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  • 1. Chapter 15 The Chromosomal Basis of Inheritance Rob Swatski Associate Professor of Biology HACC – York Campus 1
  • 2. Overview: Genes and Chromosomes We know Mendel’s “hereditary factors” are genes We know genes are located on chromosomes in specific regions (loci) - Fluorescent “tagging” 2
  • 3. Mitosis & meiosis were first described in the late 1800’s The chromosome theory of inheritance: - Mendelian genes have specific loci on chromosomes - Chromosomes undergo segregation & independent assortment Chromosome behavior during meiosis accounts for Mendel’s laws 3
  • 4. Green-wrinkled seeds ( yyrr) Yellow-round seeds (YYRR) P Generation Y Y R R r  y y r Meiosis Fertilization Gametes R Y y r All F1 plants produce yellow-round seeds (YyRr) 4
  • 5. All F1 plants produce yellow-round seeds (YyRr) F1 Generation 0.5 mm R R y r Y LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. y r Y LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. Meiosis r R Y r R y Metaphase I Y y 1 1 r R r R Y y Anaphase I Y y r R Metaphase II R r 2 2 Gametes y Y Y R R 1 4 YR r 1 3 4 yr Y Y y r y Y y Y r r 14 Yr y y R R 14 yR 3 5
  • 6. LAW OF INDEPENDENT ASSORTMENT LAW OF SEGREGATION F2 Generation 3 Fertilization recombines the R and r alleles at random. An F1  F1 cross-fertilization 9 :3 :3 :1 3 Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation. 6
  • 7. Morgan’s Experimental Evidence • The 1st solid evidence associating a specific gene with a specific chromosome came from the embryologist, Thomas Hunt Morgan • Morgan’s experiments with fruit flies provided convincing evidence that Mendel’s heritable factors are located on chromosomes 7
  • 8. Why Fruit Flies? - Fast breeding rate & lots of offspring - New generation produced every 2 weeks - The have only 4 pairs of chromosomes 8
  • 9. Wild type (normal) phenotypes were common in the fly populations Traits alternative to the wild type are called mutant phenotypes wild type mutant 9
  • 10. Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair In one mating experiment, Morgan crossed: - male white eyes (mutant) with female red eyes (wild type) - The F1 generation all had red eyes - The F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes - The white-eyed mutant allele must be on the X chromosome Morgan’s finding supported the chromosome theory of inheritance 10
  • 11. EXPERIMENT P Generation F1 Generation All offspring had red eyes. RESULTS F2 Generation 11
  • 12. CONCLUSION P Generation X X w X Y w w Eggs F1 Generation Sperm w w w w w Eggs F2 Generation w w w Sperm w w w w w w 12
  • 13. The Chromosomal Basis of Sex In humans & other mammals, there are 2 varieties of sex chromosomes: - larger X chromosome X - smaller Y chromosome Only the ends of Y chromosomes are homologous with corresponding regions of the X chromosomes Y The SRY gene on the Y chromosome codes for the development of testes 13
  • 14. 44 + XY Parents 22 + 22 + or Y X Sperm 44 + XX + 44 + XX 22 + X Egg or 44 + XY Zygotes (offspring) (a) The X-Y system: females are XX and males are XY 14
  • 15. X0 22 + XX 22 + X (b) The X-0 system (insects) 76 + ZW 76 + ZZ (c) The Z-W system (birds, fishes, insects) - Sex chromosomes are present in the egg (not sperm) 15
  • 16. 32 (Diploid) 16 (Haploid) (d) The haplo-diploid system (bees & ants) - No sex chromosomes 16
  • 17. Inheritance of Sex-Linked Genes Sex-linked gene: a gene that is located on either sex chromosome - in humans, this usually refers to x-linked genes, which are genes located on the X chromosome (there are few Y-linked genes) - X chromosomes have genes for many characters unrelated to sex 17
  • 18. X-linked genes follow specific patterns of inheritance For a recessive sex-linked trait to be expressed: - A female needs 2 copies of the allele (XnXn) = homozygous - A male needs only 1 copy of the allele (XnY) = hemizygous Sex-linked recessive disorders are much more common in males than in females – why? 18
  • 19. Transmission of Sex-Linked Recessive Traits XNXN Sperm Xn  XnY XNXn Sperm XN Y Eggs XN XNXn XNY XN XNY XNXn Sperm Xn Y Eggs (b) XN XNXN XNY Eggs XN Xn XNXn XNY (a)  XnXN XnY Xn  XnY Y XNXn XNY XnXn XnY (c) 19
  • 20. Sex-linked recessive disorders in humans include: - Color blindness - Duchenne muscular dystrophy - Hemophilia 20
  • 21. X Inactivation in Female Mammals In female mammals, 1 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 21
  • 22. X chromosomes Early embryo: 2 cell populations in adult cat: Allele for orange fur Allele for black fur Cell division & X chromosome inactivation Active X Inactive X Active X Black fur Orange fur 22
