Campbell Biology 10. A Global Approach. Chapter 15 Linkage and Chromosomes

Transcription

1 Lecture on General Biology 2 Campbell Biology 10 A Global Approach th edition Chapter 15 Linkage and Chromosomes Chul-Su Yang, Ph.D., chulsuyang@hanyang.ac.kr Infection Biology Lab., Dept. of Molecular & Life Science, Hanyang University

2 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

3 Figure 15.1

4 Figure 15.1a

5 Cytologists worked out the process of mitosis in 1875, using improved techniques of microscopy Biologists began to see parallels between the behavior of Mendel s proposed hereditary factors and chromosomes Around 1902, Sutton and Boveri and others independently noted the parallels and the chromosome theory of inheritance began to form

6 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.

7 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

8 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

9 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.

10 Concept 15.1Morgan showed that Mendelian inheritance has its physical basis in the behavior of chromosomes : Scientific inquiry Mitosis (1875) and meiosis (1890s) were first described in the late 1800s The chromosome theory of inheritance states: Mendelian genes have specific loci (positions) on chromosomes Chromosomes undergo segregation and independent assortment (1902) The behavior of chromosomes during meiosis can account for Mendel s laws of segregation and independent assortment

11 Morgan s Experimental Evidence: Scientific Inquiry The first solid evidence associating a specific gene with a specific chromosome came from Thomas Hunt Morgan, an embryologist Morgan s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel s heritable factors

12 Morgan s Choice of Experimental Organism For his work, Morgan chose to study Drosophila melanogaster, a common species of fruit fly Several characteristics make fruit flies a convenient organism for genetic studies They produce many offspring A generation can be bred every two weeks They have only four pairs of chromosomes

13 Morgan noted wild type, or normal, phenotypes that were common in the fly populations Traits alternative to the wild type are called mutant phenotypes

14 Figure 15.3

15 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

16 Figure 15.4 EXPERIMENT P Generation F 1 Generation All offspring had red eyes. RESULTS F 2 Generation CONCLUSION P Generation X X w + w + X Y w Eggs F 1 Generation w + w w + w + w Sperm F 2 Generation Eggs w + w w + w + w + w + w w Sperm w +

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

18 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 Sperm w +

19 Concept 15.2 Sex-linked genes exhibit unique patterns of inheritance Morgan s discovery of a trait that correlated with the sex of flies was key to the development of the chromosome theory of inheritance In humans and some other animals, there is a chromosomal basis of sex determination

20 The Chromosomal Basis of Sex In humans and other mammals, there are two varieties of sex chromosomes: a larger X chromosome and a smaller Y chromosome A person with two X chromosomes develops as a female, while a male develops from a zygote with one X and one Y Only the ends of the Y chromosome have regions that are homologous with corresponding regions of the X chromosome

21 Figure 15.5 X Y

22 Females are XX, and males are XY Each ovum contains an X chromosome, while a sperm may contain either an X or a Y chromosome Other animals have different methods of sex determination

23 Figure 15.6a 44 + XY Parents 44 + XX 22 + X or Sperm 44 + XX 22 + Y or 22 + X Egg 44 + XY Zygotes (offspring) (a) The X-Y system 22 + XX 22 + X (b) The X-0 system

24 Figure 15.6b 76 + ZW 76 + ZZ (c) The Z-W system 32 (Diploid) 16 (Haploid) (d) The haplo-diploid system

25 Short segments at the ends of the Y chromosomes are homologous with the X, allowing the two to behave like homologues during meiosis in males A gene on the Y chromosome called SRY (sexdetermining region on the Y) is responsible for development of the testes in an embryo

26 A gene that is located on either sex chromosome is called a sex-linked gene Genes on the Y chromosome are called Y-linked genes; there are few of these Genes on the X chromosome are called X-linked genes

27 Inheritance of X-Linked Genes X chromosome have genes for many characters unrelated to sex, whereas the Y chromosome mainly encodes genes related to sex determination

28 X-linked genes follow specific patterns of inheritance For a recessive X-linked trait to be expressed A female needs two copies of the allele (homozygous) A male needs only one copy of the allele (hemizygous) X-linked recessive disorders are much more common in males than in females

29 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)

30 Some disorders caused by recessive alleles on the X chromosome in humans Color blindness (mostly X-linked) Duchenne muscular dystrophy Hemophilia

31 Duchenne Muscular Dystrophy (DMD) Clinical Features: Progressive muscle weakness. Mainly in a wheelchair by early teens. Respiratory muscles eventually involved. Death usually in late teens, early twenties. Inheritance: X-linked recessive condition, hence males affected and females are carriers DMD Pedigree The X chromosome carrying the disease-causing mutation can be tracked through the family. Note: Shaded squares = affected males: dots in circles = carrier females.

32 Duchenne Muscular Dystrophy (DMD) (a) Affected boys stand up by bracing their arms against their legs (Gower s manoeuvre) because their proximal muscles are weak. (b) and (c) Muscle histology (Gomori trichrome stain). Normal muscle (b) shows a regular architecture of cells with dystrophin (brown stain) on all the outer membranes. (c) Shows muscle from a 10-year-old affected boy. Note the disorganisation, invasion by fibrous tissue and complete absence of dystrophin. Histology photos courtesy of Dr. Richard Charlton, Newcastle upon Tyne.

33 Duchenne Muscular Dystrophy (DMD) Immunolabeling of Muscle Biopsy Sections Dystrophin antibody staining of muscle cells Normal Control 4 year old boy with DMD No detectable dystrophin

34 Duchenne Muscular Dystrophy (DMD) The dystrophin molecule anchors the cytoskeleton of muscle cells to the extracellular matrix, via the dystrophin glycoprotein complex. This includes the sarcoglycans (mutations in which cause limbgirdle muscular dystrophies) and dystroglycans. Muscle cells that lack dystrophin are mechanically fragile, and fail after a few years, hence progressive muscle weakness.

