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Mendelian Genetics

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  1. Mendelian Genetics Genetics: science of heredity and variation • Blending Theory: Offspring blend of parents. 6’ father + 5’ mother  5’6” child • Pangenesis (Darwin): Particles in body affected by what we do  passed to sex cells by blood. • Inheritance of acquired characteristics: Skills you acquire  offspring Modern genetics: Mendel 1865: “Experiments in Plant Hybridization” Discovered in 1900 Darwin 1859: “Origin of Species”

  2. Mendelian Genetics Garden Pea • Different • True-breeding • Easy to grow. • Monoecious Cross-fertilization: P1 x P2 F1 Hybrid Self-fertilization: P1 x P1 P1 What about beans? Page 183 Figure 11.2

  3. Mendel’s Results • Parents factors  offspring Genes • Offspring receive two factors Alleles • Not all factors identical Fertilization Crossing Over Independent Assortment • One allele from each parent In the egg and sperm • Presence of allele does not mean trait expressed. Phenotype vs Genotype Dented and Smooth Pea SeedCoat DD Page 184 Figure 11.3 DD

  4. Modern Genetics Heredity:transfer of traits parent  offspring Trait:characteristic (green seed) Gene:unit of heredity (G) Locus:physical location of gene Phenotype:physical appearance (green, yellow) Genotype:alleles from mother and father (Gg) Allele:alternate forms of a gene (G or g) Dominant Allele:(R) covers recessive allele expression Recessive Allele: (r) Page 185 Figure 11.4

  5. Modern Genetics Homozygous: identical alleles at locus. Homozygous Dominant =RR Homozygous Recessive =rr Heterozygous: different alleles at locus Rr

  6. Solving Genetic Problems Punnett Square Method: • Determine dominant characteristic (Red Flower) • Code alleles (R&r) • Determine parental genotype and gametes Genotypes=RR,Rrorrr, gametes=Rorr • Assign gamete frequency Each gamete has 50% (1/2 or .5) chance of being R or r • Draw and fill in Punnett Square • List all possible sperm on one axis • List all possible eggs on other axis • List all offspring in the squares Female Eggs r R Sperm RR Rr R r rR rr

  7. Monohybrid Cross (1 gene): Complete Dominance Assume red flower color is dominant (R) to white flower color (r). You cross a homozygous dominant red flower with a homozygous white flower. What are the offspring? True Breeding=RR x rr Parents Rr = F1 generation offspring = 100% heterozygous red flower r r R Rr Rr Rr Rr R

  8. Monohybrid Cross: Complete DominanceExample • Cross two heterozygous red (Rr) F1 offspring. • What is the genotype and phenotype of their F2 offspring? • Parental genotypes = Rrx Rr • Assign gametes • Assign offspring • genotypes • Assign phenotypes • Results: • 1 RR = 25% • 2 Rr = 50% • 1 rr = 25% • Genotypic Ratio = 1:2:1 • Phenotypic Ratio = 3:1 Eggs R (.5)r (.5) Sperm 100% R(.5) Rr (.25) RR (.25) Heterozygous Red Flower Homozygous Dominant Red Flower Rr (.25) rr (.25) r(.5) Heterozygous Red Flower Homozygous Recessive White Flower

  9. One Gene Testcross: Complete DominanceExample If 100% red flowers from RRxrr Rr If 100% red flowers from RR xRR RR How can you tell the genotypes of the parents? Testcross:determines if dominant phenotype is RRorRr Tester:homozygous recessive genotype (rr) uncovers hidden recessive (r) Rrxrr RRxrr Phenotypic Ratio 1:0 Phenotypic Ratio 1:1 r r Rr Red Rr Red R R rr White r

  10. Single Gene Complete Dominant Ratios One Gene Model 3:1 = phenotypic ratio 1:2:1 = genotypic ratio 1:1 = testcross ratio for Rr 1:0 = testcross ratio for RR Two Gene Model 9:3:3:1 = phenotypic ratio 1:1:1:1 = testcross ratio forRrTt What is the clue that the trait is controlled by one or two genes?

  11. Polynomial Method or Branching Method: Complete Dominance Example

  12. Single Gene: Example 1 Problem: Cross a red rose and a white rose. Self 6 F1 plants. Count the red and white flower offspring from each F1 selfed. F1 SelfedRedWhite 1 5 19 2 9 13 3 4 21 4 4 15 5 8 15 6 4 25 34 108 • What is the inheritance of white flower color? • Which color is dominant? • What are the genotypes of the parents and F1?

  13. Single Gene: Example 2 Problem: Identify the genotype of the parents for each of the following F1 offspring phenotypes (assume red flower color is dominant and white flower color is recessive): OffspringParental CrossRedWhite Ratio Genotypes Red x White 82 78 1:1 Red x Red 118 39 3:1 White x White 0 50 0:1 Red x White 74 0 1:0 Red x Red 90 0 1:0 Whatare the parental genotypes for each cross?

  14. Dihybrid Example: Complete Dominance Parents RRTT x rrtt (gametes) (RT)(rt) F1 hybrid RrTt x RrTt Eggs (F1 gametes) ¼ RT ¼ Rt ¼ rT ¼ rt Sperm (F1 gametes) Phenotypic Ratio 9 Red-Tall 3 Red-Short 3 White-Tall 1 White-Short 16 ¼ RT RRTT Red-Tall RRTt Red-Tall RrTT Red-Tall RrTt Red-Tall ¼ Rt RRTt Red-Tall RRtt Red-Short RrTt Red-Tall Rrtt Red-Short F2 hybrids RrTT Red-Tall RrTt Red-Tall rrTT Wht-Tall rrTt Wht-Tall ¼ rT Rrtt Red-Short rrtt Wht-Short RrTt Red-Tall rrTt Wht-Tall ¼ rt

  15. Dihybrid Testcross If you have 10 red-tall flowers, how can you tell which flowers have the genotype RrTt and which are RRTT? A testcross will separate them using the rrtt tester. RRTT x rrtt RrTt x rrtt All Red-Tall 1:0 1:1:1:1 rt rt rt rt rt rt rt rt RT RT RrTt RrTt Red-Tall RT Rrtt Red-Short Rt RT rT rrTt White-Tall RT rt rrtt White-Short

  16. Autosomal Dominant: Complete Dominance Autosome: Chromosomes other than sex (X & Y) chromosomes. Caused by dominant or recessive alleles inherited from parents. Page 192 Figure 11.11

  17. Autosomal Recessive: Complete Dominance Autosome: chromosomes other than sex (X & Y) chromosomes. Page 192 Figure 11.10

  18. Autosomal Disorders in Humans Recessive • Tay-Sachs Disease (1/3,600) • Deterioration of psychomotor functions. • Lack enzyme. • Cystic Fibrosis (1/3,000) • Mucus in bronchial tubes. • Cl- can’t pass plasma membrane. • Phenylketonuria (1/5,000) • Lack enzyme for metabolism of phenylalanine. Dominant • Neurofibromatosis (1/3,500) • Tan or dark spots on skin, small benign tumors. • Chromosome 17 gene. • Huntington Disease (1/20,000) • Degeneration of brain cells, muscle spasms and personality disorders. • Chromosome 4.

  19. Incomplete Dominance Heterozygote has an intermediate phenotype. Dominant allele does not cover the recessive allele. Example: Primrose, four-o’clock and snapdragon flowers Red Flower x White Flower 1 red flower 2 pink flowers 1 white flower Page 196 Figure 11.14

  20. Incomplete Dominance in Humans • Sickle-Cell • Irregular red blood cells vs biconcave. • Glutamate (GAG) vsValine (GUG) • HbAHbA = normal • HbSHbS = sickle-cell • HbAHbS = sickle cell trait • (little or no disease • symptoms) Pitch of male voice. H1H1 = base H1H2 = baritone H2H2 = tenor Straight vs curly hair. H1H1 = curly H1H2 = wavy H2H2 = straight

  21. Multiple Alleles: Human Blood Type A = IAIA, IAi = 42% central and eastern Europe B = IBIB, IBi = 10% highest on central Asia AB = IAIB = 3% highest in Japan, China, Pakistan O = ii = 45% highest Central America, South America, Australia and Western Europe • Multiple Alleles: • More than 2 alleles of a gene. • Codominance: • More than 1 • allele expressed at • once. Page 197 Figure 11.15

  22. Human Blood Type Blood Cellular Plasma Type %AntigensDonate ToAntibodiesReceive From O+ 38% None O, A, B, AB anti-A, anti-B O O- 7% A+ 34% A A & AB anti-B O & A A- 6% B+ 9% B B & AB anti-A O & B B- 2% AB+ 3% A & B AB None O, A, B, AB AB- 1% Rh factor in independent of blood type. Single dominant gene Rh+. Rh- mother takes shots if baby is Rh+ mother’s antibodies attack baby’s blood.

  23. One Locus vs Two Loci vs Many Loci Two Loci One Locus Normal Distribution or Bell Curve Many Loci

  24. Polygenic Inheritance: Human Height Polygenic Inheritance: 2 or more genes (sets of alleles) dominant allele has additive effect on phenotype Page 198 Figure 11.17 _ X Short Tall Human Height: Very Short = aabbcc Short = Aabbcc Medium Short = AaBbcc Medium = AaBbCc Medium Tall = AABbCc Tall = AABBCc Very Tall = AABBCC

  25. Polygenic Inheritance Human Traits: Height Stature Body shape Hair color Skin color: Each dominant allele increases the color of the skin. AABBCC = Very Dark aabbcc = Very Light

  26. Epistasis Epistasis: gene at one locus interferes with gene at different locus. Coat color in Labrador retrievers (eumelanin pigment). E gene: determines if pigment present B gene: determines how dark the pigment E_ ee eebb yellow fur brown nose, lips, eye rim eeB_ yellow fur black nose, lips, eye rim E_bb chocolate Lab brown fur, nose, eye rim E_B_ Black fur, Nose, lips, Eye rim

  27. Environmental Effect on Phenotype Flower color: primrose White above 32oC Red at 24oC Uterus effect on fertilized egg Health of mother Oxygen Hormones Medication Cataracts Dominant gene + modifier gene Excessive UV light Right or left handedness “Everyone is born right handed, only the gifted overcome it”. DD = Strongly right handed Dd = Mostly right handed dd = Either right or left handed

  28. Biochemical Genotypes and PhenotypesDNA Markers F1 generation will be all A1A2 and F2 generation will be 1:2:1 ratio

  29. Summary • Complete Dominance: (R dominant overr) • Monohybrid (Rr) • Dihybrid (RrTt) • Autosomal Dominant (RR or Rr) • Autosomal Recessive (rr) • Incomplete Dominance: (Rrdiffers RR&rr) • Co-dominance: (blood type: IA and IB) both genes expressed • Multiple Alleles: (blood type A,B,and O) • Polygenic: alleles are additive (A,B,C,D,E,etc.) • Environmental Effect: