Answers to chapter 7 questions

Mastering Concepts

7.1

1. How did Griffith’s research, coupled with the work of Avery and his colleagues, demonstrate that DNA, not protein, is the genetic material?

Griffith’s research established that a lethal strain of bacteria (type S) could transfer a then-unknown molecule to nonlethal bacteria (type R) and confer the ability to kill mice. Avery and his colleagues added enzymes that destroyed either proteins or DNA to the mixtures that Griffith used in his experiments. These experiments showed that DNA, not protein, changed type R bacteria from nonlethal to lethal.

2. How did the Hershey–Chase “blender experiments” confirm Griffith’s results?

The Hershey-Chase “blender experiments” used radioactive sulfur to label the protein coats of one batch of bacteriophages and used radioactive phosphorus to label the DNA of another batch of bacteriophages. Both batches of viruses were allowed to infect bacteria. Then the solutions were separately blended at high speeds to separate viral protein coats from bacterial cells. Radioactively labeled bacteria were found only in the batches that had been infected by phages with radioactively labeled DNA. The protein-labeled phages did not transmit radioactivity to the bacteria they had infected. These experiments confirmed Griffith’s conclusion that DNA, not protein, is the genetic material.

7.2

1. What are the components of DNA and its three-dimensional structure?

A DNA molecule is composed of subunits called nucleotides. Each nucleotide is composed of a deoxyribose sugar bonded to a phosphate group and a nucleotide base (adenine, thymine, cytosine, or guanine). The three-dimensional structure of DNA is a double helix, which resembles a twisted ladder.

2. What evidence enabled Watson and Crick to decipher the structure of DNA?

The evidence included Rosalind Franklin’s X-ray diffraction photo of a crystal of DNA, plus Erwin Chargaff’s work that showed that DNA contains equal amounts of adenine and thymine and equal amounts of cytosine and guanine.

3. Identify the 3′ and 5′ ends of a DNA strand.

The 3’ and 5’ designators refer to opposite ends of a single DNA strand. The 5’ end has a phosphate group attached to the 5’ carbon atom, whereas the 3’ end has the sugar’s -OH (hydroxyl group) attached to the 3’ carbon atom.

7.3

1. What is the relationship between a gene and a protein?

A gene is a strand of DNA that encodes a protein.

2. What are the two main stages in protein synthesis?

Transcription and translation are the two main stages in protein synthesis.

3. What are the three types of RNA, and how does each contribute to protein synthesis?

Messenger RNA (mRNA) carries the instructions for building the protein; transfer RNA (tRNA) carries the appropriate amino acid to the ribosome; and ribosomal RNA (rRNA) is the major component of a ribosome, which is the structure where amino acids are assembled into polypeptides.

7.4

1. What happens during each stage of transcription?

The steps of transcription are initiation, elongation, and termination. During initiation, enzymes unzip the DNA, and RNA polymerase binds to the promoter. During elongation, RNA polymerase uses the DNA template strand to add complementary nucleotides to the 3’ end of the growing RNA strand. During termination, synthesis of the RNA molecule ends and the DNA molecule is “zipped” back into its double helix form.

2. Where in the cell does transcription occur?

Transcription occurs in the nucleus of a eukaryotic cell.

3. What is the role of RNA polymerase in transcription?

RNA polymerase is an enzyme that uses the DNA template to bind additional nucleotides to the 3’ end of the growing chain of RNA.

4. What are the roles of the promoter and terminator sequences in transcription?

The promoter signals the start of a gene, and the terminator signals the end of a gene. RNA polymerase recognizes the promoter and terminator, so it starts and stops transcription at the correct positions along the DNA template strand.

5. How is mRNA modified before it leaves the nucleus of a eukaryotic cell?

Before it leaves the nucleus of a eukaryotic cell, mRNA is altered in the following ways: a cap is added to the 5’ end of the mRNA molecule; a poly A tail is added to the 3’ end; introns are removed and exons are spliced together.

7.5

1. How did researchers determine that the genetic code is a triplet and learn which codons specify which amino acids?

Researchers knew that life uses four nucleotides and 20 amino acids.  They reasoned that the genetic code could not reflect 1-base or 2-base “words,” because neither could encode enough amino acids. A triplet code (3-base “words”) could potentially encode 64 amino acids, which is more than enough for the 20 amino acids found in biological proteins.
They deciphered the genetic code by adding synthetic mRNA molecules to test tubes containing all the ingredients needed for translation. They analyzed the sequences of the resulting polypeptides to determine which codons correspond to which amino acids. 

2. What happens in each stage of translation?

In initiation, ribosomal subunits bind to mRNA, and a tRNA carrying the first amino acid (methionine) attaches to the first codon. In elongation, the ribosome moves along the mRNA, adding new amino acids to the growing polypeptide. In termination, the ribosome reaches a stop codon and releases the last tRNA and the polypeptide. The ribosomal subunits then dissociate from the mRNA.

3. Where in the cell does translation occur?

Translation occurs at ribosomes, which are either free in the cytoplasm or attached to the rough ER.

4. How are polypeptides modified after translation?

Polypeptides must be folded to become functional proteins. In addition, sometimes amino acids are cut out of the chain, and sometimes multiple polypeptides join together.

7.6

1. What are some reasons that cells regulate gene expression?

Protein production costs a lot of energy; the regulation of gene expression avoids the production of unnecessary proteins and therefore saves energy.

2. How do proteins determine whether a bacterial operon is expressed?

A repressor protein binds to an operator and prevents the genes in the operon from being transcribed.

3. How do enhancers and transcription factors interact to regulate gene expression?

Transcription factors bind to certain DNA sequences to regulate transcription, for example by preparing a promoter site to bind RNA polymerase.  Transcription won’t occur without these factors.  Enhancers are sequences of DNA outside of the promoter.  Transcription factors can bind to the enhancers to help regulate gene expression.

