Quantitative Inheritance Overview

Quantitative Inheritance

• Quantitative traits are characteristics determined by multiple genes and influenced by environment.
• These traits are measurable and vary in magnitude within a population.
• Examples include height, weight, and metabolic traits.
• Normal distribution is a symmetrical distribution of trait values around an average.
• Calculation of the mean: ΣΧ / N (sum of all values divided by number of individuals).
• Variance measures the amount of variation around the mean: Σ(Xᵢ - X)² / (N-1).
• Covariance describes the relationship between two variables, while regression analysis predicts how one variable changes with another.
• Biometric field focuses on the statistical study of biological traits.
• Discontinuous traits are often controlled by a single gene and less influenced by environment.
• Mendelian inheritance patterns (like dominant and recessive) are often observed in discontinuous traits.
• Frequency distribution is a way to describe quantitative traits.

Polygenic Inheritance

• Polygenic inheritance involves multiple genes influencing a trait.
• William Bateson and William Castle studied polygenic inheritance, in which traits are governed by multiple genes.
• Quantitative traits are often difficult to analyze because of the influence of multiple genes and environment.
• Polygenic traits can exhibit additive effects, where genes contribute small changes in the trait, and environment interacts.
• Heritability describes the proportion of phenotypic variation due to genetic variation.
• Geneticists categorize quantitative trait variation into: genetic variation, environmental variation, interactions between genetic and environmental factors.
• Genotype-environment interaction describes responses to environmental variation that vary between genotypes.
• Genotype-environment association statistically examines the relationship between allelic frequency and relevant environmental conditions (useful in human genetics or to study adaptation).
• Heritability can be broadly categorized as all genetic factors, or more narrowly as just additive effects.

Evolution and Speciation

• Evolution is a heritable change in characteristics of a population or species.
• Microevolution: small-scale changes in allele frequencies.
• Macroevolution: large-scale changes, species formation.
• Molecular evolution: changes at the molecular level.
• Horizontal Gene Transfer: genetic material exchange directly or indirectly between organisms without direct reproduction.
• Biological species concept: groups capable of interbreeding to produce viable, fertile offspring in the wild, but not with others.
• Evolutionary lineage concept: focuses on species descent from ancestors.
• General lineage concept: emphasizes the evolution of a species over time.
• Allopatric speciation: geographical isolation leading to speciation.
• Parapatric speciation: speciation along an environmental gradient.
• Sympatric speciation: speciation within the same geographic area.
• Natural selection, random genetic drift, and migration are factors that affect population evolution.

Population Genetics

• Population: group of individuals of the same species in a specific region that can interbreed.
• Local populations (demes): smaller groups within the population that can interbreed.
• Polymorphism: existence of many forms within a trait.
• Monomorphic: all individuals have the same form of a trait.
• Single nucleotide polymorphism (SNP): genomic variation at a single base position.
• Allele frequency: proportion of a particular allele within a population (e.g., number of times an allele appears/total number of all alleles).
• Genotype frequency: proportion of a specific genotype (e.g., number of individuals with a specific genotype/ total number of individuals).
• Hardy-Weinberg equilibrium: a theoretical state where allele and genotype frequencies remain constant across generations in the absence of disturbing factors like mutation, genetic drift, migration, nonrandom mating, and natural selection.
• Founder effect: reduction in genetic variability in a population formed by individuals from a smaller, original population.
• Bottleneck effect: reduction in genetic variability caused by a sharp decrease in population size.

Factors Governing Microevolution

• Mutation: introduces new alleles (variations in a gene).
• Random genetic drift: chance changes in allele frequencies
• Migration: introduction of alleles from another population.
• Natural selection: traits favored by the environment lead to more offspring.
• Nonrandom mating: choosing mates based on phenotype or lineage.

Sources of Genetic Variation

• Independent assortment: different chromosomes independently separate and combine during gamete formation
• Crossing over
• Interspecies crosses: breeding of different species.
• Prokaryotic gene transfer: methods to transfer genetic material between prokaryotes.