Key Terms

allele

gene variations that arise by mutation and exist at the same relative locations on homologous chromosomes

autosomes

any of the non-sex chromosomes

blending theory of inheritance

hypothetical inheritance pattern in which parental traits are blended together in the offspring to produce an intermediate physical appearance

codominance

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

continuous variation

inheritance pattern in which a character shows a range of trait values with small gradations rather than large gaps between them

dihybrid

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

discontinuous variation

inheritance pattern in which traits are distinct and are transmitted independently of one another

dominant

trait which confers the same physical appearance whether an individual has two copies of the trait or one copy of the dominant trait and one copy of the recessive trait

dominant lethal

inheritance pattern in which an allele is lethal both in the homozygote and the heterozygote; this allele can only be transmitted if the lethality phenotype occurs after reproductive age

epistasis

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

F₁

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

F₂

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

genotype

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

hemizygous

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 chromosome

homozygous

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

hybridization

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

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

species or biological system used to study a specific biological phenomenon to be applied to other different species

monohybrid

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

P₀

parental generation in a cross

phenotype

observable traits expressed by an organism

product rule

probability of two independent events occurring simultaneously can be calculated by multiplying the individual probabilities of each event occurring alone

Punnett square

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

trait that appears “latent” or non-expressed when the individual also carries a dominant trait for that same characteristic; when present as two identical copies, the recessive trait is expressed

recessive lethal

inheritance pattern in which an allele is only lethal in the homozygous form; the heterozygote may be normal or have some altered, non-lethal phenotype

reciprocal cross

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

sex-linked

any gene on a sex chromosome

sum rule

probability of the occurrence of at least one of two mutually exclusive events is the sum of their individual probabilities

test cross

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

variation in the physical appearance of a heritable characteristic

X-linked

gene present on the X, but not the Y chromosome