35 Duchenne Muscular Dystrophy (DMD) PCR deletion screen 9 selected exons of the dystrophin gene have been amplified from the DNA of a panel of 20 affected boys. When the product is run on an electrophoretic gel each exon gives a band of a characteristic size. Because a boy has only a single X chromosome, any deletion shows up as missing bands. Different exon deletions can be seen in lanes 1, 5, 11, 12, 19 and 20. Lane 3 may be a large deletion or a technical failure. The boys with no deletion on this gel may have others of the 79 dystrophin exons deleted, or may have point mutations or duplications to cause loss of function of the gene.

36 Hemophilia Bleeding disorders have been recognized since ancient times The Talmud (2nd century AD) states that male babies do not have to be circumcised if two brothers have died from the procedure In 12th century Albucasis, an Arab physician, wrote about a family in which males died of excessive bleeding from minor injuries In 1803, Dr. John Otto, Philadelphia, wrote about an inherited hemorrhagic disposition affecting males In 1828 at the University of Zurich, hemophilia" was first used to describe a bleeding disorder

37 Hemophilia Vascular injury NORMAL BLEEDING DISORDER Vascular injury Vasoconstriction Vasoconstriction platelet plug incomplete platelet plug fibrin clot incomplete or delayed fibrin clot

38 Hemophilia - Royal Disease Queen Victoria (1837 to 1901) passed hemophilia on to German, Russian and Spanish royal families. Her son, Leopold, had frequent hemorrhages (British Medical Journal,1868) and died of a brain hemorrhage at 31. His grandson also died of a brain hemorrhage in 1928.

39 Molecular Genetics of Hemophilia Intrinsic pathway Extrinsic pathway Hemophilia B Factor IX deficiency IX VIII Hemophilia A Factor VIII deficiency Other Genetic Disorders with low Factor VIII Von Willebrand Disease (Type 2N) Combined Deficiency of Factor V and Factor VIII

40 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 바소체 If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character

41 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

42 Concept 15.3 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) Genes located on the same chromosome that tend to be inherited together are called linked genes

43 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

44 Figure 15.9 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

45 Morgan found that body color and wing size are usually inherited together in specific combinations (parental phenotypes) He noted that these genes do not assort independently, and reasoned that they were on the same chromosome

46 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

47 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

48 Genetic Recombination and Linkage The genetic findings of Mendel and Morgan relate to the chromosomal basis of recombination

49 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

50 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

51 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

52 Figure 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 + b vg Replication of chromosomes b vg Meiosis I b + vg + b vg b vg b vg b vg b vg b + vg + b + vg b vg + Meiosis I and II Meiosis II b vg Recombinant chromosomes Eggs b + vg + b vg b + vg b vg + Testcross offspring 965 Wild type (gray-normal) 944 Blackvestigial 206 Grayvestigial 185 Blacknormal b + vg + b vg b + vg 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 100 = 17%

53 Figure 15.10a Testcross parents Gray body, normal wings (F 1 dihybrid) b + vg + Black body, vestigial wings (double mutant) 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

54 Figure 15.10b Recombinant chromosomes Eggs b + vg + b vg b + vg b vg + Testcross offspring 965 Wild type (gray-normal) 944 Blackvestigial 206 Grayvestigial 185 Blacknormal b + vg + b vg b + vg 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 100 = 17%

55 New Combinations of Alleles: Variation for Normal Selection Recombinant chromosomes bring alleles together in new combinations in gametes Random fertilization increases even further the number of variant combinations that can be produced This abundance of genetic variation is the raw material upon which natural selection works

56 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

57 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

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

59 Genes that are far apart on the same chromosome can have a recombination frequency near 50% Such genes are physically linked, but genetically unlinked, and behave as if found on different chromosomes

60 Sturtevant used recombination frequencies to make linkage maps of fruit fly genes He and his colleagues found that the genes clustered into four groups of linked genes (linkage groups) The linkage maps, combined with the fact that there are four chromosomes in Drosophila, provided additional evidence that genes are located on chromosomes

61 Using methods like chromosomal banding, geneticists can develop cytogenetic maps of chromosomes Cytogenetic maps indicate the positions of genes with respect to chromosomal features

62 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

63 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

64 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

65 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

66 Aneuploidy 염색체이수성 results from the fertilization of gametes in which nondisjunction occurred Offspring with this condition have an abnormal number of a particular chromosome A monosomic zygote has only one copy of a particular chromosome A trisomic zygote has three copies of a particular chromosome

67 Polyploidy 염색체다배수성 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

68 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

69 Figure 15.14a (a) Deletion A B C D E F G H A deletion removes a chromosomal segment. A B C E F G H (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

70 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

71 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

72 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

73 Figure 15.15

74 Aneuploidy of Sex Chromosomes Nondisjunction of sex chromosomes produces a variety of aneuploid conditions XXX females are healthy, with no unusual physical features 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

75 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

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

77 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

78 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

79 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)

80 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

81 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

82 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

83 Figure 15.18

84 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

85 Figure 15.UN04 Sperm P generation gametes D C B A F E d c b a f e Egg The alleles of unlinked genes are either on This F 1 cell has 2n = 6 chromo- separate chromosomes somes and is heterozygous for all (such as d and e) six genes shown (AaBbCcDdEeFf ). or so far apart on the Red = maternal; blue = paternal. same chromosome Each chromosome has hundreds or thousands F D C B A E e d (c and f ) that they assort independently. Genes on the same chromo- of genes. Four (A, B, C, F ) are shown on this one. c b a f some whose alleles are so close together that they do not assort independently (such as a, b, and c) are said to be genetically linked.