time so the individual develops normal organs and organ systems.
- Discuss the role of regulatory gene mutations in cancer. Mutations in regulatory genes that normally control the cell cycle can lead to certain types of cancer. Cancer-causing mutations most often occur in two types of regulatory genes, called proto-oncogenes genes and tumor-suppressor genes. Proto-oncogenes normally help cells divide. When a proto-oncogene mutates to become an oncogene, it is expressed continuously, so the cell keeps dividing out of control, which can lead to cancer. Tumor-suppressor genes normally slow down or stop cell division. When a tumor-suppressor gene mutates, cell division can’t be slowed or stopped. The cell keeps dividing out of control, which can lead to cancer.
- Explain the relationship between proto-oncogenes and oncogenes. Proto-oncogenes are genes that normally help cells divide. An oncogene is a mutated form of a proto-oncogene that causes the gene to be expressed continuously. This can cause cancer.
- If a newly fertilized egg contained a mutation in a homeobox gene, how do you think this would affect the developing embryo? Explain your answer. Answers may vary. Sample answer: Because homeobox genes are important for the development of body structures in the embryo, including the development of organs and organ systems, I think that a mutation in one of these genes might cause the embryo to be significantly malformed. This might even result in death.
- Compare and contrast enhancers and activators. Enhancers and activators both promote gene expression. However, enhancers are distant regions of DNA and activators are proteins that bind to regulatory elements on the DNA, near the promoter region of the gene.
5.10 Mendel’s Experiments and Laws of Inheritance: Review Questions and Answers
- Why were pea plants a good choice for Mendel’s experiments? Pea plants were a good choice for Mendel’s experiments because they are fast growing and easy to raise. They also have several visible characteristics that vary, such as seed form, flower colour, and stem length.
- How did the outcome of Mendel’s second set of experiments lead to his second law? In Mendel’s second set of experiments, he investigated two characteristics at a time. For example, he crossed plants with yellow round seeds and plants with green wrinkled seeds. The plants in the F1 generation were all alike and had the same combination of characteristics (yellow round seeds) like one of the parents, whereas the plants in the F2 generation showed all possible combinations of the two characteristics, such as greenround seeds and yellow wrinkled seeds. This outcome showed that the factors controlling different characteristics are inherited independently of each other.
- Discuss the development of Mendel’s legacy. During Mendel’s lifetime, his work was largely ignored. It was only after Mendel’s results were obtained by other researchers in 1900 that his work was rediscovered and he was given the credit he was due. Now, Mendel is considered the father of genetics for his experiments and the laws he derived from them.
- If Mendel’s law of independent assortment was not correct, and characteristics were always inherited together, what types of offspring do you think would have been produced by crossing plants with yellow round seeds and green wrinkled seeds? Explain your answer. There would be only offspring with yellow round seeds and green wrinkled seeds because those characteristics would always be inherited together. The other combinations would not have been observed.
5.11 Genetics of Inheritance: Review Questions and Answers
- Define genetics. Genetics is the science of heredity.
- Why is Gregor Mendel called the father of genetics if genes were not discovered until after his death? Gregor Mendel is called the father of genetics because his laws of inheritance form the basis of the science of heredity. Mendel thought some type of “factors” control traits and are passed on to offspring, and his laws describe how the factors and the traits they control are inherited. We now call Mendel’s factors genes, and his laws of inheritance are now expressed in genetic terms.
- Imagine that there are two alleles, R and r, for a given gene. R is dominant to r. Answer the following questions about this gene:
- What are the possible homozygous and heterozygous genotypes? The homozygous genotypes would be RR and rr, and the heterozygous genotype would be Rr.
- Which genotype or genotypes express the dominant R phenotype? Explain your answer. RR and Rr would express the dominant R phenotype because only one dominant allele (in this case, R) is needed to express the dominant phenotype.
- Are R and r on different loci? Why or why not? R and r cannot be on different loci because they are alleles of the same gene.
- Can R and r be on the same exact chromosome? Why or why not? If not, where are they located? No, because there is only one version of a gene on a single chromosome. They are on homologous chromosomes.
