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Lecture 4

Lecture 4 - Mendelian Inheritance

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University of Toronto Mississauga
Esteban Parra

Mendelian Inheritance Gregor Mendel (1822-1884) - Father of Modern Genetics - Wrote “Experiments with Plant Hybrids” in 1865 - Mendel’s “Law of Segregation” - Mendel’s “Law of Independent Assortment” - Work rediscovered in 1900 The Place of Mendel in History - Mendel occupies, with Darwin, a prominent place in the fields of genetics and evolution. - The “modern synthesis of evolution” is based on Darwin’s ideas on evolution and Mendel’s principles of inheritance. o Darwin who came up with the idea of natural selection (Pangenesis is the idea of Darwin) o Mendel discovered the laws of inheritance where traits are transferred from generation to generation - My scientific labors have brought me a great deal of satisfaction, and I am convinced that before long the entire world will praise the result of these labors.“ said Mendel soon before he died in 1884. He was right!! Mendel’s Pea Phenotypes - Phenotype – observable appearance of a given trait in an organism - He tried to replicate his results in other species however peas had shorter and a faster generation time o You could see the phenotypes distinguished very clearly o Round or wrinkle, yellow or green, different ripe, different type of flower and stem Mendel’s Law of Segregation - The expression of a trait is controlled by discrete units, which occur in pairs. Offspring inherit one unit from each parent. - In today’s language, these discrete units are known as alleles. The combination of two alleles for a certain gene is a genotype. A genotype can be homozygote, resulting from the combination of the same allele form, or heterozygote, resulting from the combination of alternative allele forms. - A graphical depiction of Mendel experiments follows. - In the 2 line, the female will only have yellow alleles (since the female is homozygous for yellow) - In the 2 line, the male pea is homozygous for green, only producing green allele - Since the yellow allele is the dominant trait (denoted by the capital Y), the possible outcomes is heterozygous but displaying the yellow allele - As a result, the F1 generation is heterozygous displaying yellow Dominance, Recessiveness and Codominance - In the case of seed color, in the heterozygote one of the alleles is not expressed (green seeds are not present in the F1 generation, and resurface in the F2 generation). o Yellow seeds are said to be dominant and green seeds recessive. - Although this applies to seed color, for some other traits the phenotypic outcome of the combination of two different alleles (heterozygotes) may be different to the one described above. - Example cystic fibrosis is a recessive trait, and wont have an affect if you are carrying this trait along with a normal trait o When the disease is dominant, just having one copy of the allele, will result in the disease Possible Phenotypic Outcome of the Combination of Two Different Alleles - When two different alleles are present at a locus (heterozygote), several things can happen: - The heterozygote has the same phenotype that the phenotype of one of the homozygotes (dominance). - The heterozygote has a different phenotype that the phenotype of any of the homozygotes. Neither allele is masked by the other. There is blending (incomplete dominance). - The heterozygote have a different phenotype that the phenotype of any of the homozygotes. Both alleles are expressed, but not blended (codominance). - Examples: o Dominance: Pea’s seed color (yellow is dominant) o Incomplete dominance: flower color in
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