Chapter 15: The Chromosomal Basis of Inheritance

1. Chapter 15: The Chromosomal Basis of Inheritance

2. Essential Knowledge • 3.a.4 – The inheritance pattern of many traits cannot be explained by simple Mendelian genetics (15.1, 15.2, 15.3, 15.5). • 3.c.1 – Changes in genotype can result in changes in phenotype (15.4).

3. Sutton (1902) • Developed the “Chromosome Theory of Inheritance” 1) Mendelian factors or alleles are located on chromosomes 2) Chromosomes segregate and show independent assortment

5. Morgan • Embryologist at Columbia University • 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 • Feeds on fungus growing on fruit • Early test organism for genetic studies

7. Reasons he chose fruit fly • Small • Cheap to house and feed • Short generation time • New generation every 2 weeks • 100s of offspring produced • Few chromosomes • 4 pairs (8 total) • 3 pairs autosomes, 1 pair sex

8. Genetic Symbols • Mendel: use of uppercase or lowercase letters • T = tall • t = short • Morgan: symbol from the mutant phenotype • + = wild phenotype (natural pheno) • No symbol = mutant phenotype (any pheno different from wild)

9. Examples • Recessive mutation: • w = white eyes • w+ = red eyes • Dominant mutation: • Cy = Curly wings • Cy+ = Normal wings • Letters come from 1st mutant trait observed

10. Morgan Observed: • A male fly with a mutation for white eyes • Then, he crossed the white eye male with normal red eye female • All had red eyes • Same as Mendel’s F1 • This suggests that white eyes is a genetic recessive

11. 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 • Most red eyed flies were FEMALE • Therefore, the eye color trait appeared to be linked to sex

12. Morgan discovered: • Sex-linked traits • Genetic traits whose expression are dependent on the sex of the individual • Genes on sex chromosomes exhibited unique patterns

13. Eye color gene located on X chromo (with no corresponding gene on Y)

14. Morgan Discovered • There are many genes, but only a few chromosomes • Therefore, each chromosome must carry a number of genes together as a “package” • There was a correlation between a particular trait and an individual’s sex

15. Linked Genes • Traits that are located on the same chromosome (that tend to be inherited together) • Result: • Failure of (deviation from) Mendel's Law of Independent Assortment. • Ratios mimic monohybrid crosses.

16. Body Color and Wing type

17. Body color and wing type • Wild: gray and normal (dom +) • Mutant: Black and vestigal (rec) • This is why we use “b” for body color alleles and “vg” for wing alleles • Symbols: • Body color - b+: gray; b: black • Wings - vg+: normal; vg: vestigal

18. Example b+bvg+vg X bb vgvg #1: b+b = gray; vg+vg = normal #2: bb = black; vgvg = vestigal (b+ linked to vg+) (b linked to vg) • If unlinked: 1:1:1:1 ratio. • If linked: ratio will be altered

22. Crossing-Over • Occurs during Pro I of meiosis • Breaks up linkages and creates new ones • Recombinant offspring formed that doesn't match the parental types

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

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

25. Genetic Maps • Constructed from crossing-over frequencies • 1 map unit = 1% recombination frequency • Have been constructed for many traits in fruit flies, humans and other organism.

27. Sex Linkage in Biology • Several systems are known: • Mammals – XX and XY • Diploid insects – X and XX • Birds – ZZ and ZW • Haploid-Diploid

28. Sex Linkage in Biology • Mammals • Determined by whether sperm has X or Y • Diploid insects • Only X chromosomes present • Birds • Egg determines sex • Haploid-Diploid • Females develop from fert egg • Males develop from unfert egg

30. Chromosomal Basis of Sex in Humans • Sex determination ALWAYS 50-50 • X chromosome- medium sized chromosome with a large number of traits • Y chromosome - much smaller chromosome with only a few traits

31. Human Chromosome Sex • Eggs – only contain X • Sperm – either X or Y • Males - XYFemales - XX • Comment - The X and Y chromosomes are a homologous pair, but only for a small region at one tip

34. Sex Linkage • Inheritance of traits on the sex chromosomes • NOT TO BE CONFUSED WITH sex-linked traits!!!!! • X Linkage - common; Y- rare • Dads: only to daughters (b/c dads ONLY give X chromo to daughters) • Moms: to either sex

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

36. X-linked Disorders and Patterns • Disorders on X-chromo: • Color blindness • Duchenne's Muscular Dystrophy • Hemophilia (types a and b) • Patterns • Trait is usually passed from a carrier mother to 1 of 2 sons • Affected father has no affected sons, but passes the trait on to all daughters (who will be carriers for the trait)

37. Comment • Watch how questions with sex linkage are phrased: • Chance of children? • Chance of males? • Chance of females? • You MUST practice genetics problems w/ these traits: Hemophilia, Muscular dystrophy and colorblindness (they all work the same!)

38. Can Females be color-blind? • Yes!!! • ONLY if their mother was a carrier and their father is affected • How? Mother contributes X (with affected allele) and dad contributes all he can to make a daughter – affected X

40. Are you color blind? 25 29 45 56 6 8

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

42. Sex Limited Traits • Traits that are only expressed in one sex • Ex: prostate development, gonad specialization, fallopian tube development

43. Sex Influenced Traits • Traits whose expression differs because of the hormones of the sex • Ex: beards, mammary gland development, baldness • Baldness: • Testosterone – makes the trait act as a dominant • No testosterone – makes the trait act as a recessive • Males – have gene = bald • Females – must be homozygous to have thin hair (rare)

44. X chromosome inactivation • In every somatic cell (in females), one X chromosome is inactivated • Humans: differs/random • Kangaroos: always paternal X that is inactivated • Called Barr bodies

45. Barr Body • Inactive X chromosome observed in the nucleus • Becomes inactive during embryonic development • Way of determining genetic sex (without doing a karyotype)

46. Barr body description • Compact body which lies close to nuclear envelope • Most genes on this X are NOT expressed • Inside developed ovaries, these are reactivated (so that each ova will get an active X)

47. Lyon Hypothesis • Which X inactivated is random • Inactivation happens early in embryo development by adding CH3 groups to the DNA • Changes DNA nucleotide • Result - body cells are a mosaic/combo of X types • Some have active X from mom, others active X from dad

48. Examples • Calico Cats • Human examples are known (sweat gland disorder)

49. Question? • Why don’t you find many calico males? • They must be XB XOY and are always sterile • Why? • They MUST have an extra X chromo (to have an inactive X - you must have TWO!)

50. Chromosomal Alterations • Two types of alterations: • Changes in number • Changes in structure