Index >> Monohybrid Crosses and Law of segregation>> Genetical Terminology

It is customary in genetic studies to think of the phenotype as being the product of a reaction between its hereditary constitution or lenotype and the environment, according to the following scheme :
For example in honey bee Apis mellifera, the difference between the queen and the workers depend on nutritional differences and on pheromones, at the larval stage. Further, the flowers of hydrangea may be blue if grown in acid soil or pinkish if grown in alkaline soil, due to an interaction of gene products with the hydrogen-ion concentration (pH value) of their environment.
The genes are though stable and self replicating structures, but on certain rare occasions a change may occur simultaneously in them. This change is called a mutation. Mutation alters the coded informations of a gene and may result in the production of a defective protein or in the cessation of protein synthesis. The net result of a mutation is often seen as a change in the phenotypic trait.

Mutation may thus, change a gene into two or more alternative forms called allelomorphs or alleles. For example, the healthy people have a gene which specifies the normal protein structure of the red blood cell pigment called haemoglobin. Some anaemic individuals have an altered form of this gene, i.e., an allele, which makes a defective haemoglobin protein unable to carry the normal amount of oxygen to the body cells.
Further, each gene occupies specific position on a chromosome, called the gene locus (plural loci). All allelic forms of a gene are therefore found at corresponding positions on genetically similar (homologous) chromosomes. All the genes possessed by an individual constitute its genotype.

Genotypically, an organism may be either homozygous or heterozygous, for a particular phenotypic trait. A homozygous individual has identical alleles at both gene loci for a particular phenotypic trait. For example, a genetically pure tall pea plant has two alleles for tallness at both gene loci for the length of the stem. A homozygous individual (also called homozygote) is formed by the union of gametes carrying identical alleles and it produces only one kind of gametes during gametogenesis: