The Chromosomal Basis of Inheritance

Transcription

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 15 The Chromosomal Basis of Inheritance Lectures by Erin Barley Kathleen Fitzpatrick 2011 Pearson Education, Inc.

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3 Big Ideas 3.A.4) DNA, and some cases RNA, is the primary source of heritable information. 3.C.1) Changes in genotype can result in changes in phenotype. Illustrative Examples: o Sex-linked genes o Small Y-chromosome in mammals, flies o XX=females; XY=males (sucks to be the guy! Payback for gestation?) o Sex-limited traits: lactation (XX) and pattern baldness (XY) o Antibiotic resistance mutations o Sickle-cell disorder and heterozygote advantage 2011 Pearson Education, Inc.

4 Beyond the Scope of the AP Exam Epistasis Pleiotropy 2011 Pearson Education, Inc.

5 Overview: Variations on a Theme Living organisms are distinguished by their ability to reproduce their own kind Genetics is the scientific study of heredity and variation Heredity is the transmission of traits from one generation to the next Variation is demonstrated by the differences in appearance that offspring show from parents and siblings 2011 Pearson Education, Inc.

6 Genetic Variation: Ways to be different

7 Overview: Locating Genes Along Chromosomes Mendel s hereditary factors were genes Today we can show that genes are located on chromosomes The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene 2011 Pearson Education, Inc.

8 Figure 15.1

9 Mix & Match Review: Ch Genetics 2. Heredity 3. Variation 4. Chromosomes 5. Joff-tchoff-tchoffo-tchoffotchoff! 1. Sex chromosomes 2. Sex-linked genes 3. Law of segregation 4. F1 generation 5. F2 generation 1, 4, 7 2, 5, 8 1. Law of independent assortment 2. 9:3:3:1 ratio 3. 1:3 ratio 4. Monohybrid cross 5. Dihybrid cross 3, Pearson Education, Inc.

10 Lecture Outline 1. 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 (16.3) 2011 Pearson Education, Inc.

11 Concept #1: Mendelian inheritance has its physical basis in the behavior of chromosomes 2011 Pearson Education, Inc.

12 Concept #1: Mendelian inheritance has its physical basis in the behavior of chromosomes 2011 Pearson Education, Inc.

13 Concept #1: Mendelian inheritance has its physical basis in the behavior of chromosomes 2011 Pearson Education, Inc.

14 Figure 15.2 P Generation Y Y R R Yellow-round seeds (YYRR) y r y r Green-wrinkled seeds (yyrr) Meiosis Gametes R Y Fertilization y r All F 1 plants produce yellow-round seeds (YyRr). F 1 Generation R r Y y R r Y y LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. R Y r y Meiosis Metaphase I r Y R y LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. 1 R r r R 1 Y y Anaphase I Y y R r r R 2 2 Y y Metaphase II Y y Gametes Y Y y y Y Y y y R R r r r r R R 1 / 1 4 / 1 4 / 1 YR yr 4 Yr / 4 yr F 2 Generation An F 1 F 1 cross-fertilization 3 Fertilization recombines the R and r alleles at 3 random. 9 : 3 : 3 : 1 Fertilization results in the 9:3:3:1 phenotypic ratio in the F 2 generation.

15 Figure 15.2a P Generation Y Y R R Yellow-round seeds (YYRR) y y r r Green-wrinkled seeds (yyrr) Meiosis Gametes R Y Fertilization y r

16 Figure 15.2b All F 1 plants produce yellow-round seeds (YyRr). F 1 Generation LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. R Y r y R R y r r Y Y Meiosis Metaphase I y r Y R y LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. 1 R r r R 1 Y y Anaphase I Y y R r Metaphase II r R 2 2 Y y Y y Gametes Y Y y y Y Y y y R R r r r r R R 1 / 4 1 / 4 1 / 4 1 / 4 YR yr Yr yr

17 Figure 15.2c LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT F 2 Generation An F 1 F 1 cross-fertilization 3 Fertilization recombines the R and r alleles at random. 9 : 3 : 3 : 1 3 Fertilization results in the 9:3:3:1 phenotypic ratio in the F 2 generation.