4. What are some other ways that a cell controls which genes are expressed?

Cells can keep DNA coiled or attach methyl groups that inactivate genes. After transcription, different combinations of introns can be removed.  mRNA can be confined to the nucleus or rapidly degraded.  Proteins can also be degraded or modified in processing.

7.7

1. What is a mutation?

A mutation is a change in a DNA sequence.

2. What are the types of mutations, and how does each alter the encoded protein?

In a substitution mutation, one DNA base is replaced with another. The mutation may be have no effect on the resulting protein (silent mutation), change one amino acid (missense mutations), or create a stop codon in the middle of the mRNA (nonsense mutation). Insertions and deletions add or remove nucleotides; they often shift the “reading frame” of a gene. Such a frameshift mutation may alter many amino acids in the protein, drastically changing its shape and function. An insertion of three nucleotides adds one amino acid to the encoded protein, and a deletion of three nucleotides removes one amino acid. Expanding repeat mutations increase the number of copies of three-or four-nucleotide sequences over several generations. This causes extra amino acids to be inserted into a protein, deforming it. Large-scale mutations delete, duplicate, or invert large portions of a chromosome. The effects depend on whether genes are disrupted.

3. What causes mutations?

Mutations are often caused by DNA replication errors, exposure to chemicals or radiation, and transposons. Large-scale mutations may result from errors in meiosis.

4. What is the difference between a germline mutation and a somatic mutation?

A germline mutation is one that occurs in a cell that will give rise to a sperm or an egg cell. A somatic mutation occurs within a non-germline body cell.

5. How are mutations important?

Some mutations cause diseases. Mutations also produce genetic variability, which is the raw material of evolution. Scientists induce mutations to learn how genes normally function and to develop new varieties of crop plants.

 

7.8

1. What question about the FOXP2 gene were the researchers trying to answer?

Researchers wanted to know how the human version of the FOXP2 gene differs from that of other primates. They also wanted to know if human-specific mutations could be linked to the acquisition of language.

2. What insights could scientists gain by intentionally mutating the FOXP2 gene in a developing human? Would such an experiment be ethical?

Many answers are possible, but one idea would be to mutate the FOXP2 gene so that it is nonfunctional at different stages of development to learn whether it is active through development or just in a critical window.  Such an experiment would not be ethical.

 

Write It Out
1. Explain how Griffith’s experiment and Avery, MacLeod, and McCarty’s experiment determined that DNA in bacteria transmits a trait that kills mice.

Some strains of Streptococcus pneumoniae bacteria (type S) cause pneumonia, whereas others (type R) do not. Griffith’s experiment determined that heat-killed type S bacteria can transform type R bacteria into pneumonia-causing killers. Avery, MacLeod, and McCarty’s followup experiment determined that DNA, not proteins, from the dead type S bacteria altered the type R bacteria. When heat-killed type S bacteria were treated with a protein-destroying enzyme, the type R bacteria still became killers. But when type S bacteria were treated with DNA-destroying enzymes, the type R bacteria remained harmless.

2. Describe the three-dimensional structure of DNA.

DNA is a double helix that resembles a twisted ladder.  In this molecule, the “twin rails” of the ladder are alternating units of deoxyribose and phosphate, and the ladder’s rungs are A-T and G-C base pairs joined by hydrogen bonds.

3. Explain Chargaff’s observation that a DNA molecule contains equal amounts of A and T and equal amounts of G and C. 

DNA has two complementary strands. Each adenine (A) on one strand pairs with a thymine (T) on the opposite strand. Likewise, each guanine (G) on one strand pairs with a cytosine (C) on the other strand. Therefore, DNA has one T for every A and has one C for every G. 

4. Write the complementary DNA sequence of each of the following base sequences:
a.         A G G C A T A C C T G A G T C
b.         G T T T A A T G C C C T A C A
c.         A A C A C T A C C G A T T C A

The complementary sequences are:
a) TCCGTATGGACTCAG
b) CAAATTACGGGATGT
c) TTGTGATGGCTAAGT

5. Put the following in order from smallest to largest: nucleotide, genome, nitrogenous base, gene, nucleus, cell, codon, chromosome.

From smallest to largest, the order is nitrogenous base, nucleotide, codon, gene, chromosome, nucleus, and cell.

6.  What is the function of DNA?

The function of much of the DNA in a cell is not known, but some of it encodes the cell’s RNA and proteins. 

7. Use figure 7.9 to describe the structural and functional differences between RNA and DNA.

RNA nucleotides contain a sugar called ribose; DNA nucleotides contain a similar sugar called deoxyribose. RNA has the nitrogenous base uracil, which behaves similarly to the thymine in DNA - that is, both uracil and thymine form complementary base pairs with adenine. RNA can be single-stranded; DNA is double-stranded.  RNA can catalyze chemical reactions, a role not known for DNA.

8. Explain how information in DNA is transcribed and translated into amino acids.

Transcription copies the information encoded in a DNA base sequence into the complementary language of mRNA. Once transcription is complete and mRNA is processed, the cell is ready to translate the mRNA message into a sequence of amino acids that builds a protein. Transcription occurs in the nucleus, and translation occurs at ribosomes in the cytoplasm.

9. Some people compare DNA to a blueprint stored in the office of a construction company. Explain how this analogy would extend to transcription and translation.

Transcription would be the process of scanning or copying the blueprints so that the contractor would have a set at the construction site.  Translation would be the process of the contractor directing the assembly of all the raw materials at the site into the finished building.

10. List the three major types of RNA and their functions.

Messenger RNA (mRNA) carries the information that specifies a protein.  Ribosomal RNA (rRNA) combines with proteins to form a ribosome, the physical location of protein synthesis.  Transfer RNA (tRNA) molecules are “connectors” that bind mRNA codons at one end and specific amino acids at the other. Their role is to carry each amino acid to the ribosome at the correct spot along the mRNA molecule.

