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

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