18 Morgan s Choice of Experimental Organism Why Fruit Flies? They produce many offspring A generation can be bred every two weeks They have only four pairs of chromosomes 2011 Pearson Education, Inc.

19 Vocabulary: wild type vs mutant 2011 Pearson Education, Inc.

20 Figure 15.3 wild type vs mutant

21 Correlating Behavior of a Gene s Alleles with Behavior of a Chromosome Pair In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) The F 1 generation all had red eyes The F 2 generation showed the 3:1 red:white eye ratio, but only males had white eyes Morgan determined that the white-eyed mutant allele must be located on the X chromosome Morgan s finding supported the chromosome theory of inheritance 2011 Pearson Education, Inc.

22 Figure 15.4a EXPERIMENT P Generation F 1 Generation All offspring had red eyes. RESULTS F 2 Generation

23 Figure 15.4b CONCLUSION P Generation X X w w X Y w F 1 Generation Eggs w w w w w Sperm F 2 Generation Eggs w w w w w w w w w Sperm

24 Figure 15.4b CONCLUSION P Generation X X w w X Y w F 1 Generation Eggs w w w w w Sperm F 2 Generation Eggs w w w w w w w w w Sperm

25 Figure 15.4b CONCLUSION P Generation X X w w X Y w F 1 Generation Eggs w w w w w Sperm F 2 Generation Eggs w w w w w w w w w Sperm

26 Summing it Up Mendel proved that genes exist on chromosomes. Morgan proved that specific genes exist only on sex (X) chromosomes Pearson Education, Inc.

27 Lecture Outline 1. 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 (16.3) 2011 Pearson Education, Inc.

28 Concept #2: Sex-linked genes exhibit unique patterns of inheritance In humans and some other animals, there is a chromosomal basis of sex determination 2011 Pearson Education, Inc.

29 The Chromosomal Basis of Sex Only the ends of the Y chromosome are homologous with the X chromosome Exception (Sry gene) 2011 Pearson Education, Inc.

30 Figure 15.6a 44 XY Parents 44 XX or X Y Sperm 22 X Egg 44 XX or 44 XY (a) The X-Y system Zygotes (offspring) 22 XX 22 X No concept of Y here (p. 290) (b) The X-0 system

31 Figure 15.6b 76 ZW 76 ZZ Opposite of mammals (p. 290) (c) The Z-W system 32 (Diploid) 16 (Haploid) Ploidy determines gender (p. 290) (d) The haplo-diploid system

32 Vocabulary: sex-linked genes X-linked Y-linked 2011 Pearson Education, Inc.

33 Inheritance of X-Linked Genes X chromosomes have genes for many characters unrelated to sex Y chromosome mainly encodes genes related to sex determination 2011 Pearson Education, Inc.

34 X-linked genes follow specific patterns of inheritance Pearson Education, Inc.

35 Figure 15.7 X N X N X n Y X N X n X N Y X N X n X n Y Sperm X n Y Sperm X N Y Sperm Y X n Eggs X N X N X n X N Y Eggs X N X N X N X N Y Eggs X N X N X n X N Y X N X N X n X N Y X n X N X n X n Y X n X n X n X n Y (a) (b) (c)

36 Figure 15.7 X N X N X n Y X N X n X N Y X N X n X n Y Sperm X n Y Sperm X N Y Sperm Y X n Eggs X N X N X n X N Y Eggs X N X N X N X N Y Eggs X N X N X n X N Y X N X N X n X N Y X n X N X n X n Y X n X n X n X n Y (a) (b) (c)

37 Figure 15.7 X N X N X n Y X N X n X N Y X N X n X n Y Sperm X n Y Sperm X N Y Sperm Y X n Eggs X N X N X n X N Y Eggs X N X N X N X N Y Eggs X N X N X n X N Y X N X N X n X N Y X n X N X n X n Y X n X n X n X n Y (a) (b) (c)

38 Some disorders caused by recessive alleles on the X chromosome in humans Color blindness (mostly X-linked) Duchenne muscular dystrophy Hemophilia 2011 Pearson Education, Inc.

