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, simultaneous 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).
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; a heterozygous phenotype somewhere between both homozygous phenotypes emerges.
independent assortment
Mendelian theory that by the end of meiosis, each pair of homologous chromosomes (and linked genes on each one) are sorted before shipment to gametes independently of how the other pairs were sorted. Later modified to account for the disruptive effect of crossing over on linkages.
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
Observable trait or traits of an individual that arise from gene interactions and gene-environment interactions.
pleiotropy
Positive or negative effects on two or more traits owing to expression of alleles at a single gene locus. Effects may or may not emerge at the same time.
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 simple 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 of unknown genotype that shows dominance for 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
