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Lecture 3-More deviations from Mendelian ratios

For extra credit question, please use the index cards provided at the back of the room Print your name, TA name, and section # at top of card and place in the appropriate box at the front of the room. Thanks!. Extra-credit question:

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Lecture 3-More deviations from Mendelian ratios

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  1. For extra credit question, please use the index cards provided at the back of the room Print your name, TA name, and section # at top of card and place in the appropriate box at the front of the room. Thanks! Extra-credit question: In some cases, not all individuals with a particular genotype show the expected phenotype. The frequency with which a genotype gives rise to the expected phenotype is called the _______________ of that genotype. Lecture 3-More deviations from Mendelian ratios

  2. Xa Xa XA XA XA XA Y Y Sex Linkage: mammals, flies Heterogametic Sex Diploid XAXa XAY Adults Gametes Male XAXA XAY Female XAXa XaY

  3. W W ZB ZB ZB ZB Zb Zb Sex Linkage: birds, butterflies Homogametic Sex Heterogametic Sex Diploid ZBW ZBZb Adults Female Male Gametes Male ZBZb ZBZb Female ZBW ZbW

  4. Y-linked inheritance

  5. Hairy ears

  6. Male pattern baldness: what kind of inheritance?

  7. Sex influenced phenotype • Genotype Female Male • bb Bald Bald • bb’ Not bald Bald • b’b’ Not bald Not bald

  8. Environment-dependent dependent expression of a genotype

  9. Siamese or “Himalayan” Different allele of the C locus that causes albinism. Temperature sensitive.

  10. Phenotypes are not always a direct reflection of genotypes • Temperature-sensitive alleles: Siamese color pattern • Nutritional effects: phenylketonuria • Genetic anticipation: several genetic diseases

  11. Phenylketonuria • Nutritional defect: can’t metabolize phenylalanine. • Can lead to severe physical and mental disorders in children, but only if they consume phenylalanine. • Disease phenotype can be avoided by eliminating phenylalanine from the diet

  12. Genetic Anticipation • Huntington disease • Fragile-X syndrome • Kennedy disease • Myotonic muscular dystrophy

  13. Fragile X syndrome • Symptoms: delayed development & mental retardation. More severe in males than females • Caused by expansion of triplet repeat (CGG) in a gene on the long arm of the X chromosome • Named for breakage of X chromosome in cell preparations.

  14. Normal range: 7-52 (average=30) “Pre-mutation”: 60-200 repeats Full Mutation: > 230-1000s. DNA becomes abnormally methylated, promoter is inactivated, and gene silenced. Pre-mutation is unstable: maternally-inherited premutation with >100 repeats almost always expands to a full mutation Fragile X

  15. Most common kind of inherited mental retardation. Named for “fragile site” Due to expansion of 3-base pair repeat (CGG) in a gene near the tip of the long arm of X chromosome. Genetic Anticipation: Fragile X

  16. Pre-mutation is unstable: maternally-inherited premutation with >100 repeats almost always expands to a full mutation Fragile X

  17. Genetic Anticipation causes subsequent generations in a family to be more severely affected by a disease. It does this by increasing the number of triplet repeats in the fragile area of the X chromosome through the generations.

  18. Autosomal dominant lethal (chromosome 4) Progressive neurological deterioration First symptoms appear after reproductive age One of 8 known neurodegenerative diseases caused by expansion of (CAG) repeats All show inverse correlation with age of onset and number of repeats. Huntington Disease

  19. Autosomal dominant lethal (chromosome 4) Progressive neurological deterioration First symptoms appear after reproductive age One of 8 known neurodegenerative diseases caused by expansion of (CAG) repeats All show inverse correlation with age of onset and number of repeats. Huntington Disease

  20. Autosomal dominant lethal (chromosome 4) Progressive neurological deterioration First symptoms appear after reproductive age One of 8 known neurodegenerative diseases caused by expansion of (CAG) repeats All show inverse correlation with age of onset and number of repeats. Huntington Disease

  21. Which is the pedigree of autosomal dominant (like HD)

  22. Genetic Data Analysis I Some simple rules of probability

  23. Sum Rule • The combined probability of two events that are mutually exclusive is the sum of the individual probabilities. Clue: look for “or” Q: What’s the probability of rolling a ‘five’ or a ‘six’ on one six-sided die? A: 1/6 + 1/6 = 1/3

  24. Genetic Example: Monohybrid Cross • P: GG X gg • F1: Gg X Gg • F2: 1/4 GG: 1/2 Gg: 1/4 gg • What is the probability that the F2 offspring has the dominant phenotype (is either GG or Gg)? 1/4 GG + 1/2 Gg = 3/4 G-

  25. Genetic Example 2: Dihybrid Cross • P: GG ww X gg WW • F1: Gg Ww X Gg Ww • F2: 9/16 G-W- 3/16 G-ww 3/16 ggW- 1/16 ggww • Q: What is the probability that an F2 offspring will have the dominant phenotype (G-ww or ggW-) for only one of the two traits? 3/16 G-ww + 3/16 ggW- = 6/16=3/8

  26. Product Rule • The probability of several independent events is the product of the individual probabilities. • Two events are independent if the occurrence of the first event has no effect on the probability of the second event. Clue: look for “and”. • Q: You roll two dice. What’s the probability of getting a ‘two’ on the first one and a ‘five’ on the second one. • A: 1/6 * 1/6 = 1/36

  27. Genetic example of product rule • P: AA bb CC DD ee ff x aa BB cc dd EE FF • F1: Aa Bb Cc Dd Ee Ff x Aa Bb Cc Dd Ee Ff • Q: What proportion of F2 progeny will be • AA bb Cc DD ee Ff ? A: 1/4 * 1/4 * 1/2 * 1/4 * 1/4 * 1/2 = 1/1024

  28. Epistasis Problem 1 • Retinitis pigmentosa, a form of blindness in man may be caused either by a dominant autosomal gene, R, or a recessive autosomal gene, a. Thus only A-rr individuals are normal. An afflicted man whose parents are both normal marries a woman of genotype AaRr. What proportion of the children are expected to suffer from this affliction if R and A are inherited independently? Q1: What are the genotypes of the afflicted man’s parents? A:A- rr Aa rr Q1: What is the afflicted man’s genotype? aa rr A: Q1: Draw the cross between the afflicted man and his wife.

  29. Solution • P: Aa Rr x aa rr • F1: 1/4 Aa Rr: 1/4 Aa rr: 1/4 aa Rr: 1/4 aa rr A: 3/4 offspring will have the disorder

  30. Epistasis Problem 2 • In the summer squash (Cucurbita pepo) spherical fruit is recessive to disk, True-breeding spherical types from different geographic regions were crossed. The F1's were disk, and the F2's segregated 35 disk, 25 spherical and 4 long. Explain these results. Q: What’s the first step? A: Notice novel phenotypes: disk, long. Q: What’s the next step? A: Notice there are three F2 phenotypes. What kind of inheritance will give three F2 phenotypes? Expected F2 ratio? 1:2:1 Variation on 9:3:3:1 Incomplete dominance Epistasis

  31. Genetic Model

  32. Sex Linkage • Q: Cinnamon is a sex-linkedrecessive plumage color in chickens. A cinnamon rooster is crossed to a wild-type hen from a pure-breeding stock. What types of male and female offspring will result from this cross? Q: What’s the first step?

  33. Last class-deviations from Mendelism • Epistasis • Lethal alleles • X and Y linkage • Sex Influenced Inheritance • Temperature or nutrition sensitive alleles • Genetic anticipation

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