1 / 92

Chapter 15: The Chromosomal Basis of Inheritance

Chapter 15: The Chromosomal Basis of Inheritance. 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). Sutton (1902).

adie
Download Presentation

Chapter 15: The Chromosomal Basis of Inheritance

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

  4. 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

  5. Fruit Fly • Drosophila melanogaster • Feeds on fungus growing on fruit • Early test organism for genetic studies

  6. 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

  7. 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)

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

  9. 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

  10. 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

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

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

  13. 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

  14. 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.

  15. Body Color and Wing type

  16. 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

  17. 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

  18. 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

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

  20. 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

  21. 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.

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

  23. 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

  24. 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

  25. 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

  26. 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

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

  28. 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)

  29. 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!)

  30. 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

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

  32. 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

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

  34. 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)

  35. 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

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

  37. 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)

  38. 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

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

  40. 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!)

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

More Related