Key Terms

allele

one of two or more variants of a gene that determines a particular trait for a characteristic

codominance

in a heterozygote, complete and simultaneous expression of both alleles for the same characteristic

continuous variation

a variation in a characteristic in which individuals show a range of traits with small differences between them

dihybrid

the result of a cross between two true-breeding parents that express different traits for two characteristics

discontinuous variation

a variation in a characteristic in which individuals show two, or a few, traits with large differences between them

dominant

describes a trait that masks the expression of another trait when both versions of the gene are present in an individual

epistasis

an interaction between genes such that one gene masks or interferes with the expression of another

F₁

the first filial generation in a cross; the offspring of the parental generation

F₂

the second filial generation produced when F₁ individuals are self-crossed or fertilized with each other

genotype

the underlying genetic makeup, consisting of both physically visible and non-expressed alleles, of an organism

hemizygous

the presence of only one allele for a characteristic, as in X-linkage; hemizygosity makes descriptions of dominance and recessiveness irrelevant

heterozygous

having two different alleles for a given gene on the homologous chromosomes

homozygous

having two identical alleles for a given gene on the homologous chromosomes

hybridization

the process of mating two individuals that differ, with the goal of achieving a certain characteristic in their offspring

incomplete dominance

in a heterozygote, expression of two contrasting alleles such that the individual displays an intermediate phenotype

law of dominance

in a heterozygote, one trait will conceal the presence of another trait for the same characteristic

law of independent assortment

genes do not influence each other with regard to sorting of alleles into gametes; every possible combination of alleles is equally likely to occur

law of segregation

paired unit factors (i.e., genes) segregate equally into gametes such that offspring have an equal likelihood of inheriting any combination of factors

linkage

a phenomenon in which alleles that are located in close proximity to each other on the same chromosome are more likely to be inherited together

model system

a species or biological system used to study a specific biological phenomenon to gain understanding that will be applied to other species

monohybrid

the result of a cross between two true-breeding parents that express different traits for only one characteristic

parental generation (P)

the first generation in a cross

phenotype

the observable traits expressed by an organism

Punnett square

a visual representation of a cross between two individuals in which the gametes of each individual are denoted along the top and side of a grid, respectively, and the possible zygotic genotypes are recombined at each box in the grid

recessive

describes a trait whose expression is masked by another trait when the alleles for both traits are present in an individual

reciprocal cross

a paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and male in the other cross

recombination

the process during meiosis in which homologous chromosomes exchange linear segments of genetic material, thereby dramatically increasing genetic variation in the offspring and separating linked genes

test cross

a cross between a dominant expressing individual with an unknown genotype and a homozygous recessive individual; the offspring phenotypes indicate whether the unknown parent is heterozygous or homozygous for the dominant trait

trait

a variation in an inherited characteristic

wild type

the most commonly occurring genotype or phenotype for a given characteristic found in a population

X-linked

a gene present on the X chromosome, but not the Y chromosome