5.12 Sexual Reproduction, Meiosis, and Gametogenesis: Review Questions and Answers
- Explain how sexual reproduction happens at the cellular level. At the cellular level, sexual reproduction occurs when two parents produce reproductive cells called gametes that unite to form offspring. Gametes are haploid cells and when they unite in the process of fertilization, they produce a diploid cell called a zygote.
- Summarize what happens during Meiosis. During meiosis, homologous chromosomes separate and the cell undergoes two cell divisions to form four haploid daughter cells. Each daughter cell has just one chromosome from each homologous pair. The two cell divisions that occur during meiosis are called meiosis I and meiosis II, and each of them occurs in four phases: prophase, metaphase, anaphase, and telophase.
- Compare and contrast gametogenesis in males and females. Gametogenesis is the process in which haploid daughter cells from meiosis change to become mature gametes. Gametes produced by males are called sperm, and they mature during a process known as spermatogenesis. Gametes produced by females are called eggs, and they mature during a process known as oogenesis.
- Explain the mechanisms that increase genetic variation in the offspring produced by sexual reproduction. Mechanisms that increase genetic variation in offspring produced by sexual reproduction include crossing-over, independent assortment, and the random union of gametes. Crossing-over is the exchange of genetic material between non-sister chromatids of homologous chromosomes that may occur during meiosis. It results in new combinations of genes on each chromosome. Independent assortment refers to the way in which different chromosomes are distributed randomly to daughter cells during meiosis. It results in gametes that have unique combinations of chromosomes. Which two of the millions of possible gametes that are produced by two parents actually unite to produce an offspring is likely to be a matter of chance and is another major source of genetic variation in offspring.
- Why do gametes need to be haploid? What would happen to the chromosome number after fertilization if they were diploid? Gametes need to be haploid (i.e. half the number of chromosomes) because during fertilization, two of them join together to make a diploid (i.e. the usual number of chromosomes) zygote. If gametes were diploid, the resulting zygote would have twice the normal amount of chromosomes, which would be problematic.
- Describe one difference between Prophase I of Meiosis and Prophase of Mitosis. Answers may vary. Sample answer: In prophase I of meiosis, homologous chromosomes pair up, which does not occur in prophase of mitosis.
- Do all of the chromosomes that you got from your mother go into one of your gametes? Why or why not? This would be highly unlikely, because homologous chromosomes segregate independently from each other into daughter cells during meiosis. So the chances of all 23 of the chromosomes that you got from your mother going into one of your gametes is very low.
5.13 Mendelian Inheritance: Review Questions and Answers
- Define genetic traits and Mendelian inheritance. Genetic traits are characteristics that are encoded in DNA. Mendelian inheritance refers to the inheritance of traits controlled by a single gene with two alleles, one of which may be completely dominant to the other.
- Explain why autosomal and X-linked Mendelian traits have different patterns of inheritance. Autosomal Mendelian traits do not differ between males and females. They are inherited in the same way regardless of the sex of the parent or offspring. For example, a dominant autosomal trait will show up in anyone who inherits even one copy of the dominant allele, whereas the recessive trait will show up only in people who inherit two copies of the recessive allele. X-linked Mendelian traits, in contrast, have a different pattern of inheritance than autosomal Mendelian traits because males have just one X chromosome, which they always inherit from their mother and pass on to all of their daughters but none of their sons. A recessive X-linked trait, for example, will show up in males who inherit just one copy of the recessive allele, whereas females must inherit two copies of the recessive allele (one on each of their two X chromosomes) to express the recessive trait.
- Identify examples of human autosomal and X-linked Mendelian traits. Answers may vary. Sample answer: Examples of human autosomal Mendelian traits include albinism and Huntington’s disease. Examples of human X-linked Mendelian traits include red-green colour blindness and hemophilia.
- Imagine a hypothetical human gene that has two alleles,Q and q. Q is dominant to q and the inheritance of this gene is Mendelian. Answer the following questions about this gene.