11. List the sequences of the mRNA molecules transcribed from the following template DNA sequences:
a.         T G A A C T A C G G T A C C A T A C
b.         G C A C T A A A G A T C

The complementary sequences are:
a) ACUUGAUGCCAUGGUAUG
b) CGUGAUUUCUAG

12. How many codons are in each of the mRNA molecules that you wrote for question 11?

a. 6 codons
b. 4 codons

13. Refer to the figure to answer these questions:
a. Add labels for mRNA (including the 5’ and 3’ ends) and tRNA. In addition, draw the RNA polymerase enzyme and the ribosomes, including arrows indicating the direction of movement for each.
b. What are the next three amino acids to be added to polypeptide b?
c. Fill in the nucleotides in the mRNA complementary to the template DNA strand.
d. What is the sequence of the DNA complementary to the template strand (as much as can be determined from the figure)?
e. Does this figure show the entire polypeptide that this gene encodes? How can you tell?
f. What might happen to polypeptide b after its release from the ribosome?
g. Does this figure depict a prokaryotic or a eukaryotic cell? How can you tell?

a. Refer to figures 7.10 (Transcription Creates mRNA) and 7.15 (Translation Creates the Protein).
b. Lys-Gly-Ser
c. The remaining mRNA nucleotides are (from left to right):  CUUAGGACACC
d. The complementary DNA sequence is (from left to right):  CTTAGGACACC
e. No, because the last codon would be a stop codon (UAA, UAG, or UGA)
f. The peptide would fold into its proper shape and then either begin performing its function in the cell or be exported to the cell’s exterior.
g. The figure depicts a prokaryotic cell. In eukaryotes, the mRNA is fully synthesized in the nucleus, undergoes processing, and then is transcribed in the cytoplasm. The figure shows translation occurring simultaneously with transcription, which only occurs in prokaryotes.

14. Is changing the first nucleotide in a codon more likely or less likely to change the encoded amino acid than changing the third nucleotide in a codon?

Consult the dictionary of the genetic code. Changing the first nucleotide in a codon typically changes the encoded amino acid. In contrast, changing the third nucleotide of a codon often does not change the encoded amino acid (e.g., look at the codons for serine, proline, and alanine).

15. Titin is a muscle protein whose gene has the largest known coding sequence—80,781 DNA bases. How many amino acids long is titin?

The titan protein is 26,927 amino acids (80,781 nucleotides divided by 3 nucleotides per amino acid).

16. If a protein is 1259 amino acids long, what is the minimum size of the gene that encodes the protein? Why might the gene be longer than the minimum?

1259 x 3 = 3,777 bases plus three bases for stop codon = 3,780 bases.  The gene would have bases for the leader sequence on the mRNA and might include any number of introns.

17. How did researchers reason that a combination of at least three RNA bases must specify each amino acid?

Since RNA has four types of bases and proteins have 20 types of amino acids, one RNA base could not specify each amino acid. If a combination of two RNA bases specified one amino acid, then only 16 amino acids could be encoded (four possibilities for position 1 of the codon multiplied by four possibilities for position 2 equals 16 combinations of RNA bases). Therefore, at least three RNA bases must specify each amino acid (4x4x4=64). Later studies confirmed that each codon contains three RNA bases.

18. The roundworm C. elegans has 556 cells when it hatches. Each cell contains the entire genome but expresses only a subset of the genes. Therefore, the cells “specialize” in particular functions. List all of the ways that a roundworm cell might silence the unneeded genes.

An individual roundworm cell can keep some of its DNA coiled or attach methyl groups to inactivate genes. Transcription factors and enhancers needed for transcription might not be available. After transcription, different combinations of introns can be removed.  mRNA can be confined to the nucleus or rapidly degraded.  The proteins can also be degraded.

19. The genome of the human immunodeficiency virus (HIV) includes nine genes. Two of the genes encode four different proteins each. How is this possible?

The genes each contain several introns.  To make each protein, a different combination of introns is removed with the remaining mRNA spliced together.

20. The shape of a finch’s beak reflects the expression of a gene that encodes a protein called calmodulin. A cactus finch has a long, pointy beak; its cells express the gene more than a ground finch, which has a short, deep beak. When researchers boosted gene expression in a ground finch embryo, the bird’s upper beak was longer than normal. Develop a hypothesis that explains this finding.

One possibility is that the calmodulin gene influences the length of the upper beak. Boosting calmodulin expression in the ground finch would therefore promote additional growth in the bird’s upper beak. Perhaps the ground finch’s lower beak was unaffected because other genes influence its size.

21. If a gene is like a cake recipe, then a mutation is like a cake recipe containing an error. List the major types of mutations, and describe an analogous error in a cake recipe.

Missense: instead of baking powder, the recipe lists baking soda.  Nonsense: the recipe cuts off after a partial list of ingredients. Insertion (3 nucleotides): the recipe lists one extra ingredient. Deletion (three nucleotides): the recipe leaves out one ingredient.  Frameshift: the word spacing is altered, e.g., flour, wate, regg, ssuga, rsalt etc.  Expanding repeat: the recipe lists an ingredient repeatedly.

22. A protein-encoding region of a gene has the following DNA sequence:
T T T C A T C A G G A T G C A A C T
Determine how each of the following mutations alters the amino acid sequence:
a.         substitution of an A for the T in the first position
b.         substitution of a G for the C in the 17th position
c.         insertion of a T between the fourth and fifth DNA bases
d.         insertion of a GTA between the 12th and 13th DNA bases
e.         deletion of the first DNA nucleotide

a. Nonsense mutation; instead of encoding the amino acid lysine, the codon would recruit a release factor protein.
b. Missense mutation; instead of incorporating the amino acid cysteine, the protein would incorporate serine.
c. Frameshift mutation; valine is replaced by aspartic acid, and the remainder of the protein is disrupted.
d. Insertion mutation; the amino acid histidine is added within the protein.
e. Frameshift mutation; the entire protein is disrupted.