39 X Inactivation in Female Mammals In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development The inactive X condenses into a Barr body If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character 2011 Pearson Education, Inc.

40 Figure 15.6a 44 XY Parents 44 XX or X Y Sperm 22 X Egg 44 XX or 44 XY (a) The X-Y system Zygotes (offspring) Q: Do females produce 2X the proteins from X-linked genes (when compared with males)? 22 XX 22 X (b) The X-0 system

41 Figure 15.8a Got A Towel?

42 Figure 15.8 Early embryo: X chromosomes Allele for orange fur Allele for black fur Two cell populations in adult cat: Active X Cell division and X chromosome inactivation Inactive X Active X Black fur Orange fur

43 Figure 15.8a Got Catnip?

44 Practice Questions WHAT DO YOU KNOW?

45 Lecture Outline 1. 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 (16.3) 2011 Pearson Education, Inc.

46 Mix & Match Review: Ch Linked genes 2. Recombinant phenotype 3. Parental phenotype 4. Dihybrid cross 1, 4, 7 1. Crossing over 2. Map distance 3. Genetic map 4. Meiosis I & II 2, 5, 8 1. Eye color 2. Vestigial wings 3. Wild type 4. Body color 3, Pearson Education, Inc.

47 Concept #3: Linked genes tend to be inherited together because they are located near each other on the same chromosome Each chromosome has hundreds or thousands of genes (except the Y chromosome) Total Genome ~25K genes Genes located on the same chromosome that tend to be inherited together are called linked genes 2011 Pearson Education, Inc.

48 How Linkage Affects Inheritance Morgan did other experiments with fruit flies to see how linkage affects inheritance of two characters Morgan crossed flies that differed in traits of body color and wing size 2011 Pearson Education, Inc.

49 Figure EXPERIMENT P Generation (homozygous) Wild type (gray body, normal wings) b b vg vg Double mutant (black body, vestigial wings) b b vg vg

50 Figure EXPERIMENT P Generation (homozygous) Wild type (gray body, normal wings) b b vg vg Double mutant (black body, vestigial wings) b b vg vg F 1 dihybrid (wild type) b b vg vg TESTCROSS Double mutant b b vg vg

51 Figure EXPERIMENT P Generation (homozygous) Wild type (gray body, normal wings) b b vg vg Double mutant (black body, vestigial wings) b b vg vg F 1 dihybrid (wild type) b b vg vg TESTCROSS Double mutant b b vg vg Testcross offspring Eggs b vg b vg b vg b vg Wild type (gray-normal) Blackvestigial Grayvestigial Blacknormal b vg Sperm b b vg vg b b vg vg b b vg vg b b vg vg

52 Figure EXPERIMENT P Generation (homozygous) Wild type (gray body, normal wings) b b vg vg Double mutant (black body, vestigial wings) b b vg vg F 1 dihybrid (wild type) b b vg vg TESTCROSS Double mutant b b vg vg Testcross offspring Eggs b vg b vg b vg b vg Wild type (gray-normal) Blackvestigial Grayvestigial Blacknormal b vg Sperm PREDICTED RATIOS If genes are located on different chromosomes: b b vg vg b b vg vg b b vg vg b b vg vg 1 : 1 : 1 : 1 If genes are located on the same chromosome and parental alleles are always inherited together: 1 : 1 : 0 : 0 RESULTS 965 : 944 : 206 : 185

53 Dang! Didn t see that one! Morgan found that body color and wing size are usually inherited together in specific combinations (parental phenotypes) these genes do not assort independently (contrary to Mendel s conclusion) they were on the same chromosome 2011 Pearson Education, Inc.