- If a woman has the genotype Q q and her husband has the genotype QQ, list each of their possible gametes. What proportion of their gametes will have each allele? The woman’s gametes will be 50%Q and 50% q. Her husband’s gametes will be 100% Q.
- What are the likely proportions of their offspring being QQ, Qq, or qq? Their offspring have a 50% chance of being QQ, a 50% chance of being Qq, and a zero per cent chance of being qq. (Hint: if you are having trouble figuring this out, make a Punnett square).
- Is this an autosomal trait or an X-linked trait? How do you know? This must be an autosomal trait because the man has two alleles for the gene (i.e. he isQQ). X-linked traits have only one copy of the gene in males because males have only one X chromosome.
- What are the chances of their offspring exhibiting the dominant Q trait? Explain your answer. Their offspring will all have the dominant Q trait, because their genotypes will either be QQ or Qq, and only one copy of the dominant Q allele is needed to express the dominant trait.
- Explain why fathers always pass their X chromosome down to their daughters. Fathers pass their single X chromosome down to their daughters because women have two X chromosomes and therefore receive one X chromosome from each parent.
5.14 Non-Mendelian Inheritance: Review Questions and Answers
- What is non-Mendelian inheritance? Non-Mendelian inheritance is the inheritance of traits that have a more complex genetic basis than one gene with two alleles and complete dominance.
- Explain why the human ABO blood group is an example of a multiple allele trait with codominance. The human ABO blood group is an example of a multiple allele trait because the gene for ABO blood type has more than two commonly occurring alleles: IA, IB, and i. ABO blood group is an example of codominance because the IAand IB alleles are codominant to one another. As a result, heterozygotes who inherit one copy of each allele produce both A and B antigens, giving them type AB blood.
- What is incomplete dominance? Give an example of this type of non-Mendelian inheritance in humans. Incomplete dominance is the case in which the dominant allele for a gene is not completely dominant to a recessive allele for the gene, so an intermediate phenotype occurs in heterozygotes who inherit both alleles. A human example of incomplete dominance is Tay Sachs disease, in which heterozygotes produce half as much functional enzyme as normal homozygotes.
- Explain the genetic basis of human skin colour. Human skin colour is a polygenic trait. It is controlled by several different genes, each with more than one allele. The alleles of each gene have a minor additive effect on the phenotype, producing a whole continuum of possible phenotypes and gradations of skin colour.
- How can the human trait of adult height be influenced by the environment? The human trait of adult height is a polygenic trait, and the environment may affect the phenotypic expression of the trait. For example, if a child’s growth is negatively affected by poor nutrition or illness, the child may grow up to be shorter in stature than would otherwise be the case given the child’s genes for height.
- Define pleiotropy, and give a human example. Pleiotropy is the situation in which one gene has multiple phenotypic effects. Examples may vary. Sample answer: A human example of pleiotropy involves the gene that codes for the main protein in collagen, a substance that helps form bones and is also important in the ears and eyes. Mutations in the gene result in problems not only in bones but also in these sensory organs, which is how the gene’s pleiotropic effects were discovered.
- Compare and contrast epistasis and dominance. Epistasis is the case in which a gene affects the expression of other genes. For example, a mutation in one gene may not allow other genes to be expressed in the phenotype. This occurs with albinism, for example. Dominance is the case in which one allele masks the expression of another allele for the same gene. Epistasis is similar to dominance, except that it occurs between different genes rather than between different alleles for the same gene.
- What is the difference between pleiotropy and epistasis? Pleiotropy is when one gene affects more than one phenotypic trait. Epistasis is when one gene affects the expression of other genes.
5.15 Genetic Disorders: Review Questions and Answers
- Define genetic disorder. A genetic disorder is a disease, syndrome, or other abnormal condition that is caused by gene mutation(s) or by chromosomal alterations.
- Identify three genetic disorders caused by mutations in a single gene. Answers may vary. Sample answer: Three genetic disorders caused by mutations in a single gene are Marfan syndrome (autosomal dominant), sickle cell anemia (autosomal recessive), and hemophilia A (X-linked recessive).
