Alleles: these are the different forms of a gene. T and t are different alleles of the gene that determine the height of the pea plant. Alleles occupy the same locus or position on chromosomes. 

Allelic pair: the combination of two alleles which comprise the gene pair.

Autosomal: a locus on any chromosome excluding a sex chromosome. 

Backcross: the cross of an F1 hybrid to one of the homozygous parents; for pea plant height the cross would be Dd x DD or Dd x dd; most often, a backcross is a cross to a fully recessive parent.

Co-dominant alleles: two different alleles at one locus that are responsible for different phenotypes. Both the alleles affect the phenotype of the heterozygote.

Complete linkage: complete linkage describes the inheritance patterns for two genes on the same chromosome when the observed frequency for crossover between the loci is zero.

Dioecious: conditions where organisms produce only one type of gamete (for example in humans).

Dominant trail: a trait expressed preferentially over another trait.

Drosophila melanogaster: the fruit fly, a favorite organism for genetic analysis.

Epistasis: a condition where one gene masks the expression of a different gene for a different trait.

F 1 generation: offspring of a cross between true breeding organisms, homozygous for the trait under consideration.

F2 generation: offspring of a cross involving the F1 generation.

Gene: a unit of inheritance that is directly responsible for one trait or character.

Genotype: the genetic constitution of an organism with respect to a trait. For any single trait on an autosomal chromosome, an individual can be homozygous for the dominant trait, heterozygous, or homozygous for the recessive trait.

Hemizygous: if there is only one copy of a gene for a particular trait in a diploid organism, The organism is hemizygous for the trait, and will display a recessive phenotype. X-linked genes in fly or human males are hemizygous.

Heterozygous: differing alleles for a trait in an individual, such as Tt for the height of the plant.

Homologous chromosomes: the pair of chromosomes in a diploid individual that have the same genetic content. One member of each homologous pair of chromosomes is inherited from each parent.

Homozygous: a condition when both alleles for a trait are the same in an individual. They can be homozygous dominant (W), or homozygous recessive (yy).

Hybrid: a heterozygous condition. Usually it is referring to the offspring of two true-breeding (homozygous) individuals differing in the traits of interest.

lncomplete dominance: it refers to an intermediate phenotype in Fl but parental phenotypes reappear in F2. The flowers of the snapdragon plant can be red, pink, or white. Colour is determined at a single locus. The genotype RR results in red flowers and rr results in white flowers. The heterozygote genotype of Rr results in pink flowers. When the heterozygote has a different, intermediate phenotype compared to the homozygous dominant or homozygous recessive individuals, this is said to be called as incomplete dominance.

Lethal alleles: mutated genes that are capable of causing death.

Linkage: the sets of genes that are on the same chromosome and are physically linked to one another tend to be inherited together. Three inheritance patterns are possible: non-linkage, Partial linkage, and complete linkage.

Mendel’s law of independent assortment of alleles: this law states that alleles of different genes are assorted independently of one another during the fonnation of gametes.

Mendel’s law of segregation: this law states that the alleles segregate from one another during the formation of gametes.

Monoecious: the condition where organisms produce both male and female gametes (for example in garden pea). 

Monohybrid cross: cross involving parents differing in only one trait.

Mutation: change in the DNA sequence of a gene to some new, heritable form. Non-linkage: non-linkage describes the inheritance patterns for two genes on the same chromosome, when the expected frequency for crossover between the loci is at least one. The observed inheritance patters for non-linked genes on the same chromosome is the same for two genes on different chromosomes.

Partial linkage: partial linkage describes one of the inheritance patterns for two genes on the same chromosome, when the expected frequency for crossover between the loci is greater than zero but less than one. From partial linkage analysis, we can learn about the order and spacing of genes on the same chromosome. 

Phenotype: the physical appearance of an organism with respect to a trait, i.e., yellow (Y) or green (y) seeds in garden peas.

Pleiotropic: a condition where a single gene determines more than one phenotype for an organism.

Punnett squares: a probability diagram illustrating the possible offspring of a mating.

Pure breeding: pure breeding plants and their offspring consistently breed true for the character trait being studied. This indicates that pure-, or true-, breeders are homozygous for that specific trait.

Recessive trait: an allele whose expression is suppressed in the presence of a dominant allele.

Reciprocal cross: using male and female gametes for two different traits, alternating the source of gametes.

Sex chromosomes: chromosomes other than the autosomes. Sex determination is based on sex chromosomes.

Sex-linked: a gene coded on a sex chromosome, such as the X-chromosome linked genes of flies and man.

Test cross: generally a cross involving a homozygous recessive individual. When a single trait is being studied, a test cross is a cross between an individual with the dominant phenotype but of unknown genotype (homozygous or heterozygous) with a homozygous recessive individual. If the unknown is heterozygous, then approximately So% of the offspring should display the recessive phenotype.

True-breeding: homozygous for the true-breeding trait.

Wild-type allele: the non-mutant form of a gene, encoding the normal genetic function.

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