Genetics

Genetics Overview

  • Course: BIOL 1134 - Evolution, Ecology, and Diversity

  • Instructor: Paul Klawinski

Learning Goals

  • Describe the differences between mitosis and meiosis, especially regarding gamete production.

  • Conduct monohybrid and dihybrid crosses for trait inheritance prediction.

  • Define dominant/recessive alleles.

  • Determine allele frequencies and calculate genotypic and phenotypic ratios.

Preconditions for Evolution via Natural Selection

  • More offspring produced than can survive.

  • Interindividual phenotypic variation exists and is heritable.

  • This variation leads to differences in reproductive success or survival.

Key Concepts in Genetics

  • Transmission of genes across generations.

  • Understanding how genes are transcribed and translated into phenotypes.

  • Regulation of gene expression: when and how often genes are expressed.

  • Evolution of genes and population genetics in response to evolutionary factors.

Terminology

Ploidy

  • Ploidy: Number of copies of chromosomes within a cell.

    • Haploid: 1 set of chromosomes.

    • Diploid: 2 sets of chromosomes.

    • Triploid: 3 sets of chromosomes.

    • Tetraploid: 4 sets of chromosomes.

    • Hexaploid: 6 sets of chromosomes.

Alleles

  • Allele: Different expressions of a gene.

  • Dominant: Allele that masks the effect of another allele.

  • Recessive: Allele whose effect is masked by a dominant allele.

  • Codominant: Both alleles simultaneously express their traits in a heterozygote.

  • Genotype: The genetic makeup (set of alleles) of an individual.

  • Phenotype: The physical expression of a genotype.

Genetic Crosses

  • P Generation: Parent generation in genetics crosses.

  • F1 Generation: First filial generation, offspring of the P generation.

  • F2 Generation: Offspring of the F1 generation.

  • True Breeding: Parents with the same genotype will produce offspring with the same genotype.

Mendelian Genetics

Monohybrid Cross

  • Involves one trait.

  • Example: Purple vs. white flower colors.

    • Phenotypes: Purple flower (PP), White flower (pp).

    • Offspring from cross: 3:1 ratio of purple to white flowers.

    • Genotype ratios: WW : Ww : ww = 1 : 2 : 1.

Principle of Segregation

  • During meiosis, alleles segregate independently into gametes.

Dihybrid Cross

  • Involves two traits.

  • Example: Purple flowers (P), Yellow seeds (Y) vs. White flowers (p), Green seeds (y).

    • Produce offspring with a 9:3:3:1 ratio of traits.

Complex Dominance Patterns

Incomplete Dominance

  • Found in some phenotypes such as the Japanese four o'clock plants, exhibiting mixing of traits.

  • Prediction from crosses can yield a 1:2:1 ratio.

Codominance

  • Blood types in humans exemplify codominance (IA, IB, i).

    • Blood type examples: A, B, AB, O.

Epistasis

  • Interaction between different genes affecting phenotype ratios, like in corn color experiments leading to unexpected phenotypic ratios.

Pedigrees

  • Graphical tool for demonstrating inheritance patterns within families, allows estimating the probability of genotypes.

Probability in Genetics

Rules of Probability

  • Addition Rule: Probability of any one of two mutually exclusive outcomes.

  • Multiplication Rule: Probability of two independent events occurring together.

Hardy-Weinberg Equilibrium

  • Model to predict genetic variation from one generation to the next assuming no evolution is occurring.

  • Conditions: No selection, mutation, infinite population size, no migration, random mating.

Case Study: Manx Cats

  • Dominant lethal alleles lead to abortion of homozygous dominant individuals.

    • Effect on allele frequencies over generations, despite selection against homozygous individuals.

Polygenic Traits and Selection

  • Traits determined by multiple genes, with cumulative effects of many alleles.

  • Breeder's Equation R = h²•s used in predicting responses to selection in quantitative traits.