CHAPTER 4: HEREDITY AND EVOLUTION

Chapter Outline

I. Introduction

A. It was not until the 20th century that scientists understood how selective breeding could increase the frequency of desirable characteristics.

B. The genetic principles described by Mendel form the basis of modern genetics.

1. Anne Boleyn, mother of England’s Queen Elizabeth I and wife of Henry VIII had an extra little finger; a historic example of polydactyly.

2. The predominant belief centered on the blending of parental traits in the offspring, starting with the Greek philosophers; even Darwin held this belief

II. The Genetic Principles Discovered by Mendel

A. Historically, there have been examples of the effects of genetics, although the field of genetics did not emerge until the 19th century.

B. Gregor Mendel (1822-1884) developed his theory of heredity while working with garden pea hybrids

C. Segregation

1. The parental (P) generation was crossed to produce the first filial (F1) generation a. The F1 generation did not have intermediate traits. b. The F1 generation was then crossed to produce the F2 generation. c. Mendel concluded that discrete units, occurring in pairs and separating into different sex cells, must control the traits.

(i) One expression of the trait, shortness of the stem, for example, disappeared in the F1 generation but reappeared in the F2 generation.

(ii) The expression present in the F1 generation occurred more often in the F2 generation (in a 3:1 ratio).

(i) This is Mendel’s first principle of inheritance, the principle of segregation.

D. Dominance and Recessiveness

1. Mendel used these terms to account for the fact that the expression of one (recessive) trait in the F1 generation was masked by the expression of the other (dominant) trait. These principles of dominance and recessiveness are important concepts in genetics today.

a. Variations of genes at a locus are termed alleles.

b. Plant height in peas (but not in all plants) is controlled by two alleles at one locus.

(i) When two copies of the same allele occur at one locus, the individual is homozygous.

(ii) When two different alleles are paired at the same locus, the individual is heterozygous.

c. The genotype refers to an organism’s entire genetic makeup.

d. The phenotype is the observed physical manifestation of genes.

2. A Punnett square can be used to predict the proportions of F2 phenotypes and genotypes.

E. Independent Assortment

1. Mendel also demonstrated that different characteristics aren’t necessarily inherited together by showing that plant height and seed color are independent of each other.

a. Mendel stated this relationship as the principle of independent assortment

(i) According to this principle, the genes that code for different traits (e.g. plant height and seed color) sort out independently of each other during gamete formation.

(ii) Genes that control plant height and seed color are located on different, non partner chromosomes, and during meiosis, the chromosomes travel to newly forming cells independently of each other in a process known as random assortment

III. Mendelian Inheritance in Humans

A. Mendelian traits are also called discrete traits, controlled by alleles at only one genetic locus (or, in some cases, two or more very closely linked loci)

B. There are over 20,000 human characteristics inherited according to Mendelian principles

1. Most Mendelian traits do not have visible phenotypic expressions; the majority of the traits are biochemical in nature a. Dominant alleles cause a number of genetic disorders. b. Recessive conditions are typically associated with the lack of a substance, usually an enzyme For a person to have a recessive disorder, he/she must have two copies of the recessive allele that causes it.

2. Genetic disorders result from harmful alleles inherited in Mendelian fashion.

3. Traits may be inherited either as dominant or recessive alleles.

(i) Individuals who have one copy of the harmful recessive allele are unaffected, but can pass that allele onto offspring. They are termed carriers c. Blood groups provide one of the best examples of Mendelian traits d. Codominance exists where two different alleles are present and both are expressed.

(i) Alleles determine a person’s blood type by coding for the production of antigens.

C. There are misconceptions regarding dominance and recessiveness: a. One single, highly-conserved gene (MC1R) with two alleles makes a greater contribution to melanin production than some other melanin-producing genes. b. There are at least four other pigmentation genes.

1. Dominance and recessiveness are not all-or-nothing situations.

2. Several recessive alleles are known to have effects on the phenotype at the biochemical level, although they are not usually detectable through simple observation.

