Genetics Notes

• Mendel used the scientific approach to identify two laws of inheritance

  • Two laws of inheritance: 

    • Mendel’s Experimental, Quantitative Approach

      • Character:  a heritable feature that varies among individuals (i.e. hair color)

      • Trait: variants of character (i.e. red, black or blonde)

      • Peas were good because they were simple, had clear variation, a short generation time and produced many offspring

      • Mendel used true-breeding parents (they produced the same trait over many generations) and controlled flower fertilization

      • Mendel crossed for many generations and used large sample sizes

    • The Law of Segregation

      • Disproved the blending theory as traits reappeared after not being expressed in one generation

      • Mendel’s Model

        • Alternative versions of genes account for variations in inherited characters. Alleles arise from slight differences in the nucleotide sequences at a locus. 

        • For each character, an organism inherits two copies of a gene, one from each parent. 

        • If two alleles at a locus differ, then one, the dominant allele, determines the appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance.

        • Law of Segregation: Two alleles from gametogenesis separate into different gametes. 

      • The Test-Cross

        • The test-cross is used to determine if an organism is heterozygous or homozygous dominant for a particular trait, cross the organism with another organism which is homozygous recessive.

        • If there is a 1:1 ratio of dominant to recessive phenotypes, then the unknown organism is heterozygous

        • If there is a 1:0 ratio of dominant to recessive phenotypes, then the unknown organism is homozygous dominant

  • The Law of Independent Assortment

    • Monohybrid: organisms that are heterozygous for ONE trait

    • Dihybrid: organisms that are heterozygous for TWO traits

    • Alleles for one gene assort independently. Only applies to genes on that are on different chromosomes or are far apart on a single chromosome.

• Probability laws govern Mendelian inheritance

  • Multiplication and Addition Rules

    • Multiply probabilities for one or more independent affairs

    • Add probabilities for two or more mutually exclusive wheels

• Inheritance patterns are often more complex than predicted by simple Mendelian genetics

  • Extending Mendelian Genetics for a Single Gene: Simple Mendelian genetics do not apply when there is incomplete dominance/recession, polygenic traits or when a single gene controls multiple phenotypes

    • Degrees of Dominance

      • Incomplete Dominance: Neither allele is completely dominant nor recessive

      • Codominance: This is where both alleles are EQUALLY EXPRESSED

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    • The Relationship Between Dominance and Phenotype

      • Dominant and recessive allele plays into effect solely from genotype to phenotype

    • Frequency of Dominant Alleles

      • Dominant alleles are not inherently more common than recessive alleles or vice versa

      • Multiple Alleles

        • Most genes exist in more than two allelic forms

        • For example, blood type (IA, IB and i)

      • Pleiotropy

        • Most genes have multiple phenotypic effects

        • For example, in peas, the gene for flower color also controls the color of the seed coating

    • Extending Mendelian Genetics for Two or More Genes

      • Epistasis: one gene affects another because the gene products are related

      • Polygenic Inheritance: multiple genes independently affect a single trait

        • Quantitative Characters: traits that are not A or B, rather they exist as a gradient

        • For example, skin color and hair color.

        • Genotypes are most commonly associated with a range of phenotypes, not typically a single phenotype