addition rule                           The principle in which the probability that any one of a set of mutually exclusive events is realized equals the sum of the probabilities of the separate events.                                                                                                                                                                                                                                                                                                                                                                                                                                                       albinism                                Absence of melanin pigment in the iris, skin and hair of an animal; absence of chlorophyll in plants.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   allele                                  Any of the alternative forms of a given gene.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           antibody                                A blood protein produced in response to a specific antigen and capable of binding with the antigen.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     antigen                                 A substance able to stimulate the production of antibodies.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             backcross                               The cross of an F1 heterozygote with a partner having the same genotype as one of its parents.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          carrier                                 A heterozygote for a recessive allele.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  codominance                             The expression of both alleles in a heterozygote.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       complementation                         The phenomenon in which two recessive mutations with similar phenotypes result in a wildtype phenotype when both are heterozygous in the same genotype; complementation means that the mutations are in different genes.                                                                                                                                                                                                                                                                                                                                                                                                complementation group                   A group of mutations that fail to complement one another - usually because they occur in the same gene                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  complementation test                    A genetic test to determine whether two mutations are alleles (are present in the same functional gene).                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                dihybrid                                Heterozygous at each of two loci; progeny of a cross between true-breeding or homozygous strains that differ genetically at two loci.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   DNA typing or fingerprinting            Electrophoretic identification of individuals using DNA probes for highly polymorphic regions of the genome, such that the genome of virtually every person exhibits a unique pattern of bands.                                                                                                                                                                                                                                                                                                                                                                                                                         dominance                               Condition in which a heterozygote expresses a trait in the same manner as the homozygote for one of the alleles.  The allele, or the corresponding phenotypic trait, expressed in the heterozygote is said to be dominant.                                                                                                                                                                                                                                                                                                                                                                                              F1 generation                           The first generation of descent from a given mating.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    F2 generation                           The second generation produced by intercrossing or self-fertilizing F1 organisms.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       gamete                                  A mature reproductive cell, such as sperm or egg in animals.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            gene                                    The hereditary unit containing genetic information transcribed into an RNA molecule that is processed and either functions directly or is translated into a polypeptide chain; a gene can mutate to various forms called alleles.                                                                                                                                                                                                                                                                                                                                                                                       genotype                                The genetic constitution of an organism or virus, typically with respect to one or a few genes of interest, as distinguished from its appearance or phenotype.                                                                                                                                                                                                                                                                                                                                                                                                                                                          heterozygous                            Carrying dissimilar alleles of one or more genes; not homozygous.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       homozygous                              Having the same allele of a gene in homologous chromosomes.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             Huntington disease                      Dominantly inherited degeration of the neuromuscular system with onset in middle age.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   hybrid                                  An organism produced by the mating of genetically unlike parents; a duplex nucleic acid molecule produced of strands derived from different sources.                                                                                                                                                                                                                                                                                                                                                                                                                                                                    incomplete penetrance                   Condition in which a mutant phenotype is not expressed in all organisms with the mutant genotype.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       independent assortment                  Random distribution of unlinked genes into gametes, as with genes in different (nonhomologous) chromosomes.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             Mendelian genetics                      The mechanism of inheritance in which the statistical relations between the distribution of traits in successive generations results from (1) particulate hereditary determinants (genes), (2) random union of gametes, (3) segregation of unchanged hereditary determinants in the reproductive cells.                                                                                                                                                                                                                                                                                                                 monohybrid                              A genotype that is heterozygous for one pair of alleles; the offspring of a cross betwee genotypes that are homozygous for different alleles of a single gene.                                                                                                                                                                                                                                                                                                                                                                                                                                                          multiple alleles                        The presence in a population of more than two alleles of a gene.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        multiplication rule                     The principle in which the probability that each of a set of independent events is realized simultaneously equals the product of the probabilities of the separate events.                                                                                                                                                                                                                                                                                                                                                                                                                                              partial dominance                       A condition in which the phenotype of the heterozygote is intermediate between the corresponding homozygotes but more closely resembles one than the other.                                                                                                                                                                                                                                                                                                                                                                                                                                                             pedigree                                A diagram representing the familial relationships among relatives.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      penetrance                              The proportion of organisms having a particular genotype that actually express the corresponding phenotype; if the phenotype is always expressed, penetrance is complete, otherwise, it is incomplete.                                                                                                                                                                                                                                                                                                                                                                                                                  phenotype                               The observable properties of a cell or an organism, resulting from the interaction of the genotype and the environment.

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               Punnett square                          A cross-multiplication square used for determining the expected genetic outcome of matings (named after an early geneticist)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            recessive                               Refers to an allele, or the corresponding phenotypic trait, expressed only in homozygotes.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              reciprocal cross                        A cross in which the sexes of the parents are reversed as compared with another cross.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  segregation                             Separation of the members of a pair of alleles into different gametes in meiosis.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       sib                                     See sibling.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            sibling                                 A brother or sister, each having the same parents.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      sibship                                 A group of brothers and sisters.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        testcross                               A cross between a heterozygote and a recessive homozygote, resulting in progeny in which each phenotypic class represents a different genotype.                                                                                                                                                                                                                                                                                                                                                                                                                                                                         true-breeding                           Refers to a strain, breed, or variety of organisms that yields progeny like itselt; homozygous.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         variable expressivity                   Differences in the severity of expression of a particular genotype.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     zygote                                  The product of the fusion of a female and a male gamete in sexual reproduction; a fertilized egg.