23. Explain how a mutation in a protein-encoding gene, an enhancer, or a gene encoding a transcription factor can all have the same effect on an organism.

A mutation in the gene can lead to a polypeptide that is too short or has the wrong amino acids; in either case it will not fold properly, and therefore will not function properly.  This means that the organism will not express the effects of that protein.  A mutation to either an enhancer or a gene encoding a transcription factor can leave the transcription factor unable to bind to the gene, blocking transcription.

24. How can a mutation alter the sequence of DNA bases in a gene but not produce a noticeable change in the gene’s polypeptide product? How can a mutation alter the amino acid sequence of a polypeptide yet not alter the organism?

A mutation may alter the sequence of a gene but not produce a noticeable change in the gene’s polypeptide sequence because several different codons encode most amino acids.  A mutation may alter the amino acid sequence but not alter the phenotype because the protein’s shape may not change, other proteins may take over the altered protein’s function, or the protein may not be essential.

25. Describe the mutation shown in figure 7.26 and explain how the mutation affects the amino acid sequence encoded by the gene.

Figure 7.26 shows a deletion mutation. Since exactly three nucleotides are deleted, the reading frame of the gene remains the same. One amino acid is deleted from the protein.

26. Parkinson disease causes rigidity, tremors, and other motor symptoms. Only 2% of cases are inherited, and these tend to have an early onset of symptoms. Some inherited cases result from mutations in a gene that encodes the protein parkin, which has 12 exons. Indicate whether each of the following mutations in the parkin gene would result in a smaller protein, a larger protein, or no change in the size of the protein.
a. deletion of exon 3
b. deletion of six consecutive nucleotides in exon 1
c. duplication of exon 5
d. disruption of the splice site between exon 8 and intron 8
e. deletion of intron 2

a) Smaller protein.  b) Smaller protein.  c) Larger protein.  d) No change.  e) No change.

27. Consult the genetic code to write codon changes that could account for the following changes in amino acid sequence.
a. tryptophan to arginine
b. glycine to valine
c. tyrosine to histidine

Multiple answers are possible; these are examples. a) UGG to CGG. b) GGU to GUU.  c) UAC to CAC.

28. Researchers use computer algorithms that search DNA sequences for indications of specialized functions. Explain the significance of detecting the following sequences:
a. a promoter
b. a sequence of 75 to 80 nucleotides that folds into a backwards letter L
c. RNAs with poly A tails

a) A promoter signals the start of a gene.  b) These nucleotides compose a tRNA molecule.  c) The poly A tails signal an mRNA.

29. In a disorder called gyrate atrophy, cells in the retina begin to degenerate in late adolescence, causing night blindness that progresses to blindness. The cause is a mutation in the gene that encodes an enzyme, ornithine aminotransferase (OAT). Researchers sequenced the OAT gene for five patients with the following results:
• Patient A: A change in codon 209 of UAU to UAA
• Patient B: A change in codon 299 of UAC to UAG
• Patient C: A change in codon 426 of CGA to UGA
• Patient D: A two-nucleotide deletion at codons 64 and 65 that results in a UGA codon at position 79
• Patient E: Exon 6, including 1071 nucleotides, is entirely deleted.

a. Which patient(s) have a frameshift mutation?
b. How many amino acids is patient E missing?
c. Which patient(s) will produce a shortened protein?

a) Patient D.   b) 357 amino acids.  c) All will produce a shortened protein.

 

Pull It Together

1.  Why is protein production essential to cell function?

Cell structure and function depend on proteins.  Enzymes are proteins and are required for almost all chemical reactions to occur within a cell.  Without enzymes, the cell could not synthesize ATP, which the cell uses for energy. In addition, proteins embedded within cell membranes have several important functions such as adhesion, cell recognition, and transport of water-soluble molecules; without protein production, new cell membrane proteins could not be produced when the cell divides.

2.  Where do promoters, terminators, stop codons, transcription factors, RNA polymerase, and enhancers fit into this concept map?

Both “Transcription factors” and “RNA polymerase” can connect with the phrase “bind to” to “Promoters.” Both “Promoters” and “Terminators” can connect with the phrase “are non-coding sequences of” to “DNA.” “Promoters” can also connect with the phrase “signals the starting point for ” to “Transcription.” “Terminators” can also connect with the phrase “signals the end point for” to “Transcription.” “Stop codons” can connect with the phrase “ends the process of” to “Translation.” “Transcription factors” can connect with the phrase “bind to” to “Enhancers.”

3.  Use the concept map to explain how DNA nucleotides are related to amino acids.

DNA nucleotides are transcribed to RNA nucleotides. An mRNA molecule is divided into three-nucleotide codons, each of which corresponds to one amino acid.

4.  Use the concept map to explain why a mutation in DNA sometimes causes protein function to change.

A mutation is a change in a DNA sequence.  If the mutation leads to a change in the encoded amino acid sequence, the protein’s shape could be altered or destroyed. Therefore, mutations could lead to changes in protein function.  (A gene that undergoes a neutral mutation, however, encodes the same amino acid sequence. The protein’s function therefore does not change.)

 

Answers to Mastering Concepts Questions

10.1
1. How are chromosomes, DNA, genes, and alleles related?

Chromosomes contain tightly packed DNA and associated proteins.  DNA consists of strands of genetic material that contain genes, sequences of nucleotides that code for amino acids.  Those genes come in varieties called alleles.