3. Dominant alleles are not “stronger,” “better,” or more common than recessive alleles.

IV. Polygenic Inheritance refers to traits that are influenced by alleles at two or more loci.

A. For example, skin coloration is determined by melanin, a pigment produced by specialized cells called melanocytes.

1. The alleles coding for melanin production have additive effects.

2. The effect of multiple alleles at several loci produces continuous variation in skin tone.

B. Much of the variation in human phenotypes used in traditional racial classifications is produced by polygenic, not Mendelian, traits.

1. Most polygenic traits can be measured on a scale, hence are continuous traits.

2. Many Mendelian traits have known loci. However, polygenic traits cannot yet be traced to specific loci.

V. Genetic and Environmental Factors

A. For polygenic traits, many aspects of the phenotype are influenced by genetic-environmental interactions.

1. For many characteristics, it is not possible to identify the specific environmental components that influence the phenotype.

B. Mendelian traits are less likely to be influenced by the environment; yet even for polygenic characteristics, Mendelian principles still apply at individual loci.

VI. Mitochondrial Inheritance

A. mtDNA is transmitted to the offspring only from the mother.

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1. Because mtDNA is inherited from only one parent, meiosis and recombination do not occur.

2. This means that all the variation in mtDNA among individuals is caused by mutation, which makes mtDNA extremely useful for studying genetic change over time.

B. All the variation in mtDNA among individuals is caused by mutation (mtDNA mutation rates have been used to construct evolutionary relationships between primate species and between living human populations).

VII. Modern Evolutionary Theory

A. Modern evolutionary theory has its roots in the principles of natural selection formulated by Darwin and Wallace, plus the rediscovery of Mendelian principles of inheritance. The competing explanations of evolution offered by mutationists and selectionists were synthesized into a single theory in the mid-1930s.

B. The Modern Synthesis is the comprehensive theory of evolution that incorporates two processes: variation and natural selection.

1. The production and redistribution of variation

2. The process of natural selection acting on this variation

C. A Current Definition of Evolution is defined as a change in allele frequency from one generation to the next

1. Allele frequencies are indicators of the genetic makeup of a population, the members of which share a gene pool.

2. Short-term evolutionary changes, such as allele frequency changes from one generation to the next, are termed microevolution

3. Long-term evolutionary effects that are evident in the fossil record are termed speciation, or macroevolution.

VIII. Factors That Produce and Redistribute Variation

A. Mutations are changes in alleles. Point mutation is the substitution of one single DNA base for another, which can cause the allele to change.

1. Mutations must occur in the gametes (sperm or egg) to have evolutionary consequences.

2. Mutation rates tend to be low, and mutations alone tend to have little impact on changes in allele frequencies.

a. Evolutionary changes can occur rapidly when mutations are coupled with natural selection.

b. Mutations are the only way to produce new genes evolutionarily.

B. Gene Flow is the exchange of genes between populations.

1. Population movements (and the resulting exchange of genes) have been very high during the past 500 years.

a. Gene flow between human populations helps to explain why human speciation has been rare during the past 1 million years a. The loss of genetic diversity is termed a genetic bottleneck b. Numerous species are known to have passed though genetic bottlenecks, including the cheetah and the California condor.

C. Genetic Drift and Founder Effect are caused by random alterations of allele frequencies in populations, and are both tied to population size.

1. Genetic drift occurs solely because the population is small and an allele may completely disappear because it is not passed on to offspring.

2. Founder effect is a particular kind of drift that may occur when a small founding population colonizes a new area or when a large population is reduced to a much smaller size through war, famine, disease, or other factors.

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3. Both gene flow and genetic drift can produce evolutionary changes, yet usually these are microevolutionary changes. Natural selection is necessary to produce macroevolutionary changed such as the emergence of a new species

D. Recombination is when paired chromosomes exchange DNA during meiosis and genes sometimes find themselves in different genetic environments.

