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.