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Chapter 12

Chapter 12. Inheritance Patterns and Human Genetics. Walter Sutton - American Geneticist. Developed the Chromosome Theory of Heredity. The Chromosome Theory of Heredity. Chromosomes are located in the nucleus Factors (genes) are found on chromosomes

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Chapter 12

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  1. Chapter 12 Inheritance Patterns and Human Genetics

  2. Walter Sutton - American Geneticist Developed the Chromosome Theory of Heredity

  3. The Chromosome Theory of Heredity • Chromosomes are located in the nucleus • Factors (genes) are found on chromosomes • Sutton discovered that genes are on chromosomes in 1902

  4. Chromosome Theory of Heredity • States that genes are located on chromosomes and each gene occupies a specific place on a chromosome • Only one allele is on a chromosome • Finally explains what Mendel was talking about

  5. Thomas Hunt Morgan • Studied fruit flies – Drosophila melanogaster

  6. Fruit Flies are excellent for genetic studies because: • Reproduce quickly • Easy to raise • Many mutations • Have 8 chromosomes (n=4)

  7. Gray bodies – G Normal Wings - W Black bodies – g Small wings – w Morgan looked at TWO traits

  8. ggww GGWW P1 x GgWw F1 100%

  9. Morgan then mated the F1 back to the recessive parent GgWw x ggww Expected ratio – 1:1:1:1 25% GgWw 25% Ggww 25% ggWw 25% ggww

  10. Morgan’s Actual Results 41.5% gray normal 41.5% black small 8.5 % black normal 8.5% gray small

  11. Conclusion • Gene for body size and wing color were somehow connected or linked • Can’t undergo independent assortment

  12. Gene Linkage • Genes on the same chromosome are linked together • Inherited together – THEREFORE they do not undergo independent assortment

  13. Linkage Groups • Package of genes that are always inherited together • Genes on the same chromosome • One linkage group for each homologous pair • Fruit flies – 4 linkage groups • Humans – 23 linkage groups • Corn – 10 linkage groups

  14. So linkage groups explain the high percentages (41.5%) but What about the 8.5%??????

  15. 17% had new combinations The combinations that were expected would be: Gray normal – GW or Black small - gw

  16. When they are lined up they can become twisted and switch genes Crossing Over

  17. The 17% that had new combinations are known as Recombinants – individuals with new combinations of genes Crossing Over – gives rise to new combinations – Prophase I

  18. Chromosome Maps • The likelihood a crossover will occur that will result in the separation of two genes depends on the distance between the two genes • Chromosome maps are diagrams that show the location of genes on a chromosome • Two genes separated by crossing-over 1% of the time = one map unit apart • Alfred Sturtevant (Morgan’s student) constructed the 1st chromosome map of fruit flies

  19. WORKED WITH MEAL WORMS DISCOVERED THAT THEY HAD 20 CHROMOS. MALES HAD 19 REG. SIZE AND 1 SMALL SHE FOUND THE SAME THING IN FRUIT FLIES SHE SAID THE 19 THAT WERE THE SAME WERE AUTOSOMES, & THE MISMATCHED WERE SEX CHROMOS MALES HAVE 1 X AND 1 Y NETTIE STEVENS

  20. Sex Chromosomes • Nettie Stevens – made observations of meal worm chromosomes

  21. Sex Chromosomes • One pair • Female – XX • Male – XY

  22. Sex Determination 50/50

  23. Genes on Sex Chromosomes • Sex chromosomes determine a person’s sex • Sex chromosomes also contain other genes

  24. Sex Linked • A gene located on a sex chromosome • Usually X • Example – Fruit Fly Eye Color • The gene for eye color is on the X chromosome - not the Y

  25. Fruit Fly Sex Chromosomes X X X Y

  26. Males Females XRY XrY XRXR XRXr XrXr Red Eyed White Eyed

  27. Sex-Linked Genetic Disorders • Gene for the trait is on the X or Y • X has many genes – Y has few • Defects easy to spot – appear more in males

  28. Carrier • A heterozygous female – has the gene but does not express it – can pass it on to her children

  29. Colorblindness • Recessive X linked disorder • Cannot distinguish colors • Dominant Gene – XC • Recessive Gene - Xc

  30. Hemophilia • Recessive X linked disorder • Blood does not clot • XH – good gene • Xh – hemophilia gene

  31. Muscular Dystrophy • Results in the progressive wasting away of muscle

  32. Mutations A change in the DNA of an organism Can involve an entire chromosome or a single DNA nucleotide May take place in any cell

  33. Mutations • Germ Cell Mutations - Occur in an organism’s germ cells (gametes)- only affect offspring • Somatic Mutations - Take place in an organisms body cells and only affect the organism

  34. Mutations • Lethal Mutation: • Can cause death • Often before birth • Good Mutations: • organisms have a better chance to reproduce • have an evolutionary advantage • Provide the variation on which natural selection acts

  35. Chromosome Mutations • Changes in the structure of a chromosome • Loss or addition of an entire chromosome • Four Types: • duplication • deletion • inversion • translocation

  36. Gene Mutations • Point mutation-single nitrogen base is changed • Substitutions may not be fatal - there is redundancy in the amino acid codons

  37. Frameshift Mutation • When a nucleotide is lost or added so that the remaining codons are grouped incorrectly • This can code for the wrong amino acid and create an incorrect protein • Insertions and deletions are frameshift mutations

  38. THE FAT CAT ATE THE RAT Deletion causes a frameshift: If you delete the “E” in the THF ATC ATA TET HER AT

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