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Mendel and His Discoveries Chapter 11 (pg. 207-218) Gregor Mendel 1822-1884, Austrian monk The “Father of Genetics” Thought about researching mice genetics, but switched to plants …...decided to use Pea plants……. Advantages: - easy to grow - fast growing - high yield of seed each generation - are self-fertile (self-pollinating) - but fairly easy to cross pollinate too (by emasculating the flowers and carrying pollen from another plant) - lots of different varieties on the market that have easily recognized differences Looked at the inheritance of these 7 different traits: Seed Color = Seed Shape = Flower Color = Position of Flower = Height = Color of young pods = Pod shape = First, he made sure that each variety bred “true” (was stable for two generations). Then he started making crosses between plants that were different for the same trait…..for instance, tall and short. Observation 1 = Parents = parental generation P Tall x Tall Short x Short Tall Short First filial generation F Observation 2 = Mendel made a cross between a Tall plant and a Short plant….. X Progeny were all Tall > …not medium height! Observation 3 = If these Tall-Short hybrids were self-pollinated: F2 generation Offspring were: 75% Tall 25% Dwarf He observed this same phenomenon for all 7 traits How could such a thing happen? Mendel concluded that each plant carries two “particles” ..... One of these particles comes from female parent and the other from the male parent (even if same plant). He also concluded that one of these particles are “Dominant”over the other. He called the non-dominant ones “Recessive” It turns out there is a lot of truth to this conclusion....we now call these “particles” genes....to be specific they are alleles. Alleles are often indicated with a single letter. A CAPITAL letter denotes a dominant allele = T A lowercase letter denotes a recessive allele = t Genotype (chemical code): TT x Tt tt Phenotype (visual): Tall x Tall Dwarf Alleles are often indicated with a single letter. A CAPITAL letter denotes a dominant allele = T A lowercase letter denotes a recessive allele = t Genotype (chemical code): TT x Tt tt Phenotype (visual): Tall x Tall Dwarf In the next generation (where he self-pollinated) the offspring: Genotype (chemical code): Tt x Tt 25% TT 50% Tt 25% tt Phenotype (visual): Tall x Tall 75% Tall 25% Dwarf …see next slide …one parent (female) T t T TT Tt t Tt tt ..other Parent (male) 25% TT (tall) 50% Tt (tall) 25% tt (dwarf) P1 Yellow Seed x Green Seed F1 All Yellow Seeds Selfed the Yellow Seeds F2 6,022 yellow seeds 2,001 green seeds Monohybrid Cross ? Mendel theorized three basic Principles: “The Principle of Segregation” That each plant has two “factors” (alleles) for any genetic trait. One of these alleles comes from female parent and the other from the male parent (even if same plant). So the female sex cell (called egg or ovule) and the male sex cell (pollen or sperm)… each contain only one of these alleles. “The Principle of Dominance” He also concluded that some of these particles are “Dominant” over the other. He called the non-dominant ones “Recessive” So, the only way a plant will show the Recessive characteristic is if it has none of the Dominant factors at all. Crosses that involve one characteristic, such as seed color, is known as a monohybrid cross But Mendel when on to ask the questions, “what happens if two characteristics were breed?” F2 generation 315 smooth yellow peas 101 wrinkled yellow peas 108 smooth green peas 32 wrinkled green peas Dihybrid cross P SS YY x F1 F2 Ss Yy ss yy x Ss Yy “The Principle of Independent Assortment” ….that each pair of traits assort independently Incomplete Dominance RR rr Rr Red pigment gene produces the color red White snapdragon produces no color at all because the gene for pigment is not functional Two alleles are needed for color production Rr Rr 1 red: 2 pink : 1 white HYBRID = “progeny (offspring) resulting from a cross between parents that are very different from one another” …this could be between two organisms from completely different species (e.g.horse x donkey results in a mule) …it could also be between two organisms (two “varieties” or “breeds”) within the same species (e.g. hybrid corn, hybrid tomatoes) ….humans would not be normally considered “hybrids”… they are too similar…. (our traits only differ quantitatively, not qualitatively) PHENOTYPE = “How a genetic trait outwardly expresses itself (ie. How it influences the physical body)” Like: Tall or Short plants Green or Yellow pods Straight or Curly hair GENOTYPE = “The chemical makeup of that gene” Like: Aa YY zz ALLELE = “A single ‘factor’, ‘particle’, gene’ which ever you want to count it” Like: A Y z HOMOZYGOUS = “When both alleles are identical” Like: AA YY zz HETEROZYGOUS = “When the 2 alleles are different” Like: Aa Yy Zz BB = Homozygous Dominant Cc = ? Heterozygous tt = ? Homozygous Recessive Ww = ? Heterozygous