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Chapter 12: Inheritance Patterns and Human

Chapter 12: Inheritance Patterns and Human. 12-1 Chromosomes and Inheritance. Thomas Hunt Morgan. American geneticist Professor at Columbia University Studied the common fruit fly Drosophila melanogaster Produces many offspring quickly Tested all of Mendel’s principles

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Chapter 12: Inheritance Patterns and Human

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  1. Chapter 12: Inheritance Patterns and Human 12-1 Chromosomes and Inheritance

  2. Thomas Hunt Morgan • American geneticist • Professor at Columbia University • Studied the common fruit fly • Drosophila melanogaster • Produces many offspring quickly • Tested all of Mendel’s principles • Found they applied to other organisms; not just peas!!

  3. Thomas Hunt Morgan • Drosophila has 4 pair of homologous chromosomes (8 total) • Male & Female differed in one pair • In female, 2 chromosomes in the pair were the same • In male, one looked like female, the other was shorter & hooked. • Which chromosome was different?

  4. Thomas Hunt Morgan • Hypothesized X, Y Chromosome are sex chromosomes (Female: X Male: Y) • Meiosis I • Sex chromosomes form pairs and segregate into separate cells • Meiosis II • the gametes that form each have either an X chromosome or a Y chromosome

  5. Sex Determination • In humans and in fruit flies • Egg fertilized with a sperm (X)  female • Egg fertilized with a sperm (Y)  male • Approximately 50% • Female • Male

  6. Sex Linkage • Morgan proposed that because the X chromosome was larger, it carried more genes than Y. • Sex-linked genes • X-linked genes-carried on X chromosome • Y-linked genes-carried on Y chromosome

  7. Thomas Hunt Morgan • Morgan’s experiments confirmed the existence of X-linked traits • In Drosophila eye color is X-linked • Most fruit flies have red eyes • A few males have white

  8. Thomas Hunt Morgan • Cross Red eyed female with white eyed male • XRXR x XrY (R=red; r=white) • Results followed Mendel’s predictions • F1 Generation all had red eyes (pg 222) • F1 Generation crossed (XRXr x XRY) • F2 Results exhibited expected 3 red:1 white ratio • However all white eyed flies were males • Why no females with white eyes?? • Morgan correctly hypothesized that the gene for eye color is carried on the X chromosome; X-linked trait.

  9. Eye colorSex-linked trait

  10. Thomas Hunt Morgan • Identified more than 50 genes on 4 homologous pairs. • Research found that some were “linked” together. • Seemed to violate the principle of independent assortment.

  11. Morgan’s Conclusions • Each chromosome is actually a group of linked genes • Mendel’s principle of independent assortment holds true • It is the chromosomes that assort independently, not individual genes.

  12. Thomas Hunt Morgan • Linkage Groups • Two or more genes found on the same chromosome • More likely to be inherited together.

  13. Thomas Hunt Morgan • Drosophila Linked Genes • Body color & Wing size • (G=Gray; g=black) (L=Long; l=short) • Morgan crossed GgLl x GgLl • F1 Generation of GGLL x ggll) • Genes on different chromosomes • Should assort independently • Expected phenotype 9:3:3:1 (Dihybrid) • If on same chromosomes • Would assort together • Expected phenotype 3:1 (Monohybrid)

  14. Thomas Hunt Morgan • Results approximated 3:1 ratio • Morgan hypothesized that the genes were linked/on same chromosome • However, also produced several Gray short-winged (Ggll) and Black long-winged (ggLl) • How could this occur if genes were on same chromosome? • Alleles must have changed or rearranged

  15. Thomas Hunt Morgan • Surmised that maybe a mutation • Only 1/10,000; this occurred too often • Hypothesized that maybe something had happened during crossing over • Crossing over changes the locations of genes among the chromosomes that carry them. • Some always crossed over; some less likely Why?

  16. Gene Mapping • The likelihood that linked genes will be separated during crossing over depends on the distance they are from each other on the chromosome

  17. Crossing over

  18. Chromosome Map of the Fruit Fly Exact location on chromosomes Chromosome 2 Biology, Miller & Levine, pg 280.

  19. Gene Maps • Alfred Sturtevant • Created the 1st chromosome map showing the relative locations of each known gene of one of the Drosophila chromosomes. • Compared the frequency of crossing over for several genes.

  20. Gene Maps • Alfred Sturtevant • The percentage of crossing over between the genes for two traits is proportional to the distance between them on a chromosome. • Ex. Two genes that are separated by crossing over 1% of the time are 1 map unit apart.

  21. Comparative Scale of a Gene Map Mapping of Earth’s Features Mapping of Cells, Chromosomes, and Genes Cell Earth Chromosome Country Chromosome fragment State Gene City People Nucleotide base pairs

  22. Mutation • Basically a change in DNA • Can occur in whole or part of chromosome • Germ-Cell Mutations • Occur in gametes (germ cells) • Do not affect organism; can be passed on • Somatic Cell Mutations • Take place in body cells • Do affect organism; not passed on • Ex.) Some types of human skin cancer, leukemia • Lethal Mutations • Cause death, often before birth

  23. Chromosome Mutations • Deletion • Loss of a piece of chromosome due to breakage • All of the info carried by the piece may be lost • Inversion • Chromosomal segment breaks off and then reattaches in reverse orientation to the same chromosome • Translocation • Chromosome piece breaks off and reattaches to another, nonhomologous chromosome • Nondisjunction • Failure of a chromosome to separate from its homologue during meiosis

  24. Chromosome Mutations

  25. Gene Mutations • May involve large segments of DNA or a single nucleotide within a codon • Point mutations(DNAmRNAAmino Acid) • Substitution - one nucleotide in a codon is replaced with a different nucleotide. • Ex.) Sickle cell anemia (adenine for thymine) • Frame shift mutation – deletion or insertion of a single nucleotide. More serious; code is read in multiples of 3; will affect all coding that occurs ‘downstream’.

  26. Gene Mutations

  27. Linked Forever? • Some genes appear to be inherited together, or “linked.” If two genes are found on the same chromosome, does it mean they are linked forever? • Study the diagram, which shows four genes labeled A–E and a–e, and then answer the questions on the next slide.

  28. Linked Forever? • Some genes appear to be inherited together, or “linked.” If two genes are found on the same chromosome, does it mean they are linked forever? 1. In how many places can crossing over result in genes A and b being on the same chromosome? 2. In how many places can crossing over result in genes A and c being on the same chromosome? Genes A and e? 3. How does the distance between two genes on a chromosome affect the chances that crossing over will recombine those genes?

  29. Linked Forever? 1. In how many places can crossing over result in genes A and b being on the same chromosome? One (between A and B) 2. In how many places can crossing over result in genes A and c being on the same chromosome? Genes A and e? Two (between A and B and A and C); Four (between A and B, A and C, A and D, and A and E) 3. How does the distance between two genes on a chromosome affect the chances that crossing over will recombine those genes? The farther apart the genes are, the more likely they are to be recombined through crossing over.

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