54 Figure 15.UN01 F 1 dihybrid female and homozygous recessive male in testcross b + vg + b vg b vg b vg Most offspring b + vg + b vg or b vg b vg

55 However, nonparental phenotypes were also produced Understanding this result involves exploring genetic recombination, the production of offspring with combinations of traits differing from either parent 2011 Pearson Education, Inc.

56 Genetic Recombination and Linkage The genetic findings of Mendel and Morgan relate to the chromosomal basis of recombination 2011 Pearson Education, Inc.

57 Recombination of Unlinked Genes: Independent Assortment of Chromosomes Mendel observed that combinations of traits in some offspring differ from either parent Offspring with a phenotype matching one of the parental phenotypes are called parental types Offspring with nonparental phenotypes (new combinations of traits) are called recombinant types, or recombinants A 50% frequency of recombination is observed for any two genes on different chromosomes 2011 Pearson Education, Inc.

58 Figure 15.UN02 Gametes from yellow-round dihybrid parent (YyRr) YR yr Yr yr Gametes from greenwrinkled homozygous recessive parent (yyrr) yr YyRr yyrr Yyrr yyrr Parentaltype offspring Recombinant offspring

59 Recombination of Linked Genes: Crossing Over Morgan discovered that genes can be linked, but the linkage was incomplete, because some recombinant phenotypes were observed He proposed that some process must occasionally break the physical connection between genes on the same chromosome That mechanism was the crossing over of homologous chromosomes 2011 Pearson Education, Inc.

60 2011 Pearson Education, Inc. Animation: Crossing Over Right-click slide / select Play

61 Figure 15.10a Testcross parents Gray body, normal wings (F 1 dihybrid) Black body, vestigial wings (double mutant) b vg b vg Replication of chromosomes b vg b vg Replication of chromosomes b vg b vg b vg b vg b vg b vg Meiosis I b vg b vg b vg b vg Meiosis I and II b vg Meiosis II b vg Recombinant chromosomes Eggs b vg b vg b vg b vg b vg Sperm

62 Figure 15.10a Testcross parents Gray body, normal wings (F 1 dihybrid) Black body, vestigial wings (double mutant) b vg b vg Replication of chromosomes b vg b vg Replication of chromosomes b vg b vg b vg b vg b vg b vg Meiosis I b vg b vg b vg b vg Meiosis I and II b vg Meiosis II b vg Recombinant chromosomes Eggs b vg b vg b vg b vg b vg Sperm

63 Figure 15.10b Recombinant chromosomes Eggs b vg b vg b vg b vg Testcross offspring 965 Wild type (gray-normal) b vg 944 Blackvestigial b vg 206 Grayvestigial b vg 185 Blacknormal b vg b vg b vg b vg b vg b vg Sperm Parental-type offspring Recombinant offspring Recombination frequency 391 recombinants 2,300 total offspring %

64 Mapping the Distance Between Genes Using Recombination Data: Scientific Inquiry Alfred Sturtevant, one of Morgan s students, constructed a genetic map, an ordered list of the genetic loci along a particular chromosome Sturtevant predicted that the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency 2011 Pearson Education, Inc.

65 Figure RESULTS Chromosome Recombination frequencies 9% 9.5% 17% b cn vg

66 A linkage map is a genetic map of a chromosome based on recombination frequencies Distances between genes can be expressed as map units; one map unit, or centimorgan, represents a 1% recombination frequency Map units indicate relative distance and order, not precise locations of genes 2011 Pearson Education, Inc.

67 Genes that are far apart on the same chromosome can have a recombination frequency near 50% Big Idea: Such genes are physically linked, but genetically unlinked, and behave as if found on different chromosomes 2011 Pearson Education, Inc.

