Mendel and the Gene Idea

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1 LECTURE PRESENTATIONS For CAMPBELL BIOLOGY, NINTH EDITION Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson Chapter 4 Mendel and the Gene Idea Lectures by Erin Barley Kathleen Fitzpatrick 20 Pearson Education, Inc.

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4 Big Ideas 3.A.3) The pattern of transmission from parents to offspring is best understood via the chromosomal basis of inheritance. 4.C.2) The expression of an organism s genes is influenced by environmental factors. 4.C.4) An ecosystem s stability may be influenced by species diversity. Illustrative Examples: Any variety of genetic diseases (e.g. Tay-Sachs, X-linked color blindness, etc.) Civic/ethical issues: gene patents, insurance co. & privacy rights Any variety of genetic variation (e.g. human height, flower color, etc.) 20 Pearson Education, Inc.

5 Epistasis (4.3) Pleiotropy (4.3) 20 Pearson Education, Inc.

6 Mix & Match Review: Ch Characters/traits 2. Generations (P, F, F 2 ) 3. Homozygous 4. Heterozygous 5. Crossbreeding/cross pollination. Hybrid 2. Purebred/Pure breeding 3. Dominant 4. Recessive 5. Geno/phenotype, 4, 7 2, 5, 8. Allele 2. Law of Segregation 3. Gamete 4. Monohybrid Cross 5. Law of addition & multiplication 3, 6 20 Pearson Education, Inc.

7 Figure 4.

8 Overview: Drawing from the Deck of Genes What genetic principles account for the passing of traits from parents to offspring? Blending hypothesis: the idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green) Particulate hypothesis: the idea that parents pass on discrete heritable units (genes) This hypothesis can explain the reappearance of traits after several generations 20 Pearson Education, Inc.

9 Overview: Drawing from the Deck of Genes What genetic principles account for the passing of traits from parents to offspring? Blending hypothesis: the idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green) Particulate hypothesis: the idea that parents pass on discrete heritable units (genes) This hypothesis can explain the reappearance of traits after several generations 20 Pearson Education, Inc.

10 Lecture Outline. The Laws of Inheritance: Mendel & the scientific Method. 2. The laws of probability control Mendelian inheritance. 3. Inheritance patterns are often more complex than predicted (simple Mendelian genetics). 4. Many human traits follow Mendelian patterns of inheritance. 20 Pearson Education, Inc.

11 Concept #: The Laws of Inheritance: Mendel & the scientific Method. Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments 20 Pearson Education, Inc.

12 Mendel s Experimental, Quantitative Approach Advantages of pea plants for genetic study There are many varieties with distinct heritable features, or characters (such as flower color); character variants (such as purple or white flowers) are called traits Mating can be controlled Each flower has sperm-producing organs (stamens) and an egg-producing organ (carpel) Cross-pollination (fertilization between different plants) involves dusting one plant with pollen from another 20 Pearson Education, Inc.

13 Figure 4.2a TECHNIQUE Parental generation (P) 3 2 Stamens Carpel 4

14 Figure 4.2b RESULTS First filial generation offspring (F ) 5

15 Mendel chose to track only those characters that occurred in two distinct alternative forms (e.g. A and a) He also used varieties that were true-breeding 20 Pearson Education, Inc.

16 Figure 4.3- EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers

17 Figure EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers F Generation (hybrids) All plants had purple flowers Self- or cross-pollination

18 Figure EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers F Generation (hybrids) All plants had purple flowers Self- or cross-pollination F 2 Generation 705 purpleflowered plants 224 white flowered plants

19 ) The Law of Segregation Experiment #: o Purebred white and purple-flowered pea plants (P) o Results: 00% purple F hybrids Experiment #2: o F hybrids test-crossed o Results: ~75% purple: ~25% white 20 Pearson Education, Inc.

20 Mendel reasoned that only the purple flower factor was affecting flower color in the F hybrids Mendel called the purple flower color a dominant trait and the white flower color a recessive trait The factor for white flowers was not diluted or destroyed because it reappeared in the F 2 generation In other words, recessive traits may skip a generation 20 Pearson Education, Inc.

21 Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits What Mendel called a heritable factor is what we now call a gene 20 Pearson Education, Inc.