1. This does not cause evolution, but it provides genetic combinations upon which natural selection can act.

IX. Natural Selection Is Directional and Acts on Variation

A. Natural selection acts on variation produced by mutation, gene flow, genetic drift, and recombination.

1. Directional changes in allele frequencies are a function of the environmental context.

2. If the environment changes, selection pressures change, and ultimately, allele frequencies also change. Such a shift in allele frequencies is called adaptation

B. The best example of natural selection in human populations involves the allele for hemoglobin S.

1. The HbS mutation occurs at a low frequency in all human populations.

2. In some populations, especially in western and central Africa, the HbS allele occurs in frequencies approaching 20 percent a. The geographical correlation between the distribution of malaria and high frequencies of the HbS allele indicates a biological relationship between the two. b. Sickle-cell trait results when an individual inherits only one copy of the Hbs allele. c. Sickle-cell anemia trait results from inheriting two copies of the Hbs allele. d. Sickle-cell anemia has numerous manifestations, but basically, the abnormal hemoglobin S reduces the ability of red blood cells to transport oxygen throughout the body.

X. Review of Genetics and Evolutionary Factors

A. The evolutionary process occurs at four different levels.

1. Mutations of the DNA in the gametes (the molecular level).

2. The chromosomes assort during meiosis and are passed on to the offspring (the cellular level).

3. The individual is subject to natural selection (the level of the individual).

4. Populations are composed of individuals, and evolution is said to occur when allele frequencies change (the level of the population).

a. Genetic drift and gene flow may act to change allele frequencies in populations.

Key Terms and Concepts

Allele frequency, p. 85

Alleles, p. 77

Antigens, p. 79

Codominance, p. 79

Dominant, p. 77

Founder effect, p. 87

Gene flow, p. 86

Gene pool, p. 85

Genetic drift, p. 87

Genotype, p. 77

Heterozygous, p. 77

Homozygous, p. 77

Hybrids, p. 75

Locus, p. 77

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Macroevolution, p. 86

Mendelian traits, p. 79

Microevolution, p. 86

Modern Synthesis, p. 85

Phenotypes, p. 78

Pigment, p. 82

Polygenic, p. 81

Population, p. 85

Principle of independent assortment, p. 78

Principle of segregation, p. 76

Random assortment, p. 78

Recessive, p. 76-77

Selective breeding, p. 74

Sickle-cell anemia, p. 90

Sickle-cell trait, p. 90

Variation, p. 85

Lecture Suggestions

1. Illustrate Mendelian principles in your class by calculating the frequencies for various Mendelian traits, such as hitchhiker’s thumb or earlobe form. Then, demonstrate the nature of continuous traits by calculating stature or forearm length. The data can then be used to illustrate the different statistical approaches to Mendelian versus polygenic traits.

2. Explore genetic bottlenecking by illustrating it with in-depth case studies of those species currently affected, such as the cheetah, and those that have passed through it, such as the California condor, California elephant seals, etc. Trace the process of re-introduction and wildlife management of the species. What challenges do humans face as they attempt to return and restore a species?

3. Have students construct their own family pedigree for a particular Mendelian trait. Again, the ABO blood groups are simple to explore. Even if they do not know their own blood type, they may be able to determine their parents’ and siblings’ types and calculate their own possibilities and/or probabilities using Punnett squares.

4. Explore advances in the field of genetics and genetic testing which permit more accurate diagnosis of individual carrier status. Point out that these advances will eventually eliminate the need for pedigree analysis as more accurate methods of diagnosis become available.

Internet Exercises

1. Go to http://www.appalachianhistory.net/2010/06/blue-fugates-of-kentucky.html and read about the Fugates of Kentucky. What genetic condition is responsible for their remarkable skin color?

2. At the website http://www.nature.com/scitable/topicpage/gregor-mendel-and-the-principles-ofinheritance-593 there are numerous explanations and follow-up activities associated with Mendelian genetics.

