Genetic Terminology

Gene – Mendel used the term factor for a gene. A gene is the unit of DNA responsible for the appearance and inheritance of character.

Allelomorph or Allele – Is a Greek word which means belonging to one another. It refers to one of the two members of a gene pair. Allelomorphs or Alleles are a pair of genes that control the two alternatives of the same character and located at the same locus in the homologous chromosomes.

Gene Locus – It is the region on chromosome representing a single gene. The alleles of a gene are present on the same gene locuson the chromosome.

Heterozygous – The prefix hetero means ‘different’ and zygous means ‘a pair’. The organism in which both the genes of a character areunlike is said to be heterozygous or hybrid. Such organisms do not breed true on self fertilization .eg. Tt.

Homozygous – The prefix homo means ‘the same’ and zygous means ‘a pair’.The organism in which both the genes of a character is said to be homozygous or genetically pure for that character. It gives rise to offsprings having the same character on self breeding. .eg. TT or tt.

Dominant and Recessive – A heterozygote possesses two contrasting genes or alleles but only one of the two is able to express itself, while the other remains hidden. The gene which gains expression in F1 hybrid is known as dominant gene while its allele is unable toexpress itself in presence of the dominant gene is recessive gene.

Genotype (Johanssen) – Genotype of an individual refers to the sum total of genes inherited from both the parents irrespective of whether they are expressed or not.

OR

It is the genetic constitution of an organism which determines the characters.

Phenotype (Johanssen) – It is the appearance of an individual.

OR

Phenotype of an individual refers to the expressed or observable structural and fuctional traits produced by interaction of genes and environment.

Pureline – Generations of homozygous which produce offsprings of only one type i.e. they breed true for their phenotype and genotype. For example, tall pea plants when produce only tall plants, generation after generation on being self pollinated or cross pollinated among themselves.

Genome – Total set of genes in the haploid set of chromosomes and inherited as unit from parents to offspring.

Genepool – All the genotypes of all organisms in a population form the gene pool.

Mendel’s Experimental Plant

Mendel selected common garden pea (Pisum sativum) for his experiments, because the plant shows the presence of several contrasting traits. But it was useful in his experiments also because:

• The pea is small and annual plant. Duration of life cycle is about 2–3 months only. A large number of offsprings can be analysed within short duration.
• It has many contrasting traits.
• Self pollination is found naturally in pea plant because flowers are cleistogamous.
• Cross pollination is possible artificially.
• Pea plant is easy to cultivate.

Detailed investigations by Stig Blixt on pea plant led to the location of Mendel’s seven selected characters on 4 different chromosomes i.e 1, 4, 5 and 7.