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Extensions to Mendel

Extensions to Mendel. Mendel was right…. But not for every situation…. Going beyond Mendel. So far you have learned about Mendel’s Dominance-Recessive Mode of Inheritance… but this is not the only method of inheritance. Codominance Incomplete Dominance X-linked traits Multiple Alleles

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Extensions to Mendel

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  1. Extensions to Mendel Mendel was right…. But not for every situation…

  2. Going beyond Mendel • So far you have learned about Mendel’s Dominance-Recessive Mode of Inheritance… but this is not the only method of inheritance. • Codominance • Incomplete Dominance • X-linked traits • Multiple Alleles • Polygenic Traits

  3. Codominance • Both alleles are dominant and are fully expressed. • Examples: • Coat color in cattle • Human blood types • Cat fur

  4. Codominance Example : Cattle coat color • Red (RR) • White (WW) • Roan (RW)

  5. Punnett Squares with Codominance Cross a white heifer with a red bull • a. Determine the genotypes of the parents and the gametes they will contribute to their offspring. • b. Set up your punnett square . . .

  6. Punnett Squares with Codominance Cross a white heifer with a red bull W W Results: 100% WR Roan R WR WR WR WR R

  7. Punnett Squares with Codominance Cross two Roans • a. Determine the genotypes of the parents and the gametes they will contribute to their offspring. • b. Set up your punnett square . . .

  8. Punnett Squares with Codominance Cross two Roans WRx WR Results R W 25% White (WW) 50% Roan (WR) 25% Red (RR) W WW WR WR RR R

  9. Both alleles contribute to the phenotype….

  10. Incomplete Dominance • Neither allele is dominant. If both alleles are present than the phenotype will be an intermediate. • Example: • snapdragons flower colors (red, white, pink) • Hair texture (wavy, straight, curly)

  11. Incomplete Dominance Example: Snapdragons • Red (RR) • White (rr) • Pink (Rr)

  12. Punnett Squares with Incomplete Dominance Cross a White Snapdragon with a Red Snapdragon • a. Determine the genotypes of the parents and the gametes they will contribute to their offspring. • b. Set up your punnett square . . .

  13. Punnett Squares with Incomplete Dominance Offspring: R R 100% Pink (Rr) Rr r Rr Rr Rr r

  14. Punnett Squares with Incomplete Dominance Cross Two Pink Snapdragons • a. Determine the genotypes of the parents and the gametes they wll contribute to their offspring. • b. Set up your punnett square . . .

  15. Punnett Squares with Incomplete Dominance Cross Two Pink Snapdragons Results RrxRr r R 25% Red (RR) 50% Pink (Rr) 25% white (rr) R RR Rr r rr Rr

  16. Example of Incomplete Dominance: Hypercholesteremia

  17. ee EE Ee

  18. X-linked Traits (a.k.a. sex linked) • Traits that are located on the X-chromosome • Examples: • color-blindness color blind test • Hemophilia (blood-clotting disorder) • Calico and tortoise-shell cats • Duchenne’s muscular dystrophy

  19. Punnett Squares with X-linked traits A man with hemophilia marries a woman who is homozygous normal. In hemophilia: H - Dominant (no hemophilia)h - recessive (hemophilia)

  20. Punnett Squares with X-linked traits A man with hemophilia (h) marries a woman who is homozygous normal. • a. Determine the genotypes of the parents and the gametes they will contribute to their offspring. • b. Set up your punnett square . . .

  21. Punnett Squares with X-linked traits A man with hemophilia marries a woman who is homozygous normal. • Dad: XhY Results: Girls: (XHXh) all will be carriers with no hemophilia • Boys: (XHY) all will be normal. Xh Y • Mom : XHXH XHXh • XH XHY • XH XHXh XHY

  22. Punnett Squares with X-linked traits • Lets look at color blindness – another X-linked trait. • In color blindness: • B is dominant (not color blind) • b is recessive (color blind)

  23. Punnett Squares with X-linked traits A boy inherits his color-blindness from his mother NOT his father. Create a punnett square to show this. . . • a. Determine the genotypes of the parents and the gametes they will contribute to their offspring. • b. Set up your punnett square . . .

  24. Punnett Squares with X-linked traits So how would a boy inherit color-blindness? Create a punnett square to test your theory. • Dad: XBY Results: • 25% XBXB normal girl • 25% XBXbcarrier girl • 25% XBY normal boy • 25% XbYcolor-blind boy XB Y • Mom : XBXb XBXB • XB XBY • Xb XBXb XbY

  25. Punnett Squares with X-linked traits Even though it is rare, girls can be color-blind if they inherit it from BOTH parents! • a. Determine the genotypes of the parents and the gametes they wll contribute to their offspring. • b. Set up your punnett square . . .

