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Overview: Locating Genes Along Chromosomes Mendel’s “hereditary factors” were genes Today we can show that genes are located on chromosomes The location.

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Presentation on theme: "Overview: Locating Genes Along Chromosomes Mendel’s “hereditary factors” were genes Today we can show that genes are located on chromosomes The location."— Presentation transcript:

1 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

2 Figure 15.1

3 CONCEPT 15.1: MENDELIAN INHERITANCE HAS ITS PHYSICAL BASIS IN THE BEHAVIOR OF CHROMOSOMES

4 Mendel and Meiosis P Generation F 1 Generation Yellow-round seeds ( YYRR ) Green-wrinkled seeds ( yyrr )  Meiosis Fertilization Gametes Y Y R R Y R y y r y r All F 1 plants produce yellow-round seeds ( YyRr ). Meiosis Metaphase I Anaphase I Metaphase II R R R R R R R R R R R R rr r r rr r r r r r r YY Y Y Y Y Y Y Y Y Y Y y y y y y y y y y y y y Gametes LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation /41/4 1/41/4 1/41/4 1/41/4 YR yr Yr yR F 2 Generation 3 3 Fertilization recombines the R and r alleles at random. Fertilization results in the 9:3:3:1 phenotypic ratio in the F 2 generation. An F 1  F 1 cross-fertilization 9 : 3 : 1 r

5 P Generation Yellow-round seeds ( YYRR ) Green-wrinkled seeds ( yyrr )  Meiosis Fertilization Gametes Y Y R R Y R y y r y r r Mendel and Meiosis

6 Figure 15.2b F 1 Generation All F 1 plants produce yellow-round seeds ( YyRr ). Meiosis Metaphase I Anaphase I Metaphase II R R R R R R R R R R R R r r r r r r r r r r r r YY Y Y Y Y Y Y Y Y Y Y yy y y y y y y y y y y Gametes LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation /41/4 1/41/4 1/41/4 1/41/4 YR yr Yr yR

7 F 2 Generation 3 Fertilization recombines the R and r alleles at random. Fertilization results in the 9:3:3:1 phenotypic ratio in the F 2 generation. An F 1  F 1 cross-fertilization 9 : 3 : 1 LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT 3 Mendel and Meiosis

8 Morgan’s Experimental Evidence for Mendel 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

9 Morgan’s Choice of Experimental Organism 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

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

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

12 All offspring had red eyes. P Generation F 1 Generation F 2 Generation RESULTS EXPERIMENT

13 F 2 Generation P Generation Eggs Sperm X ww CONCLUSION X X Y ww ww ww ww ww ww ww ww ww ww w w w w w w F 1 Generation

14 CONCEPT 15.2: SEX-LINKED GENES EXHIBIT UNIQUE PATTERNS OF INHERITANCE In humans and some other animals, there is a chromosomal basis of sex determination

15 The Chromosomal Basis of Sex Only the ends of the Y chromosome have regions that are homologous with corresponding regions of the X chromosome The SRY gene on the Y codes for a protein that directs the development of male anatomical features X Y

16 Sex Determination Systems Parents or Sperm or Egg Zygotes (offspring) 44  XY 44  XX 22  X 22  Y 22  X 44  XX 44  XY 22  XX 22  X 76  ZW 76  ZZ 32 (Diploid) 16 (Haploid) (a) The X-Y system (b) The X-0 system (c) The Z-W system (d) The haplo-diploid system Human males are XY Human females are XX Other organisms use different systems!

