Presentation on theme: "Genetics and Evolution"— Presentation transcript:
1 Genetics and Evolution Welcome to IB 201!Genetics and Evolution
2 IB 201: Review QuestionA phenotype ratio of 9:3:3:1 in the offspring of a mating between two individuals that are heterozygous for two traits occurs when:A. the genes reside on the same chromosomeB. each gene contains two mutationsC. the gene pairs assort independently during meiosisD. only recessive traits are scoredE. none of the above
3 Course Outline Genetic Data Analysis: Probability & Statistics Deviations from Mendelism:Epistasis; Unusual Modes of InheritanceChromosomal Inheritance:Chromosomal Abnormalities; Sex DeterminationMapping:Gene and Genome MappingTraits Affected by Genes & EnvironmentQuantitative TraitsGenes in PopulationsGenetic Mechanisms of Evolution; Population Genetics of Disease and Disease resistance
4 Genetic Data Analysis II Some simple rules of probability
5 Sum RuleThe 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
6 Genetic Example: Monohybrid Cross P: GG x ggF1: GgGg x Gg ==>F2: 1/4 GG: 1/2 Gg: 1/4 ggWhat 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-
7 Genetic Example 2: Dihybrid Cross P: GG ww x gg WWF1: Gg WwGg Ww x Gg Ww ==>F2: 9/16 G-W- 3/16 G-ww 3/16 ggW- 1/16 ggwwWhat 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
8 Product RuleThe 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
9 Genetic example of product rule P: AA bb CC DD ee ff x aa BB cc dd EE FFF1: Aa Bb Cc Dd Ee Ff x Aa Bb Cc Dd Ee FfQ: What proportion of F2 progeny will beAA bb Cc DD ee Ff ?A: 1/4 * 1/4 * 1/2 * 1/4 * 1/4 * 1/2 = 1/1024
10 Deviations from Mendelism Lethal AllelesEpistasisUnusual sex linkageSex influenced inheritanceGenetic Anticipation
12 Lethal alleles F1: Mm x Mm F1: 1 MM 2 Mm 1 mm F2: Lethal: 2 Manx: 1 NormalF2 phenotypic ratio: 2:1 instead of 3:1
13 Other lethal mutations Achondroplasia (humans)Yellow body color (domestic mice)Curly wings (Drosophila)
14 Epistasis Genetic interaction between two (or more) loci. One gene modifies the phenotypic effects of another gene.
15 Agouti: wild type Figure: 04-00CO Caption: Agouti, yellow, and black mice.
16 F2: 9/16 B- C- 3/16 bb C- 3/16 B- cc 1/16 bb cc agouti black albino BB CC x bb ccP: agouti albinoBb CcF1: agoutiSimple dominant phenotype?F2: 9/16 B- C- 3/16 bb C- 3/16 B- cc 1/16 bb ccagoutiblackalbinoalbinoF2 Phen. ratio: 9 agouti : 3 black : 4 albinonovel phenotype
17 Epistasis Normal dihybrid ratio is altered from 9:3:3:1 to 9:3:4 C and B gene have an epistatic interaction
18 Epistasis Locus 1 Locus 2 BB Bb bb CC Cc cc agouti agouti black no effect no effect albino
19 Biochemical modelCC or Cc: tyrosinase is produced (involved in production of melanin)BB or Bb: controls distribution of the pigment
20 rrYY RRyy Codominance? R-Y- R-Y- rrY- R-yy rryy Figure 10.18bCrosses between pure lines produce novel colors.ParentalgenerationrrYY RRyyXYellowBrownCodominance?F1 generationR-Y-RedSelf-fertilizationFigure: 10.18bCaption:Inheritance of Fruit Color in Bell PeppersF2 generationR-Y- rrY R-yy rryyRedYellowBrownGreen9/163/163/161/16
21 yellow ----------------> green -----------------> brown pepper Figure 10.18cModel to explain 9 : 3 : 3 : 1 pattern observed above: Two genes interact to produce pepper color.yellow > green > brown pepperyy? chlorophyll R? red pigmentyellow > yellow > orangeY? No chlorophyll R? red pigmentGenotypeColorExplanation of colorR-Y-RedRed pigment + no chlorophyllrrY-YellowYellow pigment + no chlorophyllR-yyBrownRed pigment + chlorophyllrryyGreenYellow pigment + chlorophyllGene 1Figure: 10.18cCaption:Inheritance of Fruit Color in Bell PeppersGene 2R = RedY = Absence of green (no chlorophyll)r = Yellowy = Presence of green (+ chlorophyll)(-) = R or r(-) = Y or y