3. There is an excellent in-depth article on evolution and evolutionary principles at http://www.talkorigins.org/faqs/faq-intro-to-biology.html

4. PBS has an engaging website at http://www.pbs.org/wgbh/evolution/library/faq/cat01.html that includes an overview of evolution and also follow-up activities. Students may find these helpful as a review and deeper exploration into the topic.

Multiple Choice Questions

1. Gregor Mendel is known for which of the following?

a. He developed theories of evolutionary change, such as natural selection b. He discovered the structure of the DNA molecule. c. He studied polygenic characteristics that are influenced by several genetic loci. d. He discovered the fundamental principles of how traits are inherited. e. He developed the theory of inheritance of acquired characteristics.

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ANS: d

REF: Introduction a. They are simpler. b. Plants don’t have alleles. c. There are no differences since the basic principles are the same. d. There are no Mendelian traits in humans. e. The number of chromosomes is different; therefore, genetic principles vary

2. How do the basic principles of inheritance, identified by Mendel in plants, differ from those in humans?

ANS: c

REF: Introduction a. All the offspring were tall. b. Half the offspring were tall, the other half were short. c. All the offspring were short. d. The offspring were intermediate in height relative to the two parent plants. e. About 90 percent were tall, but the rest were short.

3. When Mendel crossed true breeding tall and short parent plants (producing first generation), what was produced?

ANS: a

REF: The Genetic Principles Discovered by Mendel a. 15 to 1 b. 3 to 1 c. ½ tall, ½ short d. 4 to 1 e. 5 to 1

4. In Mendel’s experiments, what was the ratio of tall to short plants in the F2 generation?

ANS: b

REF: The Genetic Principles Discovered by Mendel a. They were hybrids. b. Some of them were heterozygous for the traits in question. c. Their parents were all tall. d. All F1s displayed the dominant trait in their phenotype. e. All F1s displayed the recessive trait in their phenotype.

5. Which statement concerning the F1 plants in Mendel’s experiments with height is false?

ANS: e

REF: The Genetic Principles Discovered by Mendel a. Two copies of the recessive allele b. Two copies of the dominant allele c. An autosomal trisomy d. A recessive allele on the X chromosome only e. A recessive allele on the Y chromosome only

6. A person who is homozygous recessive at a locus has which of the following?

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ANS: a

REF: The Genetic Principles Discovered by Mendel

7. The position on a chromosome where a given gene occurs is called a(n): a. Mendelian trait b. phenotype c. genotype d. locus e. allele

ANS: d

REF: The Genetic Principles Discovered by Mendel

8. Mendelian traits are also called: a. discrete traits b. indiscrete traits c. continuous traits. d. heterozygous traits. e. homozygous traits.

ANS: a

REF: Mendelian Inheritance in Humans a. Phenotype b. Homozygosity c. Recessiveness d. Phenotypic ratio e. Genotype

9. What is an individual’s actual genetic make-up called?

ANS: e

REF: The Genetic Principles Discovered by Mendel a. 0 percent b. 25 percent c. 50 percent d. 75 percent e. 100 percent

10. According to the principle of independent assortment there is a __________chance that any tall pea plant will produce either yellow or green peas.

ANS: c

REF: The Genetic Principles Discovered by Mendel a. Genotype b. Phenotype c. Phenotypic ratio d. Genotypic ratio e. Independent assortment

11. What is the term used to refer to the observable, physical expression of genotypes?

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ANS: b

REF: The Genetic Principles Discovered by Mendel

12. In general, Mendelian traits: a. are governed by more than one genetic locus b. occur only in some people c. are always dominant d. are governed by one genetic locus e. are always recessive

ANS: d

REF: The Genetic Principles Discovered by Mendel

13. Gregor Mendel: a. won the Nobel Prize for his discoveries b. was formally trained as a geneticist c. did not know about chromosomes. d. was a professor at the University of Vienna. e. never published his work.