22 Table 4.

23 Mendel s Model Mendel developed a hypothesis to explain the 3: inheritance pattern he observed in F 2 offspring Four related concepts make up this model that support the existence of chromosomes. 20 Pearson Education, Inc.

24 First: alternative versions (alleles) of genes account for variations in inherited characters Second: an organism inherits two alleles for every trait (one from each parent) Third: the dominant allele (where two alleles differ) determines the organism s appearance. The recessive allele has no noticeable effect on appearance. Fourth (Law of segregation): the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes. 20 Pearson Education, Inc.

25 First: alternative versions (alleles) of genes account for variations in inherited characters Second: an organism inherits two alleles for every trait (one from each parent) Third: the dominant allele (where two alleles differ) determines the organism s appearance. The recessive allele has no noticeable effect on appearance. Fourth (Law of segregation): the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes. 20 Pearson Education, Inc.

26 First: alternative versions (alleles) of genes account for variations in inherited characters Second: an organism inherits two alleles for every trait (one from each parent) Third: the dominant allele (where two alleles differ) determines the organism s appearance. The recessive allele has no noticeable effect on appearance. Fourth (Law of segregation): the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes. 20 Pearson Education, Inc.

27 First: alternative versions (alleles) of genes account for variations in inherited characters Second: an organism inherits two alleles for every trait (one from each parent) Third: the dominant allele (where two alleles differ) determines the organism s appearance. The recessive allele has no noticeable effect on appearance. Fourth (Law of segregation): the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes. 20 Pearson Education, Inc.

28 Figure 4.4 Allele for purple flowers Locus for flower-color gene Pair of homologous chromosomes Allele for white flowers

29 Figure 4.5- P Generation Appearance: Genetic makeup: Gametes: Purple flowers White flowers PP pp P p

30 Figure P Generation Appearance: Genetic makeup: Gametes: Purple flowers White flowers PP pp P p F Generation Appearance: Genetic makeup: Gametes: / 2 Purple flowers Pp P / 2 p

31 Figure P Generation Appearance: Genetic makeup: Gametes: Purple flowers White flowers PP pp P p F Generation Appearance: Genetic makeup: Gametes: F 2 Generation / 2 Purple flowers Pp P / 2 p Sperm from F (Pp) plant P p Eggs from F (Pp) plant P p PP Pp Pp pp 3 :

32 Figure 4.6 Phenotype Genotype Purple PP (homozygous) 3 Purple Pp (heterozygous) 2 Purple Pp (heterozygous) White pp (homozygous) Ratio 3: Ratio :2:

33 The Testcross How can we tell the genotype of an individual with the dominant phenotype? Such an individual could be either homozygous dominant or heterozygous The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual If any offspring display the recessive phenotype, the mystery parent must be heterozygous 20 Pearson Education, Inc.

34 Figure 4.7 TECHNIQUE Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp Predictions If purple-flowered or If purple-flowered parent is PP parent is Pp Sperm Sperm p p p p Eggs P Pp Pp Eggs P Pp Pp P Pp Pp p pp pp RESULTS or All offspring purple / 2 offspring purple and / 2 offspring white

35 2) The Law of Independent Assortment Mendel derived the law of segregation by following a single character The F offspring produced in this cross were monohybrids, individuals that are heterozygous for one character A cross between such heterozygotes is called a monohybrid cross 20 Pearson Education, Inc.

36 Mendel identified his second law of inheritance by following two characters at the same time Crossing two true-breeding parents differing in two characters produces dihybrids in the F generation, heterozygous for both characters Q: Are two characters transmitted to offspring as a package or independently? A: Perform a dihybrid cross (a cross between F dihybrids) 20 Pearson Education, Inc.

37 Figure 4.8 EXPERIMENT P Generation YYRR yyrr Gametes YR yr F Generation YyRr Predictions Predicted offspring of F 2 generation Eggs Hypothesis of dependent assortment / 2 / 2 YR yr Sperm / 2 YR / 2 3 / 4 YYRR YyRr / 4 yr YyRr yyrr or Hypothesis of independent assortment Eggs / 4 / 4 / 4 YR Yr yr Sperm / 4 YR / 4 Yr / 4 yr / 4 yr YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyrr yyrr Phenotypic ratio 3: / 4 yr YyRr Yyrr yyrr yyrr 9 / 6 3 / 6 3 / 6 / 6 Phenotypic ratio 9:3:3: RESULTS Phenotypic ratio approximately 9:3:3:

38 Using a dihybrid cross, Mendel developed the law of independent assortment The law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation Strictly speaking, this law applies only to genes on different, non-homologous chromosomes or those far apart on the same chromosome Genes located near each other on the same chromosome tend to be inherited together 20 Pearson Education, Inc.

