The characteristic in continuous traits when different genes affecting a trait interact by adding onto one another.
Additive Genetic Variance
A component of genetic variance determined by loci, where each allele adds a fixed value to the final phenotype.
Allele
The possible forms of a gene found at a specific locus.
Back Cross
A genetic cross where an offspring is crossed with one of the parental strains, often the homozygous recessive one, or with an organism that is genetically identical to the parent.
Carrier
A heterozygous individual whose phenotype is normal but who possesses a deleterious recessive allele. The recessive allele is hidden from expression by a second normal and dominant allele.
Character
The abstraction of any physical, behavioral, ecological, or biochemical feature of an organism, like eye color or seed shape.
Chi-Squared Test
A statistical analysis that compares the observed numbers of individuals to the expected numbers of individuals: a "goodness of fit" test for genetic cross results
Chromosome
A structure housed in the nucleus and composed of DNA and proteins containing part of the genetic information of the cell.
Codominant
The characteristic where two different alleles at the same locus produce different phenotypic effects and both appear in the heterozygous state. In this case, three phenotypes are possible, as opposed to two when only dominant and recessive relationships exist. If t and t' are two alleles of a gene and are codominant, then three genotypes and three phenotypes are possible: tt, tt', and t't' will all look different, and in the heterozygote (tt'), both t and t' are expressed.
Continuous Trait
A trait that can assume any value across a continuous range, from 1 to 100, for instance, or from pure white, through many grays, to pitch black. A continuous trait can usually be described quantitatively with numbers and statistics. Height and weight are good examples.
Dihybrid Cross
In Mendel’s experiments, a cross where the two parentals differ in two characters. For example, the parentals might have different flower color and different seed shape.
Diploid
The characteristic of having two sets of chromosomes. In most cases, with the exception of some plants, all somatic, or body, cells in a sexually reproducing organism are diploid.
Discontinuous Trait
A trait that can only assume discrete values or forms, like black or white. Blood types are an example in humans: only A, O, AB, or B exist.
Dominant
A characteristic that occurs when an allele's presence determines the phenotype of a heterozygous individual. For example, if T and t are two alleles of a gene, T is dominant if Tt individuals are phenotypically identical to TT individuals. Or, to put it another way, a phenotype is dominant when both homozygous and heterozygous individuals show the same phenotype. Dominant alleles are usually represented by capital letters, as in the TT/Tt example.
Dominance Genetic Variance
A component of genetic variance determined by loci, dominant allele masks the effect of a recessive allele in a heterozygote. Therefore, the effect of the recessive allele is only observable when homozygous, while the phenotypic effect of the heterozygote is the same as the effect of the homozygous dominant.
Environmental Variance
A factor in phenotypic variance determined by the influence of environmental factors in different individuals.
Epistasis
An interaction between genes in which the presence of a particular allele at a given locus determines whether or how an allele at a second locus is expressed.
Epistatic Or Interaction Genetic Variance
A component of genetic variance determined by interactions between different loci in an epistatic manner.
Expressivity
The degree to which a genotype is expressed into the phenotype, which is often affected by the environment.
Filial Generation (F1)
In genetic experiments involving crosses, the filial generation 1 (F1), designate the offspring of the parental generation (P). This generation is special because half of its genetic material comes from one member of the parental generation, and half from the other. This helps us understand how different alleles relate to one another. For instance, at one locus, we can determine whether one allele is dominant and the other one recessive.
Filial Generation 2 (F2)
In genetic experiments involving crosses, the filial generation 2 (F2), designate the offspring of the filial generation 1(F1), and the "grandchildren"parental generation. F2 is extra special because individuals might show new combinations of the alleles and phenotypes that the parental generation, the "grandparents," carried.
Gamete
A reproductive cell or germ cell in sexually reproducing organisms. Gametes are haploid: they carry a single set of unpaired chromosomes. In females they are called eggs, and in males, they are called sperm.
Gene
A functional unit of genetic material that is transcribed and contributes to a heritable trait. A gene is a unit of biological inheritance.
Genetic-Environmental Interaction Variance
A factor in phenotypic variance determined by the interaction between genetic differences in individuals in a population and differences in environmental factors.
Genetic Linkage
The characteristic that occurs when two or more different loci tend to be inherited together because they are close to one another on the same chromosome.
Genetic Recombination
The exchange of DNA segments between sister chromosomes during meiosis that leads to new combinations of genes.
Genetic Variance
A factor in phenotypic variance determined by genetic differences between individuals in a population.
Genome
The complete set of genes in a full set of haploid chromosomes in a given organism.
Genotype
A genetic constitution of an organism. It can refer to a single trait or a set of traits.
Haploid
The characteristic of having one set of unpaired chromosomes. Gametes (eggs and sperm) in a sexually reproducing organism are haploid.
