continuous variation
a variation in a characteristic in which individuals show a range of traits with small differences between them
This hypothetical process appeared to be correct because of what we know now as continuous variation.
discontinuous variation
a variation in a characteristic in which individuals show two, or a few, traits with large differences between them
Mendel worked instead with traits that show discontinuous variation.
dominant
describes a trait that masks the expression of another trait when both versions of the gene are present in an individual
Describe the expected outcomes of monohybrid crosses involving dominant and recessive alleles.
F1
the first filial generation in a cross; the offspring of the parental generation
These offspring were called the F1, or the first filial (filial = daughter or son), generation.
F2
the second filial generation produced when F1 individuals are self-crossed or fertilized with each other
He then collected and grew the seeds from the F1 plants to produce the F2, or second filial, generation.
hybridization
the process of mating two individuals that differ, with the goal of achieving a certain characteristic in their offspring
In 1866, he published his work, Experiments in Plant Hybridization,1 in the proceedings of the Natural History Society of Brünn.
model system
a species or biological system used to study a specific biological phenomenon to gain understanding that will be applied to other species
Describe one of the reasons that made the garden pea an excellent choice of model system for studying inheritance.
P
the parental generation in a cross
The P plants that Mendel used in his experiments were each homozygous for the trait he was studying.
recessive
describes a trait whose expression is masked by another trait when the alleles for both traits are present in an individual
Describe the expected outcomes of monohybrid crosses involving dominant and recessive alleles.
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
This is called a 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.
trait
a variation in an inherited characteristic
He demonstrated that traits are transmitted faithfully from parents to offspring in specific patterns.
allele
one of two or more variants of a gene that determines a particular trait for a characteristic
Describe the expected outcomes of monohybrid crosses involving dominant and recessive alleles.
dihybrid
the result of a cross between two true-breeding parents that express different traits for two characteristics
Independent assortment of genes can be illustrated by the dihybrid cross, a cross between two true-breeding parents that express different traits for two characteristics.
genotype
the underlying genetic makeup, consisting of both physically visible and non-expressed alleles, of an organism
An organism’s underlying genetic makeup, consisting of both the physically visible and the non-expressed alleles, is called its genotype.
heterozygous
having two different alleles for a given gene on the homologous chromosomes
When P plants with contrasting traits were cross-fertilized, all of the offspring were heterozygous for the contrasting trait, meaning their genotype had different alleles for the gene being examined.
homozygous
having two identical alleles for a given gene on the homologous chromosomes
The P plants that Mendel used in his experiments were each homozygous for the trait he was studying.
law of dominance
in a heterozygote, one trait will conceal the presence of another trait for the same characteristic
Mendel’s law of dominance states that 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
The law of independent assortment states that a gamete into which an r allele is sorted would be equally likely to contain either a Y or a y allele.
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
The physical basis of Mendel’s law of segregation is the first division of meiosis in which the homologous chromosomes with their different versions of each gene are segregated into daughter nuclei.
monohybrid
the result of a cross between two true-breeding parents that express different traits for only one characteristic
Describe the expected outcomes of monohybrid crosses involving dominant and recessive alleles.
phenotype
the observable traits expressed by an organism
The observable traits expressed by an organism are referred to as its phenotype.
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
Figure 8.9 shows a Punnett square for a cross between a plant with yellow peas and one with green peas.
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
Called the test cross, this technique is still used by plant and animal breeders.
codominance
in a heterozygote, complete and simultaneous expression of both alleles for the same characteristic
A variation on incomplete dominance is codominance, in which both alleles for the same characteristic are simultaneously expressed in the heterozygote.
epistasis
an interaction between genes such that one gene masks or interferes with the expression of another
In epistasis, the interaction between genes is antagonistic, such that one gene masks or interferes with the expression of another.
hemizygous
the presence of only one allele for a characteristic, as in X-linkage; hemizygosity makes descriptions of dominance and recessiveness irrelevant
Males are said to be hemizygous, in that they have only one allele for any X-linked characteristic.
incomplete dominance
in a heterozygote, expression of two contrasting alleles such that the individual displays an intermediate phenotype
A variation on incomplete dominance is codominance, in which both alleles for the same characteristic are simultaneously expressed in the heterozygote.
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
The segregation of alleles into gametes can be influenced by linkage, in which genes that are located physically close to each other on the same chromosome are more likely to be inherited as a pair.
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
However, because of the process of recombination, or “crossover,” it is possible for two genes on the same chromosome to behave independently, or as if they are not linked.
wild type
the most commonly occurring genotype or phenotype for a given characteristic found in a population
Note that when many alleles exist for the same gene, the convention is to denote the most common phenotype or genotype in the natural population as the wild type (often abbreviated “+”).
X-linked
a gene present on the X chromosome, but not the Y chromosome
When a gene being examined is present on the X, but not the Y, chromosome, it is X-linked.
These terms were extracted from Concepts of Biology – 1st Canadian Edition by Charles Molnar and Jane Gair, eCampusOntario (originally published by BCcampus) which is licensed under a Creative Commons Attribution 4.0 International License. Chapter content adapted from Concepts of Biology by OpenStax (© Rice University), licensed under CC BY 4.0.