- Why are single-gene genetic disorders more commonly controlled by recessive than dominant mutant alleles? Single-gene genetic disorders are more commonly controlled by recessive than dominant mutant alleles because a dominant allele is always expressed. If it causes a serious genetic disorder, individuals who inherit even one copy of the allele may not live long enough to reproduce and pass on the allele to offspring. As a result, the allele is likely to die out of the population. A recessive mutant allele, in contrast, is not expressed in people who inherit just one copy of it. They carry the mutant allele and their offspring can inherit it. Thus, a recessive mutant allele is more likely than a dominant mutant allele to pass on to the next generation rather than die out.
- What is nondisjunction? Why can it cause genetic disorders? Nondisjunction is the failure of replicated chromosomes to separate properly during meiosis. Some of the resulting gametes will be missing all or part of a chromosome, while others will have an extra copy of all or part of the chromosome. If such a gamete is fertilized and forms a new individual, the individual is likely to have a serious genetic disorder.
- Explain why genetic disorders caused by abnormal numbers of chromosomes most often involve the X chromosome. Genetic disorders caused by abnormal numbers of chromosomes most often involve the X chromosome because the X and Y chromosomes are very different in size, making nondisjunction more frequent for the sex chromosomes.
- How is Down syndrome detected in utero? One way of detecting Down syndrome in utero is to extract a few fetal cells from the fluid surrounding the fetus and examine the fetal chromosomes. If an extra copy of chromosome 21 is present, the fetus has Down syndrome.
- Use the example of PKU to illustrate how the symptoms of a genetic disorder can sometimes be prevented. PKU is a genetic disorder in which the individual lacks a normal enzyme needed to break down the amino acid phenylalanine, which builds up in the body and causes the symptoms of PKU. If a low-phenylalanine diet is followed throughout life, the symptoms of PKU can be prevented.
- Explain how gene therapy works. Gene therapy works by inserting a normal gene in cells with a mutant gene, so the protein encoded by the gene can be synthesized in cells. A vector, such as a virus, is genetically engineered to deliver the normal gene by infecting cells. If the treatment is successful, the new gene delivered by the vector will allow the synthesis of a functioning protein.
- Compare and contrast genetic disorders and congenital disorders. Genetic disorders and congenital disorders both can be present at birth, but genetic disorders are specifically caused by problems in genes or chromosomes, while congenital disorders may be due to any cause.
- Explain why parents that do not have Down syndrome can have a child with Down syndrome. Answers may vary. Sample answer: Down syndrome is caused by a mistake during meiosis that produces gametes with an extra copy (complete or partial) of chromosome 21. It is only the parent’s gamete or gametes that are affected. If a gamete with this chromosomal abnormality goes on to create a zygote, the child that results will have Down syndrome.
- Hemophilia A and Turner’s syndrome both involve problems with the X chromosome. In terms of how the X chromosome is affected, what is the major difference between these two types of disorders? Answers may vary. Sample answer: Hemophilia A is a single gene mutation on the X chromosome, while Turner’s syndrome involves the loss of an entire X chromosome (XO).
- Can you be a carrier of Marfan syndrome and not have the disorder? Explain your answer. No, because Marfan syndrome is dominant. Even one copy of the gene will cause the disorder. Carriers refer to people with one copy of a recessive gene.
5.16 Genetic Engineering: Review Questions and Answers
- Define genetic engineering Genetic engineering is the use of technology to change the genetic makeup of living things for human purposes.
- What is recombinant DNA? Recombinant DNA is DNA that is formed by combining DNA from two different species of organisms.
- Identify the steps of gene cloning. The steps of gene cloning are isolation, ligation, transformation, and selection. During isolation, a gene is isolated by using an enzyme to break DNA. During ligation, another enzyme is used to combine the isolated gene with plasmid DNA from bacteria, producing recombinant DNA. In transformation, the recombinant DNA is inserted into another cell, usually a bacterial cell. During selection, transformed bacteria are grown to make sure they have the recombinant DNA and only those that do are selected for further use.