68 Sturtevant used recombination frequencies to make linkage maps of fruit fly genes Using methods like chromosomal banding, geneticists can develop cytogenetic maps of chromosomes Cytogenetic maps indicate the positions of genes with respect to chromosomal features 2011 Pearson Education, Inc.

69 Figure Short aristae Mutant phenotypes Black body Cinnabar eyes Vestigial wings Brown eyes Long aristae (appendages on head) Gray body Red eyes Normal wings Wild-type phenotypes Red eyes

70 Lecture Outline 1. Alterations of chromosome number or structure cause some genetic disorders. (15.4) 2. Some inheritance patterns are exceptions to standard Mendelian inheritance. (15.5) 2011 Pearson Education, Inc.

71 Mix & Match Review: Ch Nondisjunction 2. Aneuploidy 3. Tetraploid 4. Triploid 1, 4, 7 1. Monosomic 2. Trisomic 3. Down Syndrome 4. Genomic imprinting 1. Chromosome duplication 2. Chromosome deletion 3. Chromosome inversion 4. Chromosome translocation 2, 5, 8 3, Pearson Education, Inc.

72 Concept 15.4: Alterations of chromosome number or structure cause some genetic disorders Large-scale chromosomal alterations in humans and other mammals often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders Plants tolerate such genetic changes better than animals do 2011 Pearson Education, Inc.

73 Abnormal Chromosome Number In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy 2011 Pearson Education, Inc.

74 Figure Meiosis I Nondisjunction

75 Figure Meiosis I Nondisjunction Meiosis II Nondisjunction

76 Figure Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n 1 n 1 n 1 n 1 n 1 n 1 n n Number of chromosomes (a) Nondisjunction of homologous chromosomes in meiosis I (b) Nondisjunction of sister chromatids in meiosis II

77 Aneuploidy results from the fertilization of gametes in which nondisjunction occurred Offspring with this condition have an abnormal number of a particular chromosome 2011 Pearson Education, Inc.

78 A monosomic zygote has only one copy of a particular chromosome A trisomic zygote has three copies of a particular chromosome 2011 Pearson Education, Inc.

79 Polyploidy is a condition in which an organism has more than two complete sets of chromosomes Triploidy (3n) is three sets of chromosomes Tetraploidy (4n) is four sets of chromosomes Polyploidy is common in plants, but not animals Polyploids are more normal in appearance than aneuploids 2011 Pearson Education, Inc.

80 Alterations of Chromosome Structure Breakage of a chromosome can lead to four types of changes in chromosome structure Deletion removes a chromosomal segment Duplication repeats a segment Inversion reverses orientation of a segment within a chromosome Translocation moves a segment from one chromosome to another 2011 Pearson Education, Inc.

81 Figure (a) Deletion A B C D E F G H A B C E F G H A deletion removes a chromosomal segment. (b) Duplication A B C D E F G H A duplication repeats a segment. A B C B C D E F G H (c) Inversion A B C D E F G H An inversion reverses a segment within a chromosome. A D C B E F G H (d) Translocation A B C D E F G H M N O P Q R A translocation moves a segment from one chromosome to a nonhomologous chromosome. M N O C D E F G H A B P Q R

82 Figure 15.14a (a) Deletion A B C D E F G H A B C E F G H A deletion removes a chromosomal segment. (b) Duplication A B C D E F G H A duplication repeats a segment. A B C B C D E F G H

83 Figure 15.14b (c) Inversion A B C D E F G H An inversion reverses a segment within a chromosome. A D C B E F G H (d) Translocation A B C D E F G H M N O P Q R A translocation moves a segment from one chromosome to a nonhomologous chromosome. M N O C D E F G H A B P Q R

84 Human Disorders Due to Chromosomal Alterations Alterations of chromosome number and structure are associated with some serious disorders Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy 2011 Pearson Education, Inc.