2. How do meiosis, fertilization, diploid cells, and haploid cells interact in a sexual life cycle?

Meiosis in the adult organism creates the haploid gamete cells that combine during fertilization to form the diploid zygote cell.  That cell undergoes mitosis to make the cells that are necessary for growth into the adult form.

10.2
1. Why did Gregor Mendel choose pea plants as his experimental organism?

Mendel chose pea plants because they are easy to grow, develop quickly, produce many offspring, and have many traits that appear in two alternate forms that are easy to distinguish.  It also is easy to hand-pollinate pea plants, so an investigator can control which plants mate with one another.

2. Distinguish between dominant and recessive; heterozygous and homozygous; phenotype and genotype; wild-type and mutant.

Dominant alleles appear in a phenotype whenever they are present; recessive alleles contribute to the phenotype only if no dominant alleles are present.  An individual is homozygous for a gene if both alleles are identical; in a heterozygous individual, the two alleles for a gene are different.  An organism’s phenotype is its appearance; the genotype is the alleles an individual possesses. The wild type allele is the most common form of a gene in a population; a mutant allele arises when a gene undergoes a mutation.

10.3
1. What is a monohybrid cross, and what are the genotypic and phenotypic ratios expected in the offspring of the cross?

A monohybrid cross is a mating between two individuals that are both heterozygous for one gene. The genotypic ratio expected in a monohybrid cross is 1:2:1; the phenotypic ratio is 3:1.

2. What is a test cross, and why is it useful?

A test cross is a mating between a homozygous recessive individual and an individual of unknown genotype. The genotype of the unknown parent can be deduced from the ratio of phenotypes in the F1 generation.

3. How does the law of segregation reflect the events of meiosis?

The law of segregation reflects the movement of homologous chromosomes into separate cells during meiosis I.

10.4
1. What is a dihybrid cross, and what is the phenotypic ratio expected in the offspring of the cross?

In a dihybrid cross, two individuals that are heterozygous for two genes are mated. The phenotypic ratio that is expected is 9:3:3:1.

2. How does the law of independent assortment reflect the events of meiosis?

The law of independent assortment reflects that each homologous pair of chromosomes aligns independently of other chromosome pairs during metaphase I of meiosis. 

3. How can the product rule be used to predict the results of crosses in which multiple genes are studied simultaneously?

The product rule allows you to estimate the odds that an offspring will have a certain combination of alleles for multiple genes by multiplying the probability that each separate event will occur.

10.5
1. How do linked genes complicate patterns of inheritance?

When pairs of genes are linked, they are carried on the same chromosome and are inherited together. Crossing over complicates the inheritance of linked genes; sometimes allele combinations differ from either parent. The inheritance pattern of non-linked genes is more predictable since it is not affected by crossing over. The inheritance of non-linked genes can be visualized using a Punnett Square.

2. How do recombinant and parental chromatids arise?

Recombinant chromatids are chromosomes that have a mixture of maternal and paternal alleles instead of alleles from just a single parent. In contrast, parental chromatids carry the same combinations of alleles that were inherited from the parents. Crossing over has not altered them.

3. Explain how to use crossover frequencies to make a linkage map.

The farther apart genes are on a chromosome, the more frequently they will cross over. By comparison, genes that are close together on a chromosome are less likely to be separated. Analysis of how often the traits appear together helps to establish linkage maps, which show the relative positions of genes on chromosomes.

10.6
1. How do incomplete dominance and codominance increase the number of phenotypes?

Incomplete dominance and codominance produce phenotypes that are intermediate between or combinations of those produced by homozygous dominant or homozygous recessive individuals.

2. What is pleiotropy?

Pleiotropy occurs when a gene produces multiple phenotypic expressions. Pleiotropy results when the protein encoded by a gene enters several different biochemical pathways or affects more than one body part or process.

3. How can the same phenotype stem from many different genotypes?

Each gene encodes one protein, but many different proteins may interact in a single metabolic pathway.  A mutation in a gene encoding any of these proteins may produce a flawed metabolic pathway.  In this way, different genotypes can produce the same phenotype (failure of the metabolic pathway to operate properly).

4. How can gene interactions reduce the number of phenotypes?

If one gene affects the expression of another, the gene interaction may cause some phenotypes to appear to be missing from a population because they may not always be expressed.

10.7
1.What is the role of the Y chromosome in human sex determination?

In humans, the Y chromosome includes a sex-determining gene, which encodes a protein that acts as a master switch.  The protein turns on other genes, which direct the undeveloped testes to secrete the male sex hormone testosterone. The protein also turns on a gene encoding a protein that causes embryonic female structures to disassemble.  If the sex-determining gene is not present, an embryo will develop as a female.

2. Why do males and females express recessive X-linked alleles differently?

Each female has a pair of X chromosomes, whereas a male has only one X chromosome. Any trait a male has on its X chromosome will be expressed. Recessive alleles on an X chromosome of a female may be masked by dominant alleles on its homologous X chromosome.

3. How does X inactivation in mammals equalize the contributions of X-linked genes between the sexes?

X inactivation happens to one of the two copies of a gene on the homologous X chromosomes.  Only females have two copies of the X chromosome. Inactivation of one would make it similar to the single X in a male.

10.8
1. What is the difference between autosomal dominant and autosomal recessive modes of inheritance?

Autosomal dominant traits show affected individuals in every generation and all affected individuals have at least one affected parent.  Autosomal recessive conditions show a pattern in which affected individuals can have normal parents, and the condition often skips generations. 

2. How are pedigrees helpful in determining a disorder’s mode of inheritance?

Pedigrees track a trait through multiple generations and allow the pattern of transmission and inheritance to be studied. Pedigrees also may help predict the appearance of the trait in future generations.

10.9
1. How can the environment affect a phenotype?

Environment can affect a phenotype in a variety of ways.  Temperature can influence gene expression of temperature-sensitive alleles; infectious agents can intensify a genetic disorder; upbringing and nourishment will affect temperament and physical health.