- What is the purpose of the polymerase chain reaction? The purpose of the polymerase chain reaction is to make many copies of a gene or other DNA segment. This might be done in order to have large quantities of the gene for genetic testing.
- Make a flow chart outlining the steps involved in creating a transgenic crop. Flow charts may vary but should include the following steps in creating a transgenic crop: a. Plasmid DNA is obtained from bacteria that infect plants. b. Recombinant DNA is created by combining a desired gene with the plasmid DNA from the bacteria. c. The recombinant DNA is re-inserted into a bacterium. d. The transformed bacterium is used to insert the recombinant DNA into the chromosome of a plant cell. e. The plant cell is grown in culture. f. A plant cell clone from the culture is used to generate a plant with the desired gene.
- Explain how bacteria can be genetically engineered to produce a human protein. To genetically engineer bacteria to produce a human protein, gene cloning is used to form recombinant DNA that contains the normal human gene for the protein and plasmid DNA from bacteria. The recombinant DNA is re-inserted into the bacteria. The bacteria can multiply rapidly and produce large amounts of the human protein.
- Identify an ethical, legal, or social issue raised bygenetic engineering. State your view on the issue, and develop a logical argument to support your view. Answers may vary but should identify an ethical, legal, or social issue raised by genetic engineering; a clearly stated view on the issue; and a logical argument to support the view. Sample topics might include health, safety, privacy, and environmental issues.
- Explain what primers are and what they do in PCR. Primers are short pieces of DNA that have a complementary base sequence to a DNA strand that is being used to make copies of a gene. Primers bind to the DNA strand during the annealing stage of PCR. Then an enzyme adds nucleotides to the primer to make new DNA molecules, which contain copies of the gene.
- The enzyme Taq polymerase was originally identified from bacteria that live in very hot environments, such as hotsprings. Why does this fact make Taq polymerase particularly useful in PCR reactions? Taq polymerase is particularly useful for PCR reactions because it can function in the hot temperatures necessary for PCR, due to the fact that it comes from bacteria that live in extremely hot environments.
5.17 The Human Genome: Review Questions and Answers
- Describe the human genome. The human genome refers to all the DNA of the human species. It consists of 3.3 billion base pairs divided into 20,500 genes on 23 pairs of chromosomes.
- What is the Human Genome Project? The Human Genome Project is a multi-billion dollar, international biological research project that began in 1990, continued to 2003, and involved researchers at 20 universities in several different countries.
- Identify two main goals of the Human Genome Project. Two main goals of the Human Genome Project were to sequence all of the DNA base pairs in the human genome, and to map the location and determine the function of all the genes in the human genome.
- What is the reference genome of the Human Genome Project? What is it based on? The reference genome of the Human Genome Project is the sequence of DNA base pairs in a complete set of human chromosomes. It is based on a combined mosaic of a small number of anonymous donors, all of European origin.
- Explain how knowing the sequence of DNA bases in the human genome is beneficial for molecular medicine. Knowing the sequence of DNA bases in the human genome is beneficial for molecular medicine because it is helping researchers identify mutations linked to different forms of cancer, yielding insights into the genetic basis of many diseases, such as cystic fibrosis, and helping researchers tailor medications to individual genotypes.
- What was one surprising finding of the Human Genome Project? Answers may vary. Sample answer: One surprising finding of the HGP was the relatively small number of genes in humans.
- Why do you think scientists didn’t just sequence the DNA from a single person for the Human Genome Project? Along those lines, why do you think it is important to include samples from different ethnic groups and genders in genome sequencing efforts? Answers may vary. Sample answer: Although all humans share the same basic genes, there is some variation in the specific sequences between individuals. If only one person was sequenced, that sequence would not necessarily be a good representative of the human species as a whole. That is why scientists sequenced several individuals and came up with a composite reference sequence. This is also why different ethnic groups and genders should be included in genome sequencing efforts, because the range of human variation should be represented to better reflect the genome of the human species as a whole.