85 Down Syndrome (Trisomy 21) Down syndrome is an aneuploid condition that results from three copies of chromosome 21 It affects about one out of every 700 children born in the United States The frequency of Down syndrome increases with the age of the mother, a correlation that has not been explained 2011 Pearson Education, Inc.

86 Figure 15.15

87 Figure 15.15a

88 Figure 15.15b

89 Aneuploidy of Sex Chromosomes Nondisjunction of sex chromosomes produces a variety of aneuploid conditions Klinefelter syndrome is the result of an extra chromosome in a male, producing XXY individuals Monosomy X, called Turner syndrome, produces X0 females, who are sterile; it is the only known viable monosomy in humans 2011 Pearson Education, Inc.

90 Disorders Caused by Structurally Altered Chromosomes The syndrome cri du chat ( cry of the cat ), results from a specific deletion in chromosome 5 A child born with this syndrome is mentally retarded and has a catlike cry; individuals usually die in infancy or early childhood Certain cancers, including chronic myelogenous leukemia (CML), are caused by translocations of chromosomes 2011 Pearson Education, Inc.

91 Figure Normal chromosome 9 Normal chromosome 22 Reciprocal translocation Translocated chromosome 9 Translocated chromosome 22 (Philadelphia chromosome)

92 Lecture Outline 1. Alterations of chromosome number or structure cause some genetic disorders. (15.4) 2. Some inheritance patterns are exceptions to standard Mendelian inheritance. (15.5) 2011 Pearson Education, Inc.

93 Concept 15.5: Some inheritance patterns are exceptions to standard Mendelian inheritance There are two normal exceptions to Mendelian genetics One exception involves genes located in the nucleus, and the other exception involves genes located outside the nucleus In both cases, the sex of the parent contributing an allele is a factor in the pattern of inheritance 2011 Pearson Education, Inc.

94 Genomic Imprinting For a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traits Such variation in phenotype is called genomic imprinting Genomic imprinting involves the silencing of certain genes that are stamped with an imprint during gamete production 2011 Pearson Education, Inc.

95 Figure 15.17b Mutant Igf2 allele inherited from mother Mutant Igf2 allele inherited from father Normal-sized mouse (wild type) Normal Igf2 allele is expressed. Dwarf mouse (mutant) Mutant Igf2 allele is expressed. Mutant Igf2 allele is not expressed. Normal Igf2 allele is not expressed. (b) Heterozygotes

96 Figure 15.17b Mutant Igf2 allele inherited from mother Mutant Igf2 allele inherited from father Normal-sized mouse (wild type) Normal Igf2 allele is expressed. Dwarf mouse (mutant) Mutant Igf2 allele is expressed. Mutant Igf2 allele is not expressed. Normal Igf2 allele is not expressed. (b) Heterozygotes

97 Figure 15.17a Paternal chromosome Maternal chromosome (a) Homozygote Normal Igf2 allele is expressed. Normal Igf2 allele is not expressed. Normal-sized mouse (wild type)

98 Figure 15.17b Mutant Igf2 allele inherited from mother Mutant Igf2 allele inherited from father Normal-sized mouse (wild type) Normal Igf2 allele is expressed. Dwarf mouse (mutant) Mutant Igf2 allele is expressed. Mutant Igf2 allele is not expressed. Normal Igf2 allele is not expressed. (b) Heterozygotes

99 Figure Paternal chromosome Maternal chromosome (a) Homozygote Normal Igf2 allele is expressed. Normal Igf2 allele is not expressed. Normal-sized mouse (wild type) Mutant Igf2 allele inherited from mother Mutant Igf2 allele inherited from father Normal-sized mouse (wild type) Normal Igf2 allele is expressed. Dwarf mouse (mutant) Mutant Igf2 allele is expressed. Mutant Igf2 allele is not expressed. Normal Igf2 allele is not expressed. (b) Heterozygotes

100 It appears that imprinting is the result of the methylation (addition of CH 3 ) of cysteine nucleotides Genomic imprinting is thought to affect only a small fraction of mammalian genes Most imprinted genes are critical for embryonic development 2011 Pearson Education, Inc.