Hemizygous
An individual who has only one member of a chromosome pair instead of the usual two, and therefore, any genes present on that chromosome have only one allele. In heterogametic species like humans, this is the case for males when dealing with genes on the sex chromosomes because they only have one X.
Heredity
The process where biological properties are transferred from parent to offspring.
Heritability
The proportion of phenotypic variation in a population that can be attributed to genetic variation.
Heterogametic
The characteristic of carrying two different types of sex chromosomes, therefore producing two kinds of gametes. For example, men are heterogametic: their genotype is XY, so they produce gametes carrying either the sex chromosome X or the sex chromosome Y.
Heterozygous
The characteristic of bearing two different alleles at the same locus (Tt).
Homogametic
The characteristic of carrying only one type of sex chromosome, therefore producing only one kind of gamete. For example, women are homogametic: their genotype is XX, so they produce only gametes carrying the sex chromosome X.
Homozygous
The characteristics of earing two identical alleles at the same locus (TT or tt).
Incomplete Dominance
The characteristic that occurs when the dominant allele doesn’t completely mask the effect of the recessive allele. In this case, the heterozygous phenotype is intermediate between the homozygous dominant and homozygous recessive phenotypes. For instance, the homozygote dominant has red flowers, the homozygote recessive has white flowers, and the heterozygote has pink flowers.
Law Or Principle Of Independent Assortment
The random separation of nonhomologous chromosomes, and therefore, their different genes (and alleles) during the process of meiosis. An individual heterozygous at two loci, which are on different chromosomes—let’s call them the T and Z genes, and the possible alleles t and t', z and z'—can produce 4 types of sex cells, or gametes: tz, tz', t'z, and t'z'.
Law Or Principle Of Segregation
The separation of homologous chromosomes, and therefore their genes, during the process of meiosis. Each haploid daughter cell produced contains only one or the other copy of an allele.
Lethality
The characteristic that occurs when the presence of one or more alleles leads directly to the death of the organism.
Linked Genes
Genes that are located on the same chromosomes and tend to be inherited together.
Locus (plural Loci)
A specific location on a chromosome.
Maternal Inheritance
Inheritance of characters exclusively from the mother through factors not contained in the nucleus. Mitochondrial DNA is an example of maternal inheritance.
Meiosis
A specialized cell division resulting in germ cells or gametes. A diploid cell goes through a round of DNA replication and then two rounds of division, therefore producing four genetically different haploid gametes.
Monohybrid Cross
In Mendel’s experiments, a cross where the two parental strains differ in one character. The parentals might have different flower color, or different seed shape.
Mutant
A rare allele, genotype, or phenotype in a population. A mutant does not necessarily imply that there is something abnormal about a mutant individual—the Teenage Ninja Turtles or the X-Men, are actually bad examples for mutants. In India, for example, the wild type eye color is brown. A large fraction of Indians have brown eyes, while green and blue eyes could be called the mutant phenotype. Few people in India have green or blue eyes.
Mutation
Any change in the sequence of a gene.
Parental Generation (P)
A group that designates the original individuals or populations at the start of a genetic experiment involving crosses. The parental generation individuals are the "parents" of the filial generation 1 (F1), and the "grandparents" of the filial generation 2 (F2). This generation is extra special because, usually, the two individuals or populations are different in a character of interest. In Mendel’s experiments, the parental pea plants differed in characters like flower color or seed shape. Most often, the parental generation is true-breeding, unless you are carrying out a test cross.
Penetrance
The proportion of individuals with a given genotype that express the expected phenotype.
Phenotype
The physical, observable manifestation of genetic traits.
Phenotypic Variance
The degree of phenotypic variation in a population. Phenotypic variance can be broken down into categories of genetic, environmental, and genetic-environmental interaction variance.
Pleiotropy
The characteristic that occurs when a single gene affects multiple phenotypes.
Recessive
The characteristic that occurs when its presence determines the phenotype only when both alleles are the same. In a heterozygous individual, the effect of a recessive allele is "hidden." For example, if T and t are two alleles of a gene, t is recessive if Tt individuals are phenotypically different to tt individuals. Or, to put it another way, a phenotype is recessive when it is only present in a homozygous individual. Recessive alleles are usually represented by lowercase letters.
Test Cross
A special type of genetic cross used to identify whether an individual with a dominant phenotype has a homozygous dominant or heterozygous genotype. A test cross is performed by crossing the individual with a homozygous recessive mate.
Trait
A form of a character. For the character eye color, a trait is blue eyes.
True Breeding
When an organism produces offspring phenotypically identical to itself. For instance, a pea plant with white flowers produces only plants with white flowers.
Wild Type
The most common allele, genotype, or phenotype in a population.