Genetics And Evolution

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Hemizygous

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Where only one allele is present for a given gene, such as in sex chromosomes.

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Codominance

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When more than one dominant allele exists for a given gene (such as blood-type AB).

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Incomplete Dominance

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When a heterozygote expresses a phenotype that is intermediate between two homozygous genotypes (red and white parent flowers producing pink offspring).

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Penetrance

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Population measure defined as proportion of individuals in population carrying the allele who actually express the phenotype. Can range from full, high, reduced, low, or even nonpenetrance.

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Expressivity

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Varying phenotypes despite identical genotypes. If constant, than all individuals with a genotype express same phenotype, but if variable, than phenotypes differ.

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Mendel's First Law (of segregation)

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Genes exist in alternative forms called alleles, organism has two alleles for each gene, two alleles segregate during meiosis that results in gametes, and if two alleles of an organism are different, only one will be fully expressed.

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Mendel's Second Law (of independent assortment)

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Inheritance of one gene does not affect the inheritance of another gene, although this is complicated by existence of linked genes.

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Transposons

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Genetic elements that can insert and remove themselves from the genome.

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Genetic Leakage

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Flow of genes between species.

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Genetic Drift

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Changes in composition of gene pool due to chance, more pronounced in small populations.

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Founder Effect

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Small population of species finds itself in reproductive isolation from others as a result of natural barriers.

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Bottlenecks

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Sudden or drastic reductions in size of population available for breeding.

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Monohybrid Cross

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Cross in which only one trait is being studied.

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Test Cross

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Used to determine an unknown genotype. Unknown genotype crossed with a homozygous recessive. Also known as a back cross.

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Recombination Frequency (theta)

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Likelihood that two alleles are separated during crossing over.

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Conditions for Hardy-Weinberg Equilibrium

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Population is large, no mutations affecting gene pool, mating between individuals is random, no migration of individuals into or out of population, and the genes in the population are equally successful at reproducing.

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Hardy-Weinberg Equation

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p^2 + 2pq + q^2 = 1

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Modern Synthesis Model/Neo-Darwinism

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Adds knowledge of genetic inheritance and changes in gene pool to Darwin's original theory. Includes mutation, recombination.

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Inclusive Fitness

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Measure of an organism's success in population. This is based on the number of offspring, success in supporting offspring, ability of offspring to then support others.

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Stabilizing Selection

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Keeps phenotypes within a specific range by selecting against extremes.

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Directional Selection

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Emergence and dominance of an initially extreme phenotype.

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Disruptive Selection

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Two extreme phenotypes selected over norm.

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Adaptive Radiation

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Rapid rise of a number of different species from a common ancestor, allowing them to occupy different niches.

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Species

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Largest group of organisms capable of breeding to form a fertile offspring.

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Prezygotic vs. Postzygotic Mechanisms

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Mechanisms that prevent zygote formation, or allow gamete fusion that yield nonviable or sterile offspring.

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Molecular Clock Model

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The more similar the genomes of two species, the more recently the two species separated from each other.