22 Practice ProblemIn Labrador retrievers, coat color is controlled by two loci each with two alleles B,b and E,e respectively. When pure breeding Black labs with genotype BB EE are crossed with pure breeding yellow labs of genotype bb ee the resulting F1 offspring are black. F1 offspring are crossed (Bb Ee x Bb Ee). Puppies appear in the ratio:9/16 black; /16 chocolate; /16=1/4 yellow.What genotypes correspond to these three phenotypes?9/16 B- E /16 B- ee 3/16 bb E- 1/16 eebbB- E- B- ee bb E- and bb ee
23 Other kinds of epistasis 9/16 A-B-3/16 A-bb3/16 aaB-1/16 aabbFigure: 04-07Caption:Basis of modified dihybrid F2 phenotypic ratios.Hint: usually given numbers, not fractions27 agouti; 12 albino; 9 black28 agouti; 11 albino; 4 black
24 Practice ProblemIn the summer squash (Cucurbita pepo) fruit shape is determined by two genes. Two different true-breeding spherical types were crossed. The F1's were all disk, and the F2's segregated 35 disk, 25 spherical and 4 long. Explain these results.What’s the first step?Notice novel phenotype: disk, long.What’s the next step?Notice there are three F2 phenotypes. What kind of inheritance will give three F2 phenotypes?Genetic Model?Incomplete, codominanceEpistasisExpected F2 ratio?1:2:1Variation on 9:3:3:1
25 Practice Problem, cont.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.Are the phenotypic ratios closer 1:2:1 or to a variant of 9:3:3:1 ?If phenotypic ratios closer to a variant of 9:3:3:1, then what variant is it?Total # of individuals = = 6464/16 = 49*4 = 366*4 = 241*4 = 4Phenotypic ratio close to 9:6:1
26 Practice Problem, cont. AA bb aaBB 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.If phenotypic ratios are close to 9:6:1, then what are the genotypes associated with each phenotype?35 disk spherical long9/16 A- B /16 A- bb + 3/16 aa B- 1/16 aa bbWhat were the genotypes of the original spherical parents?AA bb aaBB
27 Sex Linkage: mammals, flies HeterogameticSexDiploid XAXa XAYAdultsGametesXAXaXAYXAYMaleXAXAXAXAYFemaleXaXAXaXaY
32 Male pattern baldness: what kind of inheritance?
33 Sex influenced phenotype Genotype Female Malebb Bald Baldbb’ Not bald Baldb’b’ Not bald Not bald
34 Environment-dependent dependent expression of a genotype
35 Siamese or “Himalayan” Different allele of the C locus that causes albinism.Temperature sensitive.
36 Phenotypes are not always a direct reflection of genotypes Temperature-sensitive alleles: Siamese color patternNutritional effects: phenylketonuriaGenetic anticipation: several genetic diseases
37 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
39 Fragile X syndromeSymptoms: delayed development & mental retardation. More severe in males than femalesCaused by expansion of triplet repeat (CGG) in a gene on the long arm of the X chromosomeNamed for breakage of X chromosome in cell preparations.
40 Fragile X Normal range: 7-52 (average=30) “Pre-mutation”: repeatsFull Mutation: > s. 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
41 Genetic Anticipation: Fragile X 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.
42 Fragile XPre-mutation is unstable: maternally-inherited premutation with >100 repeats almost always expands to a full mutation
43 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.
44 Huntington Disease Autosomal dominant lethal (chromosome 4) Progressive neurological deteriorationFirst symptoms appear after reproductive ageOne of 8 known neurodegenerative diseases caused by expansion of (CAG) repeatsAll show inverse correlation with age of onset and number of repeats.
45 Huntington Disease Autosomal dominant lethal (chromosome 4) Progressive neurological deteriorationFirst symptoms appear after reproductive ageOne of 8 known neurodegenerative diseases caused by expansion of (CAG) repeatsAll show inverse correlation with age of onset and number of repeats.
46 Huntington Disease Autosomal dominant lethal (chromosome 4) Progressive neurological deteriorationFirst symptoms appear after reproductive ageOne of 8 known neurodegenerative diseases caused by expansion of (CAG) repeatsAll show inverse correlation with age of onset and number of repeats.
47 Which is the pedigree of autosomal dominant (like HD)