- What is pharmacogenomics? Pharmacogenomics is the study of how an individual’s genes affect the way they respond to drugs.
- If a patient were to have pharmacogenomics done to optimize their medication, what do you think the first step would be? Answers may vary.Sample answer: I think the first step would be for the patient to have a test to find out the sequence of a gene or genes in their body that could affect how the medication is activated or deactivated.
- List one advantage and one disadvantage of pharmacogenomics. Answers may vary.Sample answer: One advantage of pharmacogenomics is that doctors might be able to find the most effective medication for a specific patient more quickly. One disadvantage is that this technique is currently often not covered by insurance and can be expensive.
- Explain how the sequencing of the human genome relates to ethical concerns about genetic discrimination. Answers may vary.Sample answer: By sequencing the human genome, genes associated with certain diseases can be discovered. This can lead to ethical concerns about potential discrimination against individuals with these genetic sequences, for instance by insurance companies or employers, who have a vested interest in having healthy clients or employees.
Chapter 5 Case Study Conclusion: Review Questions and Answers
- What are the differences between a sequence of DNA and the sequence of mature mRNA that it produces? Answers may vary. Sample answer: Directly after transcription, an RNA sequence is complementary to the DNA sequence that it is transcribed from, but RNA contains uracil (U) instead of the thymine (T) base that is used in DNA. Then the pre-mRNA is spliced to remove introns, possibly edited, and a “tail” of adenines is added through polyadenylation. Therefore, the mature mRNA sequence is significantly different than simply being the complementary sequence to the DNA sequence.
- Scientists sometimes sequence DNA that they “reverse transcribe” from the mRNA in an organism’s cells, which is called complementary DNA (cDNA). Why do you think this technique might be particularly useful for understanding an organism’s proteins versus sequencing the whole genome (i.e. nuclear DNA) of the organism? Answers may vary. Sample answer: I think this technique might be useful because the mRNA only contains the exons that code for amino acids and, ultimately, proteins. Nuclear DNA contains a lot of regions that do not code for proteins. Therefore, you might be able to gain insight into the proteins that an organism produces more quickly if you sequence the cDNA made from mRNA, rather than starting with the nuclear DNA of the entire genome.
- A person has a hypothetical Aa genotype. Answer the following questions about this genotype:
- What do A and a represent? A and a are different alleles of the same gene.
- If the person expresses only the phenotype associated with A, is this an example of complete dominance, codominance, or incomplete dominance? Explain your answer. Also, describe what the observed phenotypes would be if it were either of the two incorrect answers. This is an example of complete dominance because A completely dominates the phenotype over a. If it were codominance, the phenotypes for A and a would both be expressed. If it were incomplete dominance, you might see an intermediate phenotype that is between the phenotypes for A and a.
- Explain how a mutation that occurs in a parent can result in a genetic disorder in their child. Be sure to include which type of cell or cells in the parent must be affected in order for this to happen. Answers may vary. Sample answer: A gene mutation in a parent’s gametes, otherwise known as a germline mutation, can be passed down to their offspring. If this mutation results in a protein that does not function normally, it can cause a genetic disorder in the child.
- What is the term for an allele that is not expressed in a heterozygote? A recessive allele.
- What might happen if codons encoded for more than one amino acid? Answers may vary. Sample answer. If codons encoded for more than one amino acid, tRNA would bring various amino acids to the ribosome for each codon, resulting in varied proteins. These may have different functions and be detrimental to the organism.
- Explain why a human gene can be inserted into bacteria and can still produce the correct human protein, despite being in a very different organism. A human gene inserted into bacteria still produces the same human protein because the genetic code is universal, meaning that it is the same among all living organisms.
- What is gene therapy? Why is gene therapy considered a type of biotechnology? Gene therapy is an experimental technique to treat genetic disorders. In gene therapy, a normal gene is inserted into human cells to compensate for an abnormally functioning gene. This is often done using viruses as vectors to carry and insert the new DNA. Gene therapy is a type of genetic engineering because it involves changing the genetic makeup of an organism.