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