39 Lecture Outline. The Laws of Inheritance: Mendel & the scientific Method. 2. The laws of probability control Mendelian inheritance. 3. Inheritance patterns are often more complex than predicted (simple Mendelian genetics). 4. Many human traits follow Mendelian patterns of inheritance. 20 Pearson Education, Inc.

40 Concept 4.2: The laws of probability control Mendelian inheritance. Mendel s laws of segregation and independent assortment reflect the rules of probability When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss In the same way, the alleles of one gene segregate into gametes independently of another gene s alleles 20 Pearson Education, Inc.

41 The Multiplication and Addition Rules Applied to Monohybrid Crosses The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities Segregation in a heterozygous plant is like flipping a coin: Each gamete has a /2 chance of carrying the dominant allele Each gamete has a /2 chance of carrying the recessive allele 20 Pearson Education, Inc.

42 Figure 4.9 Rr Segregation of alleles into eggs Rr Segregation of alleles into sperm Sperm / 2 R / 2 r / 2 R R R R r Eggs / 4 / 4 / 2 r r R r r / 4 / 4

43 The addition rule states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities The rule of addition can be used to figure out the probability that an F 2 plant from a monohybrid cross will be heterozygous rather than homozygous The Addition Rule avoids making long Punnett Square problems 20 Pearson Education, Inc.

44 Solving Complex Genetics Problems with the Rules of Probability We can apply the multiplication and addition rules to predict the outcome of crosses involving multiple characters A dihybrid or other multi-character cross is equivalent to two or more independent monohybrid crosses occurring simultaneously In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied 20 Pearson Education, Inc.

45 Figure 4.8 EXPERIMENT P Generation YYRR yyrr Gametes YR yr F Generation YyRr Which Punnett Square would you rather solve? A or B Predictions Predicted offspring of F 2 generation Eggs Hypothesis of dependent assortment / 2 / 2 YR yr Sperm / 2 YR / 2 YYRR YyRr yr YyRr yyrr or Hypothesis of independent assortment Eggs / 4 / 4 / 4 YR Yr yr Sperm / 4 YR / 4 Yr / 4 yr / 4 yr YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyrr yyrr 3 / 4 / 4 A Phenotypic ratio 3: / 4 yr 9 / 6 YyRr Yyrr yyrr yyrr 3 / 6 3 / 6 / 6 Phenotypic ratio 9:3:3: B RESULTS Phenotypic ratio approximately 9:3:3:

46 Figure 4.UN0 Probability of YYRR = / 4 (probability of YY) / 4 (RR) = / 6 Probability of YyRR = / 2 (Yy) / 4 (RR) = / 8

47 Figure 4.UN02 ppyyrr ppyyrr Ppyyrr PPyyrr ppyyrr / 4 (probability of pp) / 2 (yy) / 2 (Rr) / 4 / 2 / 2 / 2 / 2 / 2 / 4 / 2 / 2 / 4 / 2 / 2 Chance of at least two recessive traits = / 6 = / 6 = 2 / 6 = / 6 = / 6 = 6 / 6 or 3 / 8

48 Lecture Outline. The Laws of Inheritance: Mendel & the scientific Method. 2. The laws of probability control Mendelian inheritance. 3. Inheritance patterns are often more complex than predicted (simple Mendelian genetics). 4. Many human traits follow Mendelian patterns of inheritance. 20 Pearson Education, Inc.

49 Concept 4.3: Inheritance patterns are often more complex than predicted (simple Mendelian genetics). The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied Many heritable characters are not determined by only one gene with two alleles However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance 20 Pearson Education, Inc.

50 Extending Mendelian Genetics for a Single Gene Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: When alleles are not completely dominant or recessive When a gene has more than two alleles When a gene produces multiple phenotypes 20 Pearson Education, Inc.

51 Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical (refer to pea plant traits) In incomplete dominance, the phenotype of F hybrids is somewhere between the phenotypes of the two parental varieties In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways 20 Pearson Education, Inc.