ANS: c

REF: The Genetic Principles Discovered by Mendel

14. The ABO blood type system consists of ______ alleles.

ANS: c

REF: Mendelian Inheritance in Humans a. Stature b. Skin color c. Eye color d. ABO blood type e. Hair color

15. Which of the following is not a polygenic trait?

ANS: d

REF: Polygenic Inheritance a. Recessiveness b. Codominance c. Dominance d. Homozygosity e. X-linkage

16. What is it called when a person possesses two different alleles at the same locus, and both alleles are expressed in the phenotype?

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ANS: b

REF: Mendelian Inheritance in Humans a. Codominance b. Blending c. Recessiveness d. Dominance e. Polygenic inheritance

17. What is type AB blood an example of?

ANS: a

REF: Mendelian Inheritance in Humans a. 4 b. 2 c. 3 d. 6 e. 1

18. How many ABO phenotypes (blood types) are there?

ANS: a

REF: Mendelian Inheritance in Humans

19. Mendelian traits are described as discrete, or discontinuous, because: a. their phenotypic expressions overlap. b. their phenotypic expressions do not fall into clearly defined categories. c. their genotypic expressions overlap. d. they are controlled by alleles at four different loci. e. they are controlled by alleles at a single locus.

ANS: e

REF: Mendelian Inheritance in Humans a. None b. All c. 1/4 d. 3/4 e. 1/2

20. In a hypothetical situation, B is the allele that causes brachydactyly. If a man who has normal fingers (bb) and a woman with brachydactyly (Bb) have children, what proportion of these children would you expect to have normal fingers? (Hint: Use a Punnett square).

ANS: e

REF: The Genetic Principles Discovered by Mendel a. All b. ½ c. ¾ d. ¼ e. None

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21. At a hypothetical locus, a man’s genotype is Aa. What proportion of his gametes would be expected to receive the A allele?

ANS: b

REF: The Genetic Principles Discovered by Mendel a. 3/4 b. 1/2 c. All d. 1/4 e. 2/3

22. Two people (both heterozygotes) are able to taste a chemical substance called PTC. The ability to taste PTC is caused by a dominant allele (T). The inability to taste PTC is caused by a recessive allele (t). What proportion of their children would be expected to have the ability to taste PTC?

ANS: a

REF: The Genetic Principles Discovered by Mendel

23. Polygenic traits are also called: a. indiscrete. b. discrete c. continuous. d. bimodal. e. Mendelian.

ANS: c

REF: Polygenic Inheritance

24. Scientists have used mitochondrial inheritance as a means to study all of the following except: a. ancestral relationships in the human lineage. b. development of continuous Mendelian traits. c. evolutionary relationships between species d. genetic variability among individuals. e. genetic variability within populations.

ANS: b

REF: Mitochondrial Inheritance a. Huntington Disease b. Albinism c. Cleft chin d. Tay-Sachs disease e. Skin color

25. Which of the following is not inherited in a Mendelian fashion?

ANS: e

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REF: Polygenic Inheritance a. Polygenic b. Dominant c. Mendelian d. Recessive e. Pleiotropic

26. Which of the following types of traits are governed by more than one genetic locus?

ANS: a

REF: Polygenic Inheritance a. They are governed by more than one genetic locus. b. Their expression is often influenced by genetic/environmental interactions. c. The alleles have an additive effect on the phenotype, intensifying the genes d. They are continuous traits. e. The most frequently discussed are skin, hair, and eye color.

27. Which statement concerning polygenic traits is not true?

ANS: c

REF: Polygenic Inheritance

28. Polygenic traits: a. are discrete. b. have a continuous range of expression. c. are controlled by only one genetic locus d. include the ABO blood type system and cystic fibrosis. e. are also called Mendelian traits.

