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Genetics Notes

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

      • 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



JL

Genetics Notes

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

      • 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