52 Table 4.

53 Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical In incomplete dominance, the phenotype of F hybrids is somewhere between the phenotypes of the two parental varieties In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways 20 Pearson Education, Inc.

54 Figure 4.0- P Generation Red C R C R White C W C W Gametes C R C W

55 Figure P Generation Red C R C R White C W C W Gametes C R C W F Generation Pink C R C W Gametes / 2 / 2 C R C W

56 Figure P Generation Red C R C R White C W C W Gametes C R C W F Generation Pink C R C W Gametes / 2 C R / 2 C W F 2 Generation Sperm / 2 C R / 2 C W Eggs / 2 C R C R C R C R C W / 2 C W C R C W C W C W

57 The Relation Between Dominance and Phenotype A dominant allele does not subdue a recessive allele; alleles don t interact that way Alleles are simply variations in a gene s nucleotide sequence For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype 20 Pearson Education, Inc.

58 Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain At the organismal level, the allele is recessive At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant At the molecular level, the alleles are codominant 20 Pearson Education, Inc.

59 Frequency of Dominant Alleles Dominant alleles are not necessarily more common in populations than recessive alleles For example, one baby out of 400 in the United States is born with extra fingers or toes 20 Pearson Education, Inc.

60 The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage In this example, the recessive allele is far more prevalent than the population s dominant allele 20 Pearson Education, Inc.

61 Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical In incomplete dominance, the phenotype of F hybrids is somewhere between the phenotypes of the two parental varieties In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways 20 Pearson Education, Inc.

62 Multiple Alleles (aka Codominance) Most genes exist in populations in more than two allelic forms For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: I A, I B, and i. The enzyme encoded by the I A allele adds the A carbohydrate, whereas the enzyme encoded by the I B allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither 20 Pearson Education, Inc.

63 Figure 4. (a) The three alleles for the ABO blood groups and their carbohydrates Allele I A I B i Carbohydrate A B none (b) Blood group genotypes and phenotypes Genotype I A I A or I A i I B I B or I B i I A I B ii Red blood cell appearance Phenotype (blood group) A B AB O

64 Polygenic Inheritance Quantitative characters are those that vary in the population along a continuum Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype Skin color in humans is an example of polygenic inheritance 20 Pearson Education, Inc.

65 Figure 4.3 AaBbCc AaBbCc Sperm / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 Eggs / 8 / 8 / 8 / 8 / 8 Phenotypes: Number of / 64 6 / 64 5 / / 5 64 / 64 6 / 64 / 64 dark-skin alleles:

66 Nature and Nurture: The Environmental Impact on Phenotype Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype The norm of reaction is the phenotypic range of a genotype influenced by the environment For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity and aluminum content 20 Pearson Education, Inc.

67 Figure 4.4

68 Norms of reaction are generally broadest for polygenic characters Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype 20 Pearson Education, Inc.

69 Integrating a Mendelian View of Heredity and Variation An organism s phenotype includes its physical appearance, internal anatomy, physiology, and behavior An organism s phenotype reflects its overall genotype and unique environmental history 20 Pearson Education, Inc.

70 Lecture Outline. The Laws of Inheritance: Mendel & the scientific Method. 2. The laws of probability control Mendelian inheritance. 3. Inheritance patterns are often more complex than predicted (simple Mendelian genetics). 4. Many human traits follow Mendelian patterns of inheritance. 20 Pearson Education, Inc.

71 Concept 4.4: Many human traits follow Mendelian patterns of inheritance Humans are not good subjects for genetic research Generation time is too long Parents produce relatively few offspring Breeding experiments are unacceptable However, basic Mendelian genetics endures as the foundation of human genetics 20 Pearson Education, Inc.

72 Pedigree Analysis A pedigree is a family tree that describes the interrelationships of parents and children across generations Inheritance patterns of particular traits can be traced and described using pedigrees 20 Pearson Education, Inc.

73 Figure 4.5 Key Male Female Affected male Affected female Mating Offspring st generation 2nd generation 3rd generation Ww ww ww Ww Ww ww ww Ww Ww WW or Ww ww ww st generation 2nd generation FF or Ff 3rd generation Ff Ff ff Ff ff ff Ff ff Ff FF or Ff ff Widow s peak No widow s peak Attached earlobe Free earlobe (a) Is a widow s peak a dominant or recessive trait? b) Is an attached earlobe a dominant or recessive trait?

