How Does Sexual Reproduction Generate Variation Among Offspring?

Fertilization: The union of haploid gametes to produce a diploid zygote.

Genetic Variance: Component of the phenotypic variance that is due to genetic differences among individual members of a population.

Law of Independent Assortment: Gregor Mendel's second law, stating that each allele pair segregates independently during gamete formation; applies when genes for two characteristics are located on different pairs of homologous chromosomes

Mutation: A heritable, usually rare change in the DNA of a gene. Mutations are important in creating genetic diversity.

Recombinant Chromosomes: Chromosomes created when crossing over combines the DNA from two parents into a single chromosome.

How Are Genes Inherited?

Allele: An alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome. For example, the gene for seed shape in pea plants exists in two forms, one form or allele for round seed shape (R) and the other for wrinkled seed shape (r).

Dihybrid Cross: Breeding experiments between parent generation organisms that differ in two traits, for example, AABB x aabb.

Dominant Allele: An allele that expresses its phenotypic effect even when heterozygous with a recessive allele; thus if A is dominant over a, then AA and Aa have the same phenotype.

F1 Generation Offspring: A cross between true breeding plants, homozygous for the trait of interest. Known as "First Filial Generation".

F2 Generation Offspring: A cross between members of the F1 generation. Known as "Second Filial Generation".

Gene Locus: The site on a chromosome where the gene for a particular trait is located.

Genotype: The entire genetic identity of an individual (including alleles or gene forms) that do not show as outward characteristics, or the genetic composition at a specific gene locus or set of loci.

Gregor Mendel: (1822-1884) A Czech-Austrian monk who is often called the "father of genetics" for his study of the inheritance of traits in pea plants. Mendel showed that there was particulate inheritance of traits according to his laws of inheritance. However, Mendel's work was largely ignored until the turn of the 20th century, when its significance was understood for the first time.

Homozygous: Having the same allele at the same locus on both members of a pair of homologous chromosomes. Homozygous also refers to a genotype consisting of two identical alleles of a gene for a particular trait. An individual may be homozygous dominant (AA) or homozygous recessive (aa). Individuals who are homozygous for a trait are referred to as homozygotes.

Hybrid: In this context, a hybrid is a progeny individual from any cross involving parents of differing genotypes.

Law of Independent Assortment: Gregor Mendel's second law, stating that each allele pair segregates independently during gamete formation; applies when genes for two characteristics are located on different pairs of homologous chromosomes.

Law of Segregation: Gregor Mendel's first law, stating that allele pairs separate or segregate during gamete formation, and randomly unite at fertilization.

Monohybrid Cross: Hybridization using a single trait with two alleles, for example, Aa x Aa.

Pedigree: An ancestral line of descent, as diagrammed on a chart, to show ancestral history and to analyze Mendelian inheritance of certain traits including familial diseases.

Pedigree Analysis: The formal study of the pattern of a trait in a pedigree to determine such properties as its mode of inheritance, age of onset, and variability in phenotype.

P Generation: The first generation of a cross. The letter "P" is the conventional designation for the "parental" generation.

Phenotype: The observable structural or functional characteristics of an individual organism that result from the interaction of its genotype with environmental factors.

Punnett Square: A diagrammatic representation of a particular cross used to predict the progeny of the cross, developed by British geneticist Reginald Punnett.

Recessive Allele: An allele which will only result in a specific phenotype when its counterpart allele is also recessive, or when no counterpart allele exists.

Test cross: The breeding an individual of unknown genotype to a homozygous recessive individual in order to reveal the unknown genotype.

What Are Some Complexities Of Genetics?

Co-Dominance: A type of allelic interaction in which the heterozygote simultaneously expresses traits of both homozygotes.

Incomplete Dominance: Refers to the phenotype of a heterozygote that is intermediate between the phenotypes of the two homozygotes.

Polygenic Inheritence: An additive effect of two or more gene loci on a single phenotypic character, in other words, a phenotypic characteristic that is encoded by genes at many different loci.