Terminology - SU 2

Gene

An inherited factor that helps determine a trait; often defined at the molecular level as a DNA sequence that is transcribed into an RNA molecule.

Alleles

One of two or more alternative forms of a gene.

Locus

Position on a chromosome where a specific gene is located.

Genotype

The set of alleles possessed by an individual organism.

Homozygous

Having two identical alleles at a locus.

Heterozygous

Having two different alleles at a locus.

Phenotype

Appearance or manifestation of a characteristic.

Monohybrid crosses

Cross between two individuals that differ in a single characteristic—more specifically, a cross between individuals that are homozygous for different alleles at the same locus (AA × aa); also refers to a cross between two individuals that are heterozygous for two alleles at a single locus (Aa × Aa).

P (parental) generation

The first generation in a genetic cross.

F₁ (first filial) generation

Offspring of the parents in the P generation in a genetic cross.

Reciprocal crosses

Pair of crosses in which the phenotypes of the male and female parents are reversed. For example, in one cross, a tall male is crossed with a short female, and in the other cross, a short male is crossed with a tall female.

F₂ (second filial) generation

Offspring of the F₁ generation in a genetic cross; the third generation of a genetic cross.

Dominant

Refers to an allele or a phenotype that is expressed in homozygotes (AA) and in heterozygotes (Aa); only the dominant allele is expressed in a heterozygote phenotype.

Recessive

Refers to an allele or a phenotype that is expressed only in homozygotes (aa); the recessive allele is not expressed in a heterozygote (Aa) phenotype.

The principle of segregation (Mendel’s first law)

Principle of heredity discovered by Mendel that states that each individual diploid organism possesses two alleles at a locus and that these two alleles separate when gametes are formed, one allele going into each gamete.

Concept of dominance

Principle of heredity discovered by Mendel stating that when two different alleles are present in a genotype, only one of them—the dominant allele—is observed in the phenotype.₁

Backcross

Cross between an F₁ individual and either of the parental genotypes.

Punnett square

Shorthand method of determining the outcome of a genetic cross. On a grid, the gametes of one parent are written along the upper edge, and the gametes of the other parent are written down the left side. Within the cells of the grid, the alleles in the gametes are combined to form the genotype of the progeny.

Probability

Likelihood of the occurrence of a particular event; more formally, the number of times that a particular event takes place, divided by the number of all possible outcomes. Probability values range from 0 to 1.

Multiplication rule

Rule stating that the probability of two or more independent events taking place together is calculated by multiplying their probabilities.

Addition rule

Rule stating that the probability of any of two or more mutually exclusive events is calculated by adding the probabilities of the events.

Conditional probability

Probability that is modified by additional information.

Testcross

Cross between an individual with an unknown genotype and an individual with the homozygous recessive genotype.

Wild type

The trait or allele that is most commonly found in natural (wild) populations.

Dihybrid crosses

Cross between two individuals that differ in two characteristics—more specifically, a cross between individuals that are homozygous for different alleles at two loci (AA BB × aa bb); also refers to a cross between two individuals that are both heterozygous at two loci (Aa Bb × Aa Bb).

Principle of independent assortment (Mendel’s second law)

Principle of heredity discovered by Mendel that states that genes encoding different characteristics (genes at different loci) separate independently; applies to genes located only on different chromosomes or to genes far apart on the same chromosome.

Chi-square goodness-of-fit test

Statistical test used to evaluate how well a set of observed values fit the expected values. The probability associated with a calculated chi-square value is the probability that the difference between the observed and the expected values is due to chance.