  26. Punnett Squares with X-linked traits Can girls be color-blind? How would could that happen? Create a punnet square showing your theory. Dad: XbY Results: • 25% XBXb carrier girl • 25% XbXb color-blind girl • 25% XBY normal boy • 25% XbYcolor-blind boy Xb Y • Mom : XBXb XBXb • XB XBY • Xb XbXb XbY

  27. Multiple Alleles • There are more than 2 alleles, present in a certain population, for a given trait. • This person will only have two alleles even though there are more than two possibilities! • Examples: • Rabbit fur color • Human blood types

  28. Punnett Squares with Multiple Alleles Rabbit Coat Colors • The brown allele (C) is dominant. • The light brown allele (cch) is dominant to ch and c. • The Himalayan allele (white with dark extremities) (ch) is dominant to c. • The albino allele (c) is recessive. C > cch> ch> c

  29. Punnett Squares with multiple alleles What would be the result of a cross between an albino rabbit and a light brown rabbit with a albino parent? • a. Determine the genotypes of the parents and the gametes they wll contribute to their offspring. • b. Set up your punnett square . . .

  30. Punnett Squares with Multiple Alleles Possible Genotypes • Brown (4): CC, Ccch, Cch, Cc • Light brown (3): cchcch, cchch, cchc. • Himalayan (2): chch, chc • Albino (1): cc C > cch> ch> c

  31. Punnett Squares with multiple alleles Results: 50% cchclight brown 50% cc albino • Albino c c • Light brown cch cchc cchc c cc cc

  32. Punnett Squares with multiple alleles • What would be the result of a light brown rabbit (with an albino mother) crossed with a heterozygous himalayan rabbit? • a. Determine the genotypes of the parents and the gametes they wll contribute to their offspring. • b. Set up your punnett square . . .

  33. Punnett Squares with multiple alleles Results: 25% cchch light brown 25% cchc light brown 25% chchimalayan 25% cc albino • What would be the result of a light brown rabbit (with analbino mother) crossed with a himalayan rabbit? • Light brown: cchc • Himalayan: chc cch c cchch chc ch cc c cchc

  34. Polygenic Traits • A trait that is controlled by two or more genes. • Will manifest as a range of phenotypes.

  35. Polygenic Traits • Examples include • Eyecolor • Height • Skin color

  36. Punnett Squares with polygenic traits • Even though eyecolor is controlled by at least three genes, we really understand how brown/blue/green colors work as controlled by two genes… • Eyecolor: • Gene 1: the green/blue eye color gene is located on chromosome 19. • Green is dominant (G) , blue is recessive (g) • Gene 2:the central brown eye color gene is located on chromosome 15. • Brown is dominant (B) , blue is recessive (b)

  37. Punnett Squares with polygenic traits • Eyecolor: • Gene 1:green is dominant (G), , blue is recessive (g) • Gene 2:brown is dominant (B),blue is recessive (b) Brown eyecolor: BBGG, BBGg, BbGg, Bbgg Green eyecolor:bbGG, bbGg Blue eyecolor:bbgg Brown > Green > Blue

  38. Punnett Squares with polygenic traits What would be the result of a cross between a blue-eyed person (recessive for both genes) and a brown-eyed person (heterozygous for both genes) • a. Determine the genotypes of the parents and the gametes they will contribute to their offspring. • b. Set up your punnett square . . .

  39. Punnett Squares with polygenic traits Results: Brown > Green > Blue • bbgg bg BG BbGg • BbGg Bbgg Bg bG bbGg bg bbgg

  40. Punnett Squares with polygenic traits • What is the result when a brown-eyed man (heterozygous for both genes) is crossed with a heterozygous green eyed woman? • a. Determine the genotypes of the parents and the gametes they wll contribute to their offspring. • b. Set up your punnett square . . .

  41. Punnett Squares with polygenic traits Results:Geno: Pheno:1/8 BbGG4/8 brown 2/8 BbGg3/8 green 1/8 Bbgg1/8 blue 1/8 bbGG 2/8 bbGg 1/8 bbgg • BbGg X bbGg bG bg BG BbGG BbGg Bg BbGg Bbgg bG bbGG bbGg bg bbGg bbgg

  42. Polygenic trait – Skin color Hypothetically found on 3 genes: Human Genome project proposes its actually found on many more.

  43. “Notable Notes”

  44. Pleiotropy • Exact opposite of polygenic inheritance: A single gene affects many phenotypic characteristics • Example: Sickle-cell allele • When present on both homologous chromosomes can cause sickle-cell anemia • Heterozygotes areresistant to malaria(why allele has survived)

  45. Pleiotropy 1 GENE Can affect MANY phenotypes

  46. Linked genes • The number of genes in a cell is far greater than the number of chromosomes; in fact, each chromosome has hundreds or thousands of genes. • Genes located close together on the same chromosome tend to be inherited together and are called linked genes. • Linked genes generally do not follow Mendel’s law of independent assortment.

  47. Gene mapping • Thomas Hunt Morgan • Drosophila fly (fruit fly) • Used % outcome of crossing over (recombination frequency) to map out where genes loci (location) are on chromosomes

  48. Genetics and Your Environment Many characteristics (phenotypes) result from a combination of heredity and environment. For humans, nutrition influences height; exercise alters build; sun-tanning darkens the skin, and experience improves performance on intelligence tests. Risk of heart disease and cancer and susceptibility to alcoholism and schizophrenia–are influenced by both genes and environment.

  49. The End

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