ANS: b

REF: Polygenic Inheritance a. Nuclear DNA b. 46 chromosomes c. An X but never a Y chromosome d. Several copies of a ring-shaped DNA molecule, or chromosome e. A Y but never an X chromosome

29. What does each mitochondrion contain?

ANS: d

REF: Mitochondrial Inheritance a. Mutation and polygenic inheritance only b. Independent assortment and recombination only c. Recombination followed by mutation d. Production of a fixity of species e. Production and redistribution of variation

30. Evolution can be described as a two-stage process that includes which of the following?

ANS: e

REF: Modern Evolutionary Theory

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31. Evolution can be most succinctly defined as: a. the appearance of a new species b. the change from one species to another in one generation c. the change in allele frequency from one generation to the next. d. any type of genetic mutation. e. genetic drift

ANS: c

REF: Modern Evolutionary Theory a. Mutation b. Genetic drift c. Founder effect d. Migration e. Natural selection

32. What is the only source of new genetic material in any population?

ANS: a

REF: Factors that Produce and Redistribute Variation a. Natural selection b. Recombination c. Mutation d. Migration e. Genetic drift

33. What produces completely new alleles at a locus?

ANS: c

REF: Factors that Produce and Redistribute Variation

34. In order for a mutation to be passed on to offspring, the mutation must: a. occur in a gamete. b. be beneficial. c. occur in a somatic cell. d. result in additional chromosomes. e. have negative evolutionary consequences

ANS: a

REF: Factors that Produce and Redistribute Variation a. Production of new alleles b. Production of new genetic material c. Chance loss of alleles in a population d. Exchange of genes between populations e. Differential reproductive success of individuals

35. What is gene flow defined as?

ANS: d

REF: Factors that Produce and Redistribute Variation

36. Genetic drift is: a. the change in allele frequencies produced by random factors. b. the result of large populations c. the opposite of founder effect d. not associated with evolutionary change. e. the change in allele frequencies produced by nonrandom factors.

ANS: a

REF: Factors that Produce and Redistribute Variation a. There is no geographic correlation between the distribution of the HbS allele and malaria. b. Heterozygotes have greater resistance to malaria than homozygous individuals. c. The malarial parasite does not reproduce very well in the red blood cells of heterozygotes. d. Malaria is caused by the Plasmodium parasite. e. Most people are homozygous for the HbA allele

37. Which of the statements below is false regarding the relationship between malaria and the HbS allele?

ANS: a

REF: Natural Selection Is Directional and Acts on Variation a. Sickle-cell anemia b. Genetic drift c. Migration d. Increased mutation rates e. Natural selection

38. The HbS allele increased in frequency in West African populations due to which of the following?

ANS: e

REF: Natural Selection Is Directional and Acts on Variation

True/False Questions

1. All human genetic disorders are inherited as recessive traits.

ANS: False

REF: The Genetic Principles Discovered by Mendel

2. Recessive genotypic expressions are associated with homozygous alleles..

ANS: True

REF: The Genetic Principles Discovered by Mendel

3. Dominance and recessiveness are all-or-nothing situations because the recessive allele has no phenotypic effects in heterozygotes.

ANS: False

REF: The Genetic Principles Discovered by Mendel

4. Melanin production is the result of interactions between several different loci.

ANS: True

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REF: Polygenic Inheritance

5. Polygenic traits account for few, if any, of the readily observable phenotypic variation seen in humans.

ANS: False

REF: Polygenic Inheritance

6. The genotype sets limits and potentials for development and interacts with the environment to produce the phenotype.

ANS: True

REF: Genetic and Environmental Factors

7. Mitochondrial DNA (mtDNA) is extremely useful for studying genetic changes over time.

ANS: True

REF: Mitochondrial Inheritance

8. New alleles are always the results of mutations.

ANS: True

REF: Factors that Produce and Redistribute Variation

9. Genetic drift is the random factor in evolution.

ANS: True

REF: Factors that Produce and Redistribute Variation

10. The relationship between malaria and the HbS allele is an example of natural selection as a factor that can cause directional change in allele frequencies.

