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Mendelian Genetics Gregor Mendel •Monk •Lived in modern-day Czech Republic •“father of modern genetics” •Wrote a famous paper called Experiments on plant hybridization •In mid-1800s, he studied heredity in pea plants 7 traits of pea plants studied Seed Shape Round R Wrinkled r Flower Position Seed Color Yellow Green Flower Color Axial Terminal Stem Height Purple White Pod Shape Inflated Constricted Pod Color Tall Green Yellow Short P1 F1 P1 F1 Which color is dominant? • A purple and white plant are crossbred • The resulting peas are planted • All four F1 generation plants are purple • Which allele do you think is the dominant allele? Remember: each parent donates 1 allele to each offspring What if he had crossed Pp x pp? p Genotype: Pp Phenotype: P Purple p Genotype: Pp Phenotype: Purple Genotype: pp p Phenotype: white Genotype: pp Phenotype: White This F1 Generation ≠ 4 purple flowered plants, therefore the P1 generation must have been PP x pp. Now, cross two from the F1 generation. P Genotype: PP Phenotype: P Purple p Genotype: Pp Phenotype: Purple Genotype: Pp p Phenotype: purple Genotype: pp Phenotype: White The white flowers show back up—the gene wasn’t lost. It was just hidden!!! •If the phenotype is a recessive trait, the organism must be ___ for that trait. •homozygous recessive (aa) •If the phenotype is a dominant trait, the organism could be ____ or ___ for that trait. •Homozygous dominant (AA) or heterozygous (Aa) Gene: flower position Possible Allele Combinations: AA or Aa Axial position of flower aa Terminal position of flower Think through it… If the offspring have a phenotypic ratio of 4 axial: 0 terminal A then the genotype of the dominant parent must be A or a AA If the genotype of the dominant parent is Aa, then the phenotypic ratio of the offspring is 2 axial: 2 terminal a a _a _a _a _a Law of Segregation—alleles for a trait separate in forming eggs and sperm ! Law of Independent Assortment— alleles for different traits are distributed to eggs and sperm independently !!! Independent Assortment Animation Law of Independent Assortment—alleles on separate chromosomes assort independently Incomplete Dominance—a blend is obtained Codominance—Roan color horse-Red and white hairs! Red Roan Black and white hairs! Blue Roan Incomplete Dominance OR CoDominance SEX LINKED TRAITS FRUIT FLIES Some alleles are located on the sex chromosomes instead of the autosomes (chromosome pairs 1-22 for humans). These traits are called sex-linked traits. Fruit fly eye color is determined by an allele on the X chromosome. Fruit fly eyes may be red(R) or white(r). XR = red eyes Xr = white eyes XRXR or XRXr Red eyed female XrXr White eyed female XR Y Red eyed male XrY White eyed male Cross a homozygous pure red eyed female with a white eyed male. Give the GR and PR results. XR XR Xr XRXr XRXr Y XRY XRY GR: 50% XRXr, 50% XRY PR: 50% Red eyed females, 50% Red eyed males
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