RR

Genetics Notes

  • Mendel used the scientific approach to identify 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.
  • 14.2: 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
  • 14.3: 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