  • 23. Each chromosome has 100’s or 1000’s of genes (except for the Y chromosome) Linked genes: located near each other on the same chromosome - Linked genes are usually inherited together 23
  • 24. How Linkage Affects Inheritance Morgan experimented with fruit flies to see how linkage affects the inheritance of 2 characters He crossed flies that differed in: - body color & wing size 24
  • 25. EXPERIMENT P Generation (homozygous) Wild type (gray body, normal wings) Double mutant (black body, vestigial wings) b b vg vg b b vg vg F1 dihybrid (wild type) Double mutant TESTCROSS b b vg vg b b vg vg Testcross offspring Eggs b vg b vg b vg Wild type Black(gray-normal) vestigial b vg Blacknormal Grayvestigial b vg Sperm b b vg vg b b vg vg b b vg vg b b vg vg PREDICTED RATIOS If genes are located on different chromosomes: 1 : 1 : 1 : 1 If genes are located on the same chromosome and parental alleles are always inherited together: 1 : 1 : 0 : 0 965 : 944 : 206 : 185 RESULTS 25
  • 26. Conclusion: body color & wing size are usually inherited together in specific combinations (parental phenotypes) These genes do not assort independently & Morgan reasoned that they were on the same chromosome 26
  • 27. b+ vg+ b vg b vg b vg b+ vg+ F1 dihybrid female & homozygous recessive male in testcross b vg Most offspring or b vg b vg Results: a much higher % of parental phenotypes than would be expected by independent assortment 27
  • 28. However, non-parental phenotypes were also produced Understanding this result involves exploring genetic recombination - the production of offspring with combinations of traits different from either parent Both findings of Mendel & Morgan relate to the chromosomal basis of recombination 28
  • 29. Recombination of Unlinked Genes: Independent Assortment of Chromosomes Mendel observed that combinations of traits in some offspring differ from either parent - Parental types: offspring with a phenotype matching 1 of the parental phenotypes - Recombinant types (recombinants): offspring with non-parental phenotypes (new combinations of traits) A 50% frequency of recombination is observed for any 2 genes on different chromosomes 29
  • 30. Gametes from yellow-round heterozygous parent (YyRr) YR Gametes from greenwrinkled homozygous recessive parent ( yyrr) yr Yr yR YyRr yyrr Yyrr yyRr yr Parentaltype offspring Recombinant offspring 30
  • 31. Recombination of Linked Genes: Crossing Over Morgan discovered that genes can be linked, but the linkage was incomplete (as shown by the appearance of recombinant phenotypes) Morgan proposed that some process must sometimes break the physical connection between genes on the same chromosome = the crossing over of homologous chromosomes 31
  • 32. Black body, vestigial wings (double mutant) Gray body, normal wings (F1 dihybrid) Testcross parents b+ vg+ b vg Replication of chromosomes b vg b vg b+ vg+ b+ vg+ b vg b vg b vg b vg Meiosis I b vg b vg b+ vg+ Crossingover Replication of chromosomes Meiosis I and II b+ vg b vg+ b vg Meiosis II Recombinant chromosomes b+ vg+ b vg Eggs b+ vg b vg+ b vg Sperm 32
  • 33. Recombinant chromosomes Eggs Testcross offspring b+ vg+ 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 b vg b vg b vg Sperm Parental-type offspring Recombinant offspring Recombination 391 recombinants = frequency 2,300 total offspring  100 = 17% 33
  • 34. 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 34
  • 35. Genetic Maps Alfred Sturtevant, one of Morgan’s students, constructed a genetic map: an ordered list of loci along a particular chromosome Sturtevant predicted that: the farther apart 2 genes are, the higher the probability that a crossover will occur between them & therefore the higher the recombination frequency 35
  • 36. Linkage Maps Genetic map of a chromosome based on recombination frequencies Distances between genes expressed as map units: 1 map unit (centimorgan) = 1% recombination frequency Map units indicate relative distance & order, not precise locations of genes 36
  • 37. RESULTS Recombination frequencies 9% Chromosome 9.5% 17% b cn vg 37
  • 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, & behave as if found on different chromosomes 38
  • 39. Sturtevant used recombination frequencies to make linkage maps of fruit fly genes Using methods like chromosomal banding, geneticists can develop cytogenetic maps of chromosomes - Indicate positions of genes with respect to chromosomal features 39
  • 40. Mutant phenotypes Cinnabar Vestigial Black eyes wings body Short aristae 48.5 0 Long aristae (appendages on head) Gray body 57.5 67.0 Red eyes Normal wings Brown eyes 104.5 Red eyes Wild-type phenotypes A partial linkage map of a Drosophila chromosome 40
