Genetic Terminology - Part 1 - Principles of inheritance and variations

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Genetic Terms

• Character. The term character is used for the feature of the individual such as stem height , flower colour, type of hair, etc.
• Trait. It is an inherited character such as tall or dwart , purple or white colour of flower , curly or straight hair.
• Unit Factor. This term was used by Mendel. It is a unit of inheritance which controls a trait singly in case of haploid and alongwith another factor of similar type in case of diploid.
• Gene. The unit factor of Mendel was called gene by Johannsen in 1909. A gene is the unit of inheritance which is carried from parent by a gamete . Gene occurs in a chromosome and controls the expression of a character in cooperation with other genes and environment. Chemically gene is a segment of DNA called cistron that has a particular function . One cistron controls the production of one polypeptide.

• Symbols used for Genes. Each trait is given a symbol . There are two types of conven tions for allotting symbols.
• After the Dominant Tralt. The first letter of the dominant trait is used in the capital form for representing the dominant trait . The recessive trait is then shown with the small letter of the same symbol , e.g. , T for tallness and t for dwarfness.
• After the Recessive Trait. The first letter of the recessive trait is used in the small form as a symbol for the recessive trait . The capital form of this symbol is used for representing the dominant trait , e.g. , d for dwarfness and D for tallness.

• Alleles ( The abbreviated form of the term " allelomorphs " Gr . allelon- of one another). The term alleles was given by Bateson in 1905. Alleles are slightly different forms of the same gene which are found at a particular place or locus on homologous chromosomes and control the same character, e.g. , tallness and dwarfness. Now a days the term allele is used for any form of the gene, similar or dissimilar , e.g. , TT, Tt, tt.
• Locus ( pl . loci ). A fixed position or place on a chromosome that is occupied by a given gene or one of its alleles . The allelic genes occupy the corresponding loci in a pair of homologous chromosomes
• Dominant Factor or Allele. It is one of a pair of alleles which can express itself whether present in homozygous or heterozygous state , e.g. , the factor for tallness in hybrid and homozygous states or Tt and TT.
• Recessive Factor or Allele. The factor of an allelic or allelomorphic pair which is unable to express its effect in the presence of its contrasting factor in a heterozygote is called recessive factor or allele, e.g. , the allele of t in hybrid tall Pea plant Tt. The effect of recessive factor becomes known only when it is present in the pure or homozygous state, e.g. , At in dwarf Pea plant
• Wild and Mutant Alleles ( Wild and Mutant Phenotypes ). Wild allele is the one which was originally present in the population, is dominant and usually widespread. The recessive allele is less common and is believed to be formed through mutation of wild allele. It is , therefore , also called mutant allele.

• William Bateson (1861 - 1926) gave the terms genetics, allele, homozygous, heterozygous, F1 and F2.
• Homozygote (Homozygous Individual). The term was introduced by Bateson and Saunders in 1902. It is an individual which contains identical alleles of a gene or factors of a character on its homologous chromosomes. The homozygote is pure for the character and breeds true, that is, it gives rise to offspring having the same trait on self breeding, e.g., TT or tt. It is of two types, homozygous dominant (e.g., TT) and homozygous recessive (e.g., tt).
• Heterozygote (Heterozygous Individual; Bateson and Saunders, 1902). It is an individual which contains the two contrasting factors of a character or two different alleles of a gene on its homologous chromosomes. It is not pure and is called hybrid for that character. Heterozygote does not breed true on self fertilization, e.g. Tt.

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