Glossary of Chapter 11

Gene Sequence of DNA or unit of information for a heritable trait, passed from parents to offspring.

Allele One of a number of different forms of a gene.

Heterozygous Term used to refer to an organism that has two different alleles for the same.

Phenotype Physical characteristics or observable trait or traits of an organism.

Codominance In heterozygotes, refers to a relationship between two alleles of a gene. It occurs when both of the contributions of both alleles are visible and specify different phenotypes. continuous variation Of a population, a more or less continuous range of small differences in a given trait among its individuals.

F1 The offspring of an initial genetic cross. F2 The offspring of parents who are the first filial generation from a genetic cross.

dihybrid cross Is a cross between F1 offspring (first generation offspring) of two individuals that differ in two traits of particular interest.

Epistasis Interaction among the products of two or more gene pairs.

Genotype Genetic makeup of an organism; a single gene pair or the sum total of an individual's genes.

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 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.

multiple allele system Three or more slightly different molecular forms of a gene that occur among individuals of a population.

Pleiotropy Positive or negative effects on two or more traits owing to expression of alleles at a single gene locus.

probability Likelihood that a particular event will occur and that it is proportional to the number of ways the outcome can be reached.

Punnett-square method Diagram showing the gene combinations that might result from a genetic cross.

segregation Mendelian theory. Separation of alleles during gamete formation.

testcross Experimental cross to determine whether an individual of unknown genotype 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.