74 Pedigrees can also be used to make predictions about future offspring We can use the multiplication and addition rules to predict the probability of specific phenotypes 20 Pearson Education, Inc.

75 Inheritance Patterns Autosomal Recessive: Many genetic disorders are inherited in a recessive manner Autosomal Dominant: Some human disorders are caused by dominant alleles Polygenetic Inheritance (Multifactorial): more than one gene influences the phenotype 20 Pearson Education, Inc.

76 Recessively Inherited Disorders Many genetic disorders are inherited in a recessive manner These range from relatively mild to lifethreatening 20 Pearson Education, Inc.

77 The Behavior of Recessive Alleles Recessively inherited disorders show up only in individuals homozygous for the allele Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal; most individuals with recessive disorders are born to carrier parents Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair 20 Pearson Education, Inc.

78 Figure 4.6 Normal Aa Sperm Parents Normal Aa Eggs A a A AA Normal Aa Normal (carrier) a Aa Normal (carrier) aa Albino

79 If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele Most societies and cultures have laws or taboos against marriages between close relatives 20 Pearson Education, Inc.

80 Cystic Fibrosis Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes leading to a buildup of chloride ions outside the cell Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine 20 Pearson Education, Inc.

81 Sickle-Cell Disease: A Genetic Disorder with Evolutionary Implications Sickle-cell disease affects one out of 400 African-Americans The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells In homozygous individuals, all hemoglobin is abnormal (sickle-cell) Symptoms include physical weakness, pain, organ damage, and even paralysis 20 Pearson Education, Inc.

82 Fig. 4-UN Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptoms About one out of ten African Americans has sickle cell trait, an unusually high frequency of an allele with detrimental effects in homozygotes Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous 20 Pearson Education, Inc.

83 Inheritance Patterns Autosomal Recessive: Many genetic disorders are inherited in a recessive manner Autosomal Dominant: Some human disorders are caused by dominant alleles Polygenetic Inheritance (Multifactorial): more than one gene influences the phenotype 20 Pearson Education, Inc.

84 Dominantly Inherited Disorders Some human disorders are caused by dominant alleles Dominant alleles that cause a lethal disease are rare and arise by mutation Achondroplasia is a form of dwarfism caused by a rare dominant allele 20 Pearson Education, Inc.

85 Figure 4.7 Dwarf Dd Parents Normal dd Eggs d Sperm D Dd Dwarf d dd Normal d Dd Dwarf dd Normal

86 Huntington s Disease: A Late-Onset Lethal Disease The timing of onset of a disease significantly affects its inheritance Huntington s disease is a degenerative disease of the nervous system The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age Once the deterioration of the nervous system begins the condition is irreversible and fatal 20 Pearson Education, Inc.

87 Inheritance Patterns Autosomal Recessive: Many genetic disorders are inherited in a recessive manner Autosomal Dominant: Some human disorders are caused by dominant alleles Polygenetic Inheritance (Multifactorial): more than one gene influences the phenotype 20 Pearson Education, Inc.

88 Multifactorial Disorders Many diseases, such as heart disease, diabetes, alcoholism, mental illnesses, and cancer have both genetic and environmental components Little is understood about the genetic contribution to most multifactorial diseases 20 Pearson Education, Inc.

89 Figure 4.UN03 Relationship among alleles of a single gene Description Example Complete dominance of one allele Heterozygous phenotype same as that of homozygous dominant PP Pp Incomplete dominance of either allele Codominance Multiple alleles Pleiotropy Heterozygous phenotype intermediate between the two homozygous phenotypes Both phenotypes expressed in heterozygotes In the whole population, some genes have more than two alleles One gene is able to affect multiple phenotypic characters C R C R C R C W C W C W I A I B ABO blood group alleles I A, I B, i Sickle-cell disease

90 Lecture Outline. Mendelian inheritance has its physical basis in the behavior of chromosomes 2. Sex-linked genes exhibit unique patterns of inheritance 3. Linked genes tend to be inherited together because they are located near each other on the same chromosome 4. A chromosome consists of a DNA molecule packed together with proteins (6.3) 20 Pearson Education, Inc.

91 Please complete the following problems from your Practice Exam: # Pearson Education, Inc.