| allele | One of two or more molecular forms of a gene that arise by mutation and code for different versions of the same trait. |
| codominance | In heterozygotes, expression of a pair of nonidentical alleles that specify different phenotypes |
| continuous variation | Of a population, a more or less continuous range of small differences in a given trait among its individuals. |
| dihybrid cross | An intercross between two F1 heterozygotes that are identical for two gene loci; the dihybrids are offspring of parents that bred true for different versions of two traits. |
| epistasis | Interaction among the products of two or more gene pairs. |
| F1 | The offspring of an initial genetic cross. |
| F2 | The offspring of parents who are the first filial generation from a genetic cross. |
| gene | Unit of information for a heritable trait, passed from parents to offspring. |
| genotype | Genetic constitution of an individual; a single gene pair or the sum total of an individual's genes. |
| heterozygous | Having a pair of nonidentical alleles at a gene locus (that is, on a pair of homologous chromosomes). |
| homozygous dominant | Having a pair of dominant alleles at a gene locus (on a pair of homologous chromosomes). |
| homozygous recessive | Having a pair of recessive alleles at a gene locus (on a pair of homologous chromosomes). |
| hybrid offspring | Of a genetic cross, offspring having a pair of nonidentical alleles for a trait. |
| incomplete dominance | Condition in which one allele of a pair is not fully dominant. |
| independent assortment | Formation of random combinations of chromosomes in meiosis and of genes on different pairs of homologous chromosomes by the passage according to the laws of probability. |
| monohybrid cross | Intercross between two F1 heterozygotes that are identical for one gene locus; offspring of two parents that breed true for different forms of a trait. |
| multiple allele system | Three or more slightly different molecular forms of a gene that occur among individuals of a population. |
| phenotype | the appearance of an organism resulting from the interaction of the genotype and the environment. |
| pleiotropy | The phenomenon of one gene being responsible for or affecting more than one phenotypic characteristic. |
| probability | The chance that each outcome of a given event will occur is proportional to the number of ways the outcome can be reached. |
| Punnett-square method | Construction of a diagram as a way to predict probable outcomes of a genetic cross. |
| segregation | Mendelian theory. Sexually reproducing organisms inherit pairs of genes (on pairs of homologous chromosomes), the two genes of each pair are separated from each other at meiosis, and they end up in separate gametes. |
| testcross | Experimental cross to determine whether an individual a trait is either homozygous dominant or heterozygous. |
| true-breeding lineage | Of sexually reproducing species, a lineage in which only one version of a trait appears over the generations in all parents and their offspring
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