2. What is a polygenic trait?

A polygenic trait is one that is controlled by many genes.

10.10
1. Explain the logic of planting non-Bt-crop buffer strips around fields planted with Bt crops.

The buffer strip creates a feeding zone for non-Bt-resistant larvae.  These moths are likely to mate with the Bt-resistant varieties surviving in the field.  Since both Bt-resistant and non-Bt-resistant moths survive in the population, this strategy should reduce the likelihood that Bt-resistance will increase significantly in the population.

2. How did the researchers use a feeding experiment to show that Bt resistance alleles in pink bollworms are recessive?

Researchers used individuals that had been fed Bt-laced meals so resistance to Bt was known.  Researchers knew that if the resistant allele was recessive then matings between moths heterozygous for resistance with moths fully resistant (homozygous recessive) should show a phenotypic ratio in the offspring of approximately 1:1.  Experiments revealed that ,indeed, approximately 50% of the offspring thrived while 50% either died or were quite small.

3. If farmers stop planting buffer strips, how will the incidence of resistance alleles in pink bollworm populations change?

Without the refuge strip resistant, moths would only have other resistant moths to mate with; allele frequencies would shift toward the recessive (resistance) allele in future generations.  If this occurs then Bt as a pesticide in corn will no longer be useful.

 

Answers to Write It Out Questions

1. What advantages do pea plants have for studies of inheritance? Why aren’t humans equally suitable?

Pea plants are easy to grow, develop rapidly, produce many offspring, and have many traits that appear in two easily distinguishable forms. In addition, it is easy to control genetic crossing in pea plants.  Humans cannot be used because they take longer to reach sexual maturity, do not produce an abundance of offspring, and cannot be forced to mate to suit the objectives of an experiment.

2. Some people compare a homologous pair of chromosomes to a pair of shoes. Explain the similarity. How would you extend the analogy to the sex chromosomes for females and for males?

Shoes come in all kinds of varieties: sandals, boots, sneakers, but they are paired with their matching shoe which will be the same size, and have straps or laces, rubber treads or uppers all in the same places and of the same materials.  Similarly, homologous chromosomes are the same length and shape with the same genes in the same places.  The sex chromosomes of males are not homologous, however, and would be like an adult size 11 sneaker paired with a child’s size 3 sandal.  In a female, the shoes would be homologous and would match.

3. In an attempt to breed winter barley that is resistant to barley mild mosaic virus, agricultural researchers cross a susceptible domesticated strain with a resistant wild strain. The F1 plants are all susceptible, but when the F1 plants are crossed with each other, some of the F2 individuals are resistant. Is the resistance allele recessive or dominant? How do you know?

The resistance allele is recessive because it was not expressed in the F1 generation but was expressed in some of the plants in the F2 generation.

4. Given the relationship between genes, alleles, and proteins, how can a recessive allele appear to “hide” in a heterozygote?

A recessive allele often encodes a nonfunctional protein.  A heterozygous individual has one dominant and one recessive allele, but the recessive allele appears to “hide” because the cell has enough of the normal protein (encoded by the dominant allele) to function properly.

5. Many plants are polyploid (see chapter 9); that is, they have more than two sets of chromosomes. How would having four (rather than two) copies of a chromosome more effectively mask expression of a recessive allele?

The extra chromosomes will provide additional opportunities for a dominant allele to mask the expression of a recessive allele.

6. Springer spaniels often suffer from canine phosphofructokinase (PFK) deficiency. The dogs lack an enzyme that is crucial in extracting energy from glucose molecules. Affected pups have extremely weak muscles and die within weeks. A DNA test is available to identify male and female dogs that are carriers. Why would breeders wish to identify carriers if these dogs are not affected?

It would be beneficial because breeders could prevent carriers from mating, thus reducing the incidence of this disease in the dogs.

7. How did Mendel use evidence from monohybrid and dihybrid crosses to deduce his laws of segregation and independent assortment? How do these laws relate to meiosis?

From his series of monohybrid crosses, Mendel concluded that genes occur in alternative forms (alleles) and that each individual inherits two alleles for each gene. His law of segregation states that two alleles of the same gene separate as they are packaged into gametes. This law reflects meiosis because homologous chromosomes are pulled into separate cells during meiosis I.  From his series of dihybrid crosses, Mendel developed the law of independent assortment, which states that during gamete formation, the segregation of the alleles of one gene does not influence the segregation of the alleles for another gene. This law reflects meiosis (as long as the two genes being studied reside on different chromosomes) because the orientation of each homologous pair of chromosomes does not affect the orientation of other homologous pairs during meiosis I. 

8. In a dihybrid cross, the predicted phenotype ratio is 9:3:3:1; the “9” represents the proportion of plants expressing at least one dominant allele for both traits. How would you use test crosses to determine whether these plants are homozygous dominant or heterozygous for one or both genes?

A test cross is a mating with a homozygous recessive individual.  In this case, you would obtain a plant that was homozygous recessive for both alleles.  If a plant is homozygous dominant for both genes, all of the offspring will have the dominant phenotype for both traits.  If the plant is heterozygous for either gene, about half the offspring will exhibit the recessive phenotype for that trait. 

9. A white woman with fair skin, blond hair, and blue eyes and a black man with dark brown skin, dark hair, and brown eyes have fraternal twins. One twin has blond hair, brown eyes, and light skin, and the other has dark hair, brown eyes, and dark skin. What Mendelian law does this real-life case illustrate?

This scenario represents Mendel’s principle of independent assortment.

10. The radish has nine groups of traits. Within each group, dihybrid crosses do not yield a 9:3:3:1 phenotypic ratio. Instead, such crosses yield an overabundance of phenotypes like those of the parents. What does this information reveal about the chromosomes of this plant?

