Chapter 15 The Chromosomal Basis of Inheritance

1. Chapter 15 The Chromosomal Basis of Inheritance

2. Timeline • 1866- Mendel's Paper • 1875- Mitosis worked out • 1890's- Meiosis worked out • 1902- Sutton, Boveri et. al. connect chromosomes to Meiosis.

3. Sutton • Developed the “Chromosome Theory of Inheritance”. • Mendelian factors or alleles are located on chromosomes. • Chromosomes segregate and show independent assortment.

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

6. Fruit Fly • Drosophila melanogaster • Early test organism for genetic studies.

7. Reasons • Small • Cheap to house and feed • Short generation time • Many offspring • Few chromosomes

8. Genetic Symbols • Mendel - use of uppercase or lowercase letters. T = tall t = short • Morgan: symbol from the mutant phenotype. + = wild phenotype

9. Examples • Recessive mutation: • w = white eyes • w+ = red eyes • Dominant Mutation • Cy = Curly wings • Cy+ = Normal wings

10. Morgan Observed: • A male fly with a mutation for white eyes.

11. Morgan crossed • The white eye male with a normal red eye female.

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

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

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

15. Fruit Fly Chromosomes

17. Sex linked traits • Sex linked traits in humans will be covered in a few minutes.

18. Morgan Discovered • There are many genes, but only a few chromosomes. • Therefore, each chromosome must carry a number of genes together as a “package”.

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

20. Body Color and Wing type

21. Example b+b vg+vg X bb vgvg (b+ linked to vg+) (b linked to vg) If unlinked: 1:1:1:1 ratio. If linked: ratio will be altered.

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

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

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

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

29. Comment - only good for genes that are within 50 map units of each other. Why?

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

32. Sex Linkage in Biology • Several systems are known: • Mammals – XY and XX • Diploid insects – X and XX • Birds – ZZ and ZW • Social insects – haploid and diploid

34. Homework • Read parts of Chapter 15 • Lab – Genetics of Organisms • Chapter 48 – part II • Chapter 15 Homework • Due in by Dec. 12th noon

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

36. Human Chromosome Sex • Males - XYFemales - XX • Comment - The X and Y chromosomes are a homologous pair, but only for a small region at one tip.

39. SRY • Sex-determining Region Y chromosome gene. • If present - male • If absent - female • SRY codes for a cell surface receptor.

40. Sex Linkage • Inheritance of traits on the sex chromosomes. • X- Linkage (common) • Y- Linkage (very rare if exists at all)

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

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

43. Samples of X-linked patterns:

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

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

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

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

48. Sex Limited Traits • Traits that are only expressed in one sex. • Ex – prostate gland

49. Sex Influenced Traits • Traits whose expression differs because of the hormones of the sex. • These are NOT on the sex chromosomes. • Ex. – beards, mammary gland development, baldness

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