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

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

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

20 X-linked Inheritance X chromosomes code for many genes not related to sex Some disorders caused by recessive alleles on the X chromosome in humans ◦ Color blindness (mostly X-linked) ◦ Duchenne muscular dystrophy ◦ Hemophilia

21 Transmitting X-linked genes: Colorblindness Eggs Sperm (a) (b) (c) XNXNXNXN XnYXnY XNXnXNXn XNYXNY XNXnXNXn XnYXnY XnXn Y XNXN Y Y XnXn XnXn XnXn XNXN XNXN XNXN XNXN XNXnXNXn XNYXNY XNYXNY XNYXNYXNYXNY XnYXnY XnYXnY XNXnXNXn XNXnXNXn XNXnXNXn XNXNXNXN XnXnXnXn X-linked recessive disorders are much more common in males than in females 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)

22 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

23 Early embryo: X chromosomes Allele for orange fur Allele for black fur Two cell populations in adult cat: Cell division and X chromosome inactivation Active X Inactive X Active X Black fur Orange fur Female Mosaics: X Inactivation

24 CONCEPT 15.3: LINKED GENES TEND TO BE INHERITED TOGETHER BECAUSE OF LOCATION 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

25 Gene Linkage Linked genes assort together

26 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 Wild type (gray body, normal wings) Double mutant (black body, vestigial wings)

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

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

29 Figure P Generation (homozygous) Wild type (gray body, normal wings) F 1 dihybrid (wild type) Testcross offspring TESTCROSS b  b  vg  vg  b  b vg  vg b b vg vg Double mutant (black body, vestigial wings) Double mutant Eggs Sperm EXPERIMENT Wild type (gray-normal) Black- vestigial Gray- vestigial Black- normal b  vg  b vg b  vgb vg  b  b vg  vg b b vg vgb  b vg vgb b vg  vg b vg

30 Figure P Generation (homozygous) Wild type (gray body, normal wings) F 1 dihybrid (wild type) Testcross offspring TESTCROSS b  b  vg  vg  b  b vg  vg b b vg vg Double mutant (black body, vestigial wings) Double mutant Eggs Sperm EXPERIMENT RESULTS PREDICTED RATIOS Wild type (gray-normal) Black- vestigial Gray- vestigial Black- normal b  vg  b vg b  vgb vg  b  b vg  vg b b vg vgb  b vg vgb b vg  vg If genes are located on different chromosomes: If genes are located on the same chromosome and parental alleles are always inherited together: : : : : : : : : : b vg

31 Most offspring F 1 dihybrid female and homozygous recessive male in testcross or b + vg + b vg b + vg + b vg Morgan found body color and wing size are usually inherited together in specific combinations (parental phenotypes)

32 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

33 Genetic Recombination and Linkage Offspring with a phenotype matching one of the parental phenotypes are called parental types Offspring with nonparental phenotypes are called recombinant types, or recombinants A 50% frequency of recombination is observed for any two genes on different chromosomes

34 Parental vs. Recombinant Phenotypes Gametes from green- wrinkled homozygous recessive parent ( yyrr ) Gametes from yellow-round dihybrid parent ( YyRr ) Recombinant offspring Parental- type offspring YR yr Yr yR yr YyRr yyrr Yyrr yyRr

35 Recombination of Linked Genes: Crossing Over Morgan discovered that genes can be linked, but the linkage was incomplete, because some recombinant phenotypes were observed Some process must occasionally break the physical connection between genes on the same chromosome

36 Crossing Over and Recombination Testcross parents Replication of chromosomes Gray body, normal wings (F 1 dihybrid) Black body, vestigial wings (double mutant) Replication of chromosomes Meiosis I Meiosis II Meiosis I and II Recombinant chromosomes Eggs b  vg  b vg b  vg  b  vg b vg  b vg b  vg  b  vg b vg b vg  Testcross offspring 965 Wild type (gray-normal) 944 Black- vestigial 206 Gray- vestigial 185 Black- normal Sperm Parental-type offspring Recombinant offspring Recombination frequency 391 recombinants 2,300 total offspring  100  17%  b  vg  b vg  b  vg b vg Recombinant chromosomes bring alleles together in new combinations in gametes Genetic variation drives evolution!