The information reveals that at least some of the genes are located on the same chromosome.

11. How does gene linkage interfere with Mendel’s law of independent assortment? Why doesn’t the inheritance pattern of linked genes disprove Mendel’s law?

Within each linkage group, dihybrid crosses did not produce the proportions of offspring that Mendel’s law of independent assortment predicts. Scientists eventually realized that each linkage group was simply a set of genes transmitted together on the same chromosome.  This observation does not disprove Mendel’s law of independent assortment, which applies only when genes are located on different chromosomes.

12. How does crossing over “unlink” genes?

Crossing over separates alleles that occurred together on the same chromatid, so that alleles that were previously linked are no longer transmitted together.

13. If two different but linked genes are located very far apart on a chromosome, how may the inheritance pattern create the appearance of independent assortment?

Since the genes are very far apart on the chromosome, they have a high probability of being separated by crossing over.

14. Explain how each of the following appears to disrupt Mendelian ratios: incomplete dominance, codominance, pleiotropy.

Incomplete dominance:the heterozygote’s phenotype is intermediate between those of the two homozygotes.  This goes against the idea that two alleles should produce only two phenotypes, with one allele dominant over the other.  Instead of a 3:1 phenotypic ratio, the ratio is 1:2:1.
Codominance:  the heterozygote fully expresses two different and equally expressed alleles. This goes against the idea that two alleles should produce only two phenotypes, with one allele dominant over the other. Instead of a 3:1 phenotypic ratio, the ratio is 1:2:1.
Pleiotropy:  one gene has multiple phenotypic expressions. Mendel’s laws imply that each gene controls only one trait.  One allele can therefore change the phenotype in multiple ways.

15. Suppose a single trait is controlled by a gene with four codominant alleles. A person can inherit any combination of two of the four alleles. How many phenotypes are possible for this trait?

If the alleles are labeled A, B, C, and D, the following allele combinations are possible:  AA, AB, AC, AD, BB, BC, BD, CC, CD, and DD.  Ten phenotypes are possible.

16. What is the role of the Y chromosome in human sex determination?

The Y chromosome contains the SRY gene that acts as a switch for other sex determining genes that then activate in the embryo so that it develops as a male and dismantles all female embryonic structures.

17. Do you agree with the statement that all alleles on the Y chromosome are dominant? Why or why not?

No. One might be tempted to answer “yes” because the Y chromosome is not homologously paired, so all alleles on the Y chromosome are expressed. However, recessive alleles are still “recessive” even if no dominant allele can mask them. A recessive allele encodes a nonfunctional protein. If an allele on the Y chromosome encodes a nonfunctional protein, then the allele is recessive. 

18. Suppose a fetus has X and Y chromosomes but lacks receptors for the protein encoded by Y chromosome’s sex-determining gene. Will the fetus develop as a male or as a female? Explain your answer.

The fetus will develop as a female. Without these receptors, the signal to develop as a male will never be received.

19. How are X-linked genes inherited differently in male and female humans?

Whereas a female inherits two X chromosomes, a male inherits his single X chromosome from his mother.  A male expresses every allele (dominant or recessive) on his X chromosome because he lacks a second allele that could mask the expression of recessive alleles.

20. What does X inactivation accomplish?

In X inactivation, all but one X chromosome is shut off in each cell, a process that happens early in the embryonic development of a mammal. Which X chromosome is inactivated is a random event.  This prevents female mammals with two X chromosomes from expressing more X-linked genes than a male.

21. Rett syndrome is a severe X-linked recessive disorder that affects mostly female children. How does X inactivation explain this observation?

Because the disorder is severe, most males die as a result of inheriting the recessive allele.  Females who are heterozygous, however, will have the dominant allele inactivated in some cells, leaving the recessive allele to be expressed.  The effects may not be lethal since the recessive allele is inactivated in about half of the cells, but the disease will be severely debilitating.

22. A family has an X-linked dominant form of congenital generalized hypertrichosis (excessive hairiness). Although the allele is dominant, males are more severely affected than females. Moreover, the women in the family often have asymmetrical, hairy patches on their bodies. How does X chromosome inactivation explain this observation?

A female is a mosaic for X-linked genes because the maternal or paternal X chromosome is inactivated at random in each cell.

23. Why are male calico cats rare?

In cats, the genes encoding black and orange fur are located only on the X chromosome.  Calico cats result from the random inactivation of black and orange alleles.  Male calico cats are unusual because they would have to be XXY. 

24. Study the following pedigree.  Is the disorder’s mode of inheritance autosomal dominant, autosomal recessive, or X-linked recessive? Explain your reasoning.

The mode of inheritance is autosomal dominant. The disorder cannot be X-linked recessive since individual 7 on line II, who received only one X from her affected father, expresses the disorder. Notice that the pedigree has no carriers; every individual that inherits an allele of the disorder expresses the disorder. The allele conferring the disorder must therefore be dominant.

25. Pedigree charts can sometimes be difficult to construct and interpret. People may refuse to supply information, and adoption or serial marriages can produce blended families. Artificial insemination may involve anonymous sperm donors. Many traits are strongly influenced by the environment. How does each of these factors complicate the use of pedigrees?

If people refuse to supply medical information, it can be impossible to tell who is affected and who is not.  Blended families and artificial insemination make it impossible to trace parentage. 

26. Explain the following “equation”:
Genotype + Environment = Phenotype

Genotype represents what proteins will be produced and how they will interact with each other, but the environment often affects how those proteins will express themselves or when the genes will be activated and inactivated.  The combination of all these factors will determine the actual physical expression, or phenotype.

27. Mitochondria and chloroplasts contain DNA that encodes some proteins essential to life. These organelles are inherited via the female parent’s egg. Do you expect these genes to follow Mendelian laws of inheritance? Explain your answer.

