1. Chapter 15 The Chromosomal Basis of Inheritance

2. Morgan u Chose to use fruit flies as a test organism in genetics. u Allowed the first tracing of traits to specific chromosomes.

3. Fruit Fly u Drosophila melanogaster u Early test organism for genetic studies.

4. Life Cycle u Egg u Larva u Pupa u Adult

5. Reasons u Small u Cheap to house and feed u Short generation time u Many offspring u Few chromosomes

6. Fruit Fly Chromosomes

7. Morgan Observed: u A male fly with a mutation for white eyes.

8. Morgan crossed u The white eye male with a normal red eye female.

9. The F1 offspring: u All had red eyes. u This suggests that white eyes is a genetic _________? u Recessive.

10. F1 X F1 = F2 u Morgan expected the F2 to have a 3:1 ratio of red:white u He got this ratio, however, all of the white eyed flies were MALE. u Therefore, the eye color trait appeared to be linked to sex.

11. Morgan discovered: u Sex linked traits. u Genetic traits whose expression are dependent on the sex of the individual.

12. Sex Linkage in Biology u Several systems are known: 1. Mammals – XX and XY 2. Diploid insects – X and XX 3. Birds – ZZ and ZW

13. Chromosomal Basis of Sex in Humans u X chromosome - medium sized chromosome with a large number of traits. u Y chromosome - much smaller chromosome with only a few traits.

14. Human Chromosome Sex u Males - XY Females - XX u Comment - The X and Y chromosomes are a homologous pair, but only for a small region at one tip.

17. Sex Linkage u Inheritance of traits on the sex chromosomes. u X- Linkage (common) u Y- Linkage (rare)

18. Males u Hemizygous - 1 copy of X chromosome. u Show ALL X traits (dominant or recessive). u More likely to show X recessive gene problems than females.

19. X-linked Disorders u Color blindness u Duchenne's Muscular Dystrophy u Hemophilia (types a and b)

20. Samples of X-linked patterns:

21. X-linked Patterns u Trait is usually passed from a carrier mother to 1/2 of sons. u Affected father has no affected children, but passes the trait on to all daughters who will be carriers for the trait.

22. Comment u Watch how questions with sex linkage are phrased: u Chance of children? u Chance of males?

23. Can Females be color-blind? u Yes, if their mother was a carrier and their father is affected.

24. Y-linkage u Hairy ear pinnae. u Comment - new techniques have found a number of Y-linked factors that can be shown to run in the males of a family. u Ex: Jewish priests

25. Sex Limited Traits u Traits that are only expressed in one sex. u Ex – prostate glands

26. Sex Influenced Traits u Traits whose expression differs because of the hormones of the sex. u Ex. – beards, mammary gland development, baldness

27. Baldness u Testosterone – makes the trait act as a dominant. u No testosterone – makes the trait act as a recessive. u Males – have gene = bald u Females – must be homozygous to have thin hair.

28. Barr Body u Inactive X chromosome observed in the nucleus. u Way of determining genetic sex without doing a karyotype.

29. Lyon Hypothesis u Which X inactivated is random. u Inactivation happens early in embryo development by adding CH 3 groups to the DNA. u Result - body cells are a mosaic of X types.

30. Examples u Calico Cats. u Human examples are known such as a sweat gland disorder.

31. Calico Cats u X B = black fur u X O = orange fur u Calico is heterozygous, X B X O.

33. Question? u Why don’t you find many calico males? u They must be X B X O Y

34. Linked Genes u Traits that are located on the same chromosome. u Result: u Failure of Mendel's Law of Independent Assortment. u Ratios mimic monohybrid crosses.

38. Crossing-Over u Breaks up linkages and creates new ones. u Recombinant offspring formed that doesn't match the parental types.

39. If Genes are Linked: u Independent Assortment of traits fails. u Linkage may be “strong” or “weak”.

40. Linkage Strength u Degree of strength related to how close the traits are on the chromosome. u Weak - farther apart u Strong - closer together

41. Genetic Maps u Constructed from crossing- over frequencies. u 1 map unit = 1% recombination frequency.

43. Genetic Maps u Have been constructed for many traits in fruit flies, humans and other organisms.

45. Chromosomal Alterations u Changes in number. u Changes in structure.

46. Number Alterations u Aneuploidy - too many or too few chromosomes, but not a whole “set” change. u Polyploidy - changes in whole “sets” of chromosomes.

47. Aneuploidy u Caused by nondisjunction, the failure of a pair of chromosomes to separate during meiosis.

49. Comment u Nondisjunction in Meiosis I produces 4 abnormal gametes. u Nondisjunction in Meiosis II produces 2 normal and 2 abnormal gametes.

50. Types of Aneuoploidy u Monosomy: 2N – 1 (very rare) u Trisomy: 2N + 1 (more common)

51. Turner Syndrome u 2N - 1 or 45 chromosomes Genotype: X_ or X0. u Phenotype: female, but very poor secondary sexual development.

52. Characteristics u Short stature. u Extra skin on neck. u Broad chest. u Usually sterile u Normal mental development except for some spatial problems.

53. Question u Why are Turner Individuals usually sterile? u Odd chromosome number. u Two X chromosomes needed for ovary development.

54. Other Sex Chromosome changes u Kleinfelter Syndrome u Meta female u Supermale

55. Kleinfelter Syndrome u 2N + 1 (2N + 2, 2N + 3) u Genotype: XXY (XXXY, XXXXY) u Phenotype: male, but sexual development may be poor. Often taller than average, mental development fine (XXY), usually sterile. u More X = more mental problems

57. George Washington u May have been a Kleinfelter Syndrome individual. u Much taller than average. u Produced no children.

58. Meta female u 2N + 1 or 2N + 2 u Genotype: XXX or XXXX u Phenotype: female, but sexual development poor. Mental impairment common.

59. Super male u 2N + 1 or 2N + 2 u Genotype: XYY or XYYY u Phenotype: male, usually normal, fertile.

61. Trisomy events u Trisomy 21: Downs Syndrome u Trisomy 13: Patau Syndrome u Both have various physical and mental changes.

63. Question? u Why is trisomy more common than monosomy? u Fetus can survive an extra copy of a chromosome, but being hemizygous is usually fatal.

64. Question? u Why is trisomy 21 more common in older mothers? u Maternal age increases risk of nondisjunction.

65. Polyploid u Triploid= 3N u Tetraploid= 4N u Usually fatal in animals.

66. Question? u In plants, even # polyploids are often fertile, why odd # polyploids are sterile. Why? u Odd number of chromosomes can’t be split during meiosis to make spores.

67. Structure Alterations u Deletions u Duplications u Inversions u Translocations

68. Result u Loss of genetic information. u Position effects: a gene's expression is influenced by its location to other genes.

70. Cri Du Chat Syndrome u Part of p arm of #5 missing. u Good survival. u Severe mental retardation. u Small sized heads common.

71. Parental Imprinting of Genes u Gene expression and inheritance depends on which parent passed on the gene. u Usually caused by different methylations of the DNA.

72. Example: u Prader-Willi Syndrome and Angelman Syndrome u Both lack a small gene region from chromosome 15. u Male imprint: Prader-Willi Female imprint: Angelman

73. Cause: u Imprints are "erased" in gamete producing cells and re-coded by the body according to its sex.

74. Why have parental imprinting? u Method to detect that TWO different sets of chromosomes are in the zygote.

75. Summary u Know about linkage and crossing-over. u Sex chromosomes and their pattern of inheritance. u Variations of chromosomes and inheritance patterns.

76. Summary u Be able to work genetics problems for this chapter.