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102 Defective Paternal 15 Defective Maternal 15 Prader-Willi Syndrome Angelman Syndrome

103 Certain genes are imprinted in some way each generation, and the imprint is different depending on whether genes come from females versus males. In each new generation, parental imprints are erased in testes, ovaries and recoded (DNA methylation) according to one s gender. HYPOTHESIS:

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106 Molecular Genetics Test DNA Methylation: most sensitive test for Angelman Syndrome. Moms and Dads tag (imprint) their DNA differently via methyl (-CH 3 ) groups added in different patterns.

107 Figure 15.18

108 Inheritance of Organelle Genes Extranuclear genes (or cytoplasmic genes) are found in organelles in the cytoplasm Mitochondria, chloroplasts, and other plant plastids carry small circular DNA molecules Extranuclear genes are inherited maternally because the zygote s cytoplasm comes from the egg The first evidence of extranuclear genes came from studies on the inheritance of yellow or white patches on leaves of an otherwise green plant 2011 Pearson Education, Inc.

109 Mitochondrial DNA Mary Claire King & the Grandmothers of the Plaza de Mayo (Argentina) iga_09/lecture15slides.pdf

110 Practice Questions WHAT DO YOU KNOW?

111 Some defects in mitochondrial genes prevent cells from making enough ATP and result in diseases that affect the muscular and nervous systems For example, mitochondrial myopathy and Leber s hereditary optic neuropathy 2011 Pearson Education, Inc.

112 Lecture Outline 1. 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 (16.3) 2011 Pearson Education, Inc.

113 Concept #4: A chromosome consists of a DNA molecule packed together with proteins The bacterial chromosome is a double-stranded, circular DNA molecule associated with a small amount of protein Eukaryotic chromosomes have linear DNA molecules associated with a large amount of protein In a bacterium, the DNA is supercoiled and found in a region of the cell called the nucleoid 2011 Pearson Education, Inc.

114 2011 Pearson Education, Inc. Animation: DNA Packing Right-click slide / select Play

115 Chromatin, a complex of DNA and protein, is found in the nucleus of eukaryotic cells Chromosomes fit into the nucleus through an elaborate, multilevel system of packing 2011 Pearson Education, Inc.

116 Figure 16.22a DNA double helix (2 nm in diameter) Nucleosome (10 nm in diameter) DNA, the double helix Histones Histones Histone tail H1 Nucleosomes, or beads on a string (10-nm fiber)

117 Figure 16.22b Chromatid (700 nm) 30-nm fiber Loops Scaffold 300-nm fiber 30-nm fiber Looped domains (300-nm fiber) Replicated chromosome (1,400 nm) Metaphase chromosome

118 Figure 16.22c DNA double helix (2 nm in diameter)

119 Figure 16.22d Nucleosome (10 nm in diameter)

120 Figure 16.22e 30-nm fiber

121 Figure 16.22f Loops Scaffold

122 Figure 16.22g Chromatid (700 nm)

123 Chromatin undergoes changes in packing during the cell cycle Interphase: some chromatin is organized into a 10-nm fiber but much is compacted into a 30-nm fiber, through folding and looping Though interphase chromosomes are not highly condensed, they still occupy specific restricted regions in the nucleus 2011 Pearson Education, Inc.

124 Most chromatin is loosely packed in the nucleus during interphase and condenses prior to mitosis Loosely packed chromatin is called euchromatin During interphase a few regions of chromatin (centromeres and telomeres) are highly condensed into heterochromatin Dense packing of the heterochromatin makes it difficult for the cell to express genetic information coded in these regions 2011 Pearson Education, Inc.

125 Lecture Outline 1. 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 (16.3) 2011 Pearson Education, Inc.