Mendelian laws of inheritance rely on the separation of homologous pairs (law of independent assortment) and alleles within a gene pair (law of segregation).  Both of these separation events are the result of spindle fibers separating chromosomes in the stages of meiosis.  Chloroplasts and mitochondria do not undergo meiosis and so their DNA is not subject to the Mendelian laws of inheritance.

 

Answers to Genetics Problems

1. Wild-type canaries are yellow. A dominant mutant allele of the color gene, designated W, causes white feathers. Inheriting two dominant alleles is lethal to the embryo. If a yellow canary is crossed to a white canary, what is the probability that an offspring will be yellow? What is the probability that it will be white?

The probability is 50% chance for each color.

2. In humans, more than 100 forms of deafness are inherited as recessive alleles on many different chromosomes. Suppose that a woman who is heterozygous for a deafness gene on one chromosome has a child with a man who is heterozygous for a deafness gene on a different chromosome. Does the child face the general population risk of inheriting either form of deafness or the 25% chance that Mendelian ratios predict for a monohybrid cross? Explain your answer.

No.  The child faces a 25% chance of inheriting both recessive alleles.  The chance that both of those alleles are of the same gene, and lead to a dominant phenotype is much lower.

3. A man and a woman each have dark eyes, dark hair, and freckles. The genes for these traits are on separate chromosomes. The woman is heterozygous for each of these genes, but the man is homozygous. The dominance relationships of the alleles are as follows:
B = dark eyes; b = blue eyes
H = dark hair; h = blond hair
F = freckles; f = no freckles

a. What is the probability that their child will share the parents’ phenotype?
b. What is the probability that the child will share the same genotype as the mother? As the father?
Use the product rule or a Punnett square to obtain your answers.  Which method do you think is easier?

1. 100%
2. 1/8 chance that a child will have the same genotype as either parent

The product rule is an easier method.

4. Genes J, K, and L are on the same chromosome. The crossover frequency between J and K is 19%, the crossover frequency between K and L is 2%, and the crossover frequency between J and L is 21%. Use this information to create a linkage map for the chromosome.

The largest crossover frequency indicates the two genes that are farthest apart.  The smallest frequency indicates the two closest genes.  So the map is
_L_K____________J_.

5. A particular gene in dogs contributes to coat color. The two alleles exhibit incomplete dominance. Dogs with genotype mm have normal pigmentation; genotype Mm leads to “dilute” pigmentation; genotype MM produces an all-white dog. If a breeder mates a normal dog with a white dog, what will be the genotypes and phenotypes of the puppies?
If two Mm dogs are mated, what is the probability that a puppy will be all white?

Normal (mm)  x  all white (MM) = all dilute (Mm) pups

Mm x Mm = 25% normal, 50% dilute, 25% all white

6. Three babies are born in the hospital on the same day. Baby X has type AB blood; Baby Y has type B blood; Baby Z has type O blood. Use the information in the table below to determine which baby belongs to which couple. (Assume that all individuals are homozygous dominant for the H gene.)

Baby Z, O blood, belongs to couple 2, because an AB parent cannot produce an O child.
Baby X, AB blood, belongs to couple 1, because an O parent cannot produce an AB child.
Baby Y, B blood, therefore belongs to couple 3.

7. Consider a woman whose brother has hemophilia A but whose parents are healthy.  What is the chance that she has inherited the hemophilia allele?  What is the chance that the woman will conceive a son with hemophilia?

If the woman’s brother has hemophilia A, but both parents appear healthy, then their mother must be a carrier.  In that case, there is a 50% chance that the woman has inherited one copy of the allele (that is, she is a carrier herself).  As a result, each son has a 50% chance of having hemophelia. 

8. New parents Gloria and Michael were startled when their son Will’s diapers turned blue when he urinated. Fortunately, this occurred for the first time in the hospital, where tests determined that the newborn had inherited “blue diaper syndrome.” Because of abnormal transport of the amino acid tryptophan across the small intestinal lining, urine contains a compound that turns blue on contact with the air. Gloria’s sister Edith was pregnant at the time of Will’s diagnosis and became concerned that her child might inherit the disorder. The family doctor assured Gloria and her sister that this wasn’t possible because each parent had to be a carrier. However, Edith and Archie’s son Aaron also was born with blue diaper syndrome. Draw a pedigree for this family and describe how this disorder is most likely inherited. How was the doctor’s explanation incorrect?

If the baby has the disorder but the parents do not, the disorder must be inherited in a recessive manner.  This would mean that both Gloria and Michael had to be carriers for the disease.  If Gloria is a carrier, it is likely that her sister is also a carrier since the affected gene must have come from one of the girls’ parents.  Since Aaron was also born with the syndrome, Archie must also have been a carrier.  Your pedigree should show all of the parents above as carriers, both babies as affected, and at least one of Edith and Gloria’s parents as a carrier.

 

Answers to Pull It Together Questions

1. Which cells in the human body are haploid? Which cells are diploid?

Gametes are haploid cells and nearly all other cells are diploid.  Some cells, like red blood cells, lack a nucleus, and are therefore not haploid or diploid.

2. What is the difference between genotype and phenotype?

A genotype describes the genetic makeup of an individual and a phenotype describes the expression of its genetic makeup.

3. Add meiosis, gametes, mutations, incomplete dominance,
codominance, and pleiotropy, to this concept map.

 “Meiosis” leads to “Gametes” with “produces”, which leads to “HAPLOID CELLS” with “are”. “Genes” leads to “Mutation” with “can undergo”, which leads to “Alleles” with “results in new”. “Pleiotropy” leads to “Phenotype” with “is when genes have multiple effects on the”. “Codominance” can lead to “Dominant” with “occurs when multiple alleles for a gene are”. “Incomplete dominance” can lead to “Phenotype” with “occurs when heterozygotes have an intermediate”.