  • 41. Alteration of Chromosome Number or Structure Cause Some Genetic Disorders Large-scale chromosomal alterations often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders Plants tolerate such genetic changes better than animals do 41
  • 42. Abnormal Chromosome Number Nondisjunction: pairs of homologous chromosomes do not separate normally during meiosis - As a result, one gamete receives 2 of the same type of chromosome & another gamete receives no copy 42
  • 43. 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 43
  • 44. Aneuploidy: results from the fertilization of gametes in which nondisjunction occurred - offspring have an abnormal number of a particular chromosome Monosomic zygote: has only 1 copy of a particular chromosome Trisomic zygote: has 3 copies of a particular chromosome 44
  • 45. Polyploidy: organism has more than 2 complete sets of chromosomes - Common in plants, but not animals - Polyploids are more normal in appearance than aneuploids - Triploidy (3n): 3 sets of chromosomes - Tetraploidy (4n): 4 sets of chromosomes 45
  • 46. Alterations of Chromosome Structure Breakage of a chromosome can lead to 4 types of changes in chromosome structure: - Deletion: removes a chromosomal segment - Duplication: repeats a segment - Inversion: reverses the orientation of a segment within a chromosome - Translocation: moves a segment from 1 chromosome to another 46
  • 47. (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 (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 47
  • 48. Human Disorders Due to Chromosomal Alterations Some types of aneuploidy have higher survival rates, resulting in individuals surviving to birth & beyond These survivors have a set of symptoms (syndrome) characteristic of the type of aneuploidy 48
  • 49. Down Syndrome (Trisomy 21) Aneuploidy due to 3 copies of chromosome 21 - affects 1 out of every 700 US children - frequency increases with mother’s age… 49
  • 50. 50
  • 51. Aneuploidy of Sex Chromosomes Due to nondisjunction of sex chromosomes Klinefelter syndrome: results from an extra X chromosome in a male, producing XXY individuals Monosomy X (Turner syndrome): produces sterile X0 females - the only known viable monosomy in humans 51
  • 52. Klinefelter Syndrome 52
  • 53. Turner Syndrome 53
  • 54. Disorders Caused by Structurally Altered Chromosomes Cri du chat syndrome: due to a specific deletion in chromosome 5 - mental retardation, catlike cry - individuals usually die in infancy or early childhood 54
  • 55. Disorders Caused by Structurally Altered Chromosomes Certain cancers are caused by translocations of chromosomes - Chronic myelogenous leukemia (CML) 55
  • 56. Normal chromosome 9 Normal chromosome 22 Reciprocal translocation Translocated chromosome 9 Translocated chromosome 22 (Philadelphia chromosome) 56
  • 57. There are 2 normal exceptions to Mendelian genetics 1. One involves genes located inside the nucleus (genomic imprinting) 2. The other involves genes located outside the nucleus (extranuclear genes) In both cases, the sex of the parent contributing an allele is a factor in the pattern of inheritance 57
  • 58. Genomic Imprinting For a few mammalian developmental traits, phenotype depends on which parent passed along those alleles - this phenotype variation is called genomic imprinting - involves the silencing of certain genes that are “stamped” with an imprint during gamete production - due to the methylation (addition of –CH3) of cysteine nucleotides of DNA 58
  • 59. Paternal chromosome Normal Igf2 allele is expressed Maternal chromosome Normal Igf2 allele is not expressed Wild-type mouse (normal size) (a) Homozygote Genomic imprinting of the mouse Igf2 gene 59
  • 60. Mutant Igf2 allele inherited from mother Mutant Igf2 allele inherited from father Normal size 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
  • 61. Inheritance of Organelle Genes Extranuclear (cytoplasmic) genes: found in organelles in the cytoplasm - mitochondria, chloroplasts, & 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 61
  • 62. 62
  • 63. Some defects in mitochondrial genes prevent normal ATP synthesis - result in diseases that affect the muscular & nervous systems - mitochondrial myopathy 63
  • 64. Credits by Rob Swatski, 2013 Visit my website for more Biology study resources! Please send your comments and feedback to: Images used in this work bear a Creative Commons license and are attributed to their original authors. This work bears an Attribution-Noncommercial Share Alike Creative Commons license. 64