ANS: True

REF: Natural Selection Is Directional and Acts on Variation

11. A manifestation of sickle-cell anemia is the abnormal hemoglobin S reduces the ability of red blood cells to transport oxygen throughout the body.

ANS: True

REF: Natural Selection Is Directional and Acts on Variation

12. In regions where malaria is present, it acts as a selective agent that favors the heterozygous phenotype, because people with sickle-cell trait produce more offspring than those with only normal hemoglobin, who may die of malaria.

ANS: True

REF: Natural Selection Is Directional and Acts on Variation

Short Answer Questions

1. What is selective breeding?

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ANS: It is a practice in which animal or plant breeders choose which individuals to mate (plant) so that they can create desirable traits in the offspring.

REF: Introduction

2. Explain Mendel’s principle of independent assortment.

ANS: It is the principle which argues that the distribution of one pair of alleles into gametes does not the influence the distribution of another pair. Genes controlling different traits are assorted separately.

REF: The Genetic Principles Discovered by Mendel

3. What are the typical Mendelian phenotypic and genotypic ratios in the F2 generation for a cross of two heterozygous tall plants, where tall is a dominant trait? Why are these ratios typical?

ANS: Using a Punnett square diagram the genotypes to expect are ¼ homozygous tall, ¼ homozygous short, and ½ heterozygous. The phenotypes (given that tall is dominant) would be ¾ tall and ¼ short.

REF: The Genetic Principles Discovered by Mendel

4. What kind of research is done with mitochondrial DNA? What is particular or specific about mitochondrial DNA? (In other words, how is it different from most other types of inheritance?)

ANS: Mitochondrial DNA provides ways to study genetic change over time both within a species and between species because it is inherited from one parent only (mother) and is only subject to mutation.

REF: Mitochondrial Inheritance

5. What is meant by the “modern synthesis”?

ANS: It is a synthesis of multiple lines of evidence to integrate Mendelian genetics and natural selection within evolutionary theory.

REF: Modern Evolutionary Theory

6. Explain how two parents who do not express a particular trait in their phenotype can nevertheless produce children who express the trait. Give an example of a specific trait or disease where this could occur.

ANS: The answer is the difference between genotype (carrying the trait) and phenotype (expressing the trait). Parents can be heterozygous, leaving their children a 25 percent chance of expressing a carried trait. Examples include the ABO blood system.

REF: The Genetic Principles Discovered by Mendel

7. Define genetic drift. How are founder effect and genetic drift related?

ANS: Genetic drift is change in allele frequencies produced by random factors in small populations and founder effect is a type of genetic drift in which allele frequencies are altered in small populations take from or remnants of a larger population.

REF: Factors that Produce and Redistribute Variation

8. What is a genetic bottleneck and how is it caused?

ANS: It refers to the loss of genetic diversity within a species caused by a decline in numbers and constrained breeding.

REF: Factors that Produce and Redistribute Variation

9. What is meant by the statement, “natural selection is the one factor that can cause directional change in allele frequency relative to specific environmental factors”?

ANS: It means that if the environment changes, then selective pressures change and allele frequencies shift in response.

REF: Natural Selection Is Directional and Acts on Variation

Essay Questions

1. Why is mutation an important element in accounting for the variation in mtDNA? What are the factors that redistribute genetic variation?

ANS: Will vary

REF: Factors that Produce and Redistribute Variation

2. Using the HbS allele to illustrate, describe why fitness levels are a function of the environment.

ANS: Will vary

REF: Natural Selection Is Directional and Acts on Variation

3. Discuss the differences between Mendelian and polygenic modes of inheritance. Provide an example of a Mendelian and a polygenic trait.

ANS: Will vary

REF: Polygenic Inheritance

4. Allele frequencies are indicators of the genetic makeup of a population Use the example of ABO blood types to show how allele frequencies change.

ANS: Will vary

REF: Mendelian Inheritance in Humans

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