37 Chromosome Recombination frequencies 9% 9.5% 17% b cn vg Mapping Distance Between Genes Using Recombination Data A linkage map is a genetic map of a chromosome based on recombination frequencies The farther apart two genes are, the higher the probability crossover will occur and therefore the higher the recombination frequency Distances between genes can be expressed as map units. 1 map unit = 1% recombination frequency

38 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

39 Genetic Linkage Map: Cytogenic Mapping Mutant phenotypes Short aristae Black body Cinnabar eyes Vestigial wings Brown eyes Long aristae (appendages on head) Gray body Red eyes Normal wings Red eyes Wild-type phenotypes

40 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

41 Chromosomal Changes and Disorders Abnormal Chromosome Number Alteration of Chromosome Structure

42 Meiosis I Nondisjunction Abnormal Chromosome Number In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis

43 Meiosis I Meiosis II Nondisjunction Non- disjunction Abnormal Chromosome Number As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy

44 Meiosis I Meiosis II Nondisjunction Non- disjunction Gametes Number of chromosomes Nondisjunction of homo- logous chromosomes in meiosis I (a) Nondisjunction of sister chromatids in meiosis II (b) n  1 n  1 n  1 n  1 n  1 n n Abnormal Chromosome Number

45 Aneuploidy The fertilization of gametes in which nondisjunction occurred Offspring with this condition have an abnormal number of a particular chromosome

46 A monosomic zygote has only one copy of a particular chromosome A trisomic zygote has three copies of a particular chromosome

47 Polyploidy: More than 2 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 Triploidy (3n) Hexaploidy (6n) Octoploidy (8n)

48 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

49 (a) Deletion (b) Duplication A deletion removes a chromosomal segment. A duplication repeats a segment. BACDEFGH ABCDEFGH ABCDEFGHBC ABCEFGH Alterations of Chromosome Structure

50 (c) Inversion (d) Translocation An inversion reverses a segment within a chromosome. A translocation moves a segment from one chromosome to a nonhomologous chromosome. ABCDEFGH ADCBEFGH ABC D EFGH MNOPQR G MNOC HFED ABPQ R Alterations of Chromosome Structure

51 Human Disorders Due to Chromosomal Alterations Most mammalian aneuploidy are lethal but some survive to birth and beyond These surviving individuals have a set of symptoms, or syndrome.

52 Down Syndrome (Trisomy 21) Down syndrome is an aneuploid condition that results from three copies of chromosome 21 It affects about 1in 700 children born in the US The frequency of Down syndrome increases with the age of the mother

53 Aneuploidy of Sex Chromosomes SyndromeGenotypeOccurrenceSymptoms Klinefelter Syndrome XXY1/500 ~ 1/1000 Underdeveloped testes Male is sterile Some secondary female characteristics XYY SyndromeXYY1/1000 Normal sexual development Taller than average Increased aggression Turner Syndrome (Monosomy X) XO1/2500 Only viable monosomy in humans Sex organs do not develop; sterile Trisomy XXXX1/1000 Learning disabled Fertile Slightly taller

54 Turner’s SyndromeKlinefelter Syndrome

55 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

56 CML: Chronic Myelogenous Leukemia Normal chromosome 9 Normal chromosome 22 Reciprocal translocation Translocated chromosome 9 Translocated chromosome 22 (Philadelphia chromosome) CML caused by translocations of chromosomes

57 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

58 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

59 Figure (a) Homozygote Paternal chromosome Maternal chromosome 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) Dwarf mouse (mutant) Normal Igf2 allele is expressed. Mutant Igf2 allele is expressed. Mutant Igf2 allele is not expressed. Normal Igf2 allele is not expressed. (b) Heterozygotes

60 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

61 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

62 Figure 15.18

63 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

64 Figure 15.UN03 P generation gametes Sperm Egg This F 1 cell has 2 n  6 chromo- somes and is heterozygous for all six genes shown ( AaBbCcDdEeFf ). Red  maternal; blue  paternal. Each chromosome has hundreds or thousands of genes. Four ( A, B, C, F ) are shown on this one. The alleles of unlinked genes are either on separate chromosomes (such as d and e ) or so far apart on the same chromosome ( c and f ) that they assort independently. Genes on the same chromo- 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. D C B A F E d c b a f e D C B A F e d E f c b a

65 Figure 15.UN04

66 Figure 15.UN05

67 Figure 15.UN06


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