Gene Terminology

1. Character. It is a distinct morphological and physiological feature of an individual, e.g., Stem height, flower colour, position of flower, shape our, position of flower, shape of seed, type of hair, colour of eyes, colour of skin, etc. 

2. Trait. It is an expression of a character like tallness, dwarfness, curly or straight hair.

3. Unit factor. It is a unit of inheritance which controls a trait singly in case of haploid and alongwith ctor or similar or allelic type in case of diploid e.g., d(dwarfness), D(tallness) or T (tallness) and warmness). The unit factor has been called gene by Johannsen (1909). Genes occur in a linear sequence on a chromosome. The Chemically gene were the segments of DNA that has a particular function, the common being of synthesis of polypeptide.

4. Alleles (Bateson, 1905). They are the various forms of a gene which are found at the same place o locus on homologous chromosomes and control the same character. In common usage the alleles represent two different expressions of the same character, e.g., tallness and dwarfness. They are also calle allelomorphs or allelomorphic pair. These days the allele word is used for any form of the gene, similar or dissimilar e.g.,DD,Dd, dd

5. Gene symbol. Each trait is given a symbol. There are two types of conventions for allotting symbols:

Dominant Trait.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 aries. (in) After the Recessive Trait. The first letter of the recessive trait is used in the small Tool 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.

6. Phenotype (Gk. phanein - appearance, typos - type). It is observable external manifestation of morphological or physiological character or characters. The term was given by Johannsen (1909). Phenotype is the same for dominant homozygous and heterozygous states. It is different in case of homozygous recessive trait. Phenotype changes to certain extent with environment and age. 

(i) There is a slow change when infant grows into a child, child and adolescent, adolescent and adult and adult into the aged. 

(ii) Excessive exposure to sunlight causes tanning of skin while polar colour appears in persons living indoors.

(iii) In Primrose red flower colour is dominant trait. However, it is unable to exert a phenotypic effect and flower colour appears white if the temperature is above 30°C. (iv) Himalayan rabbit has coloured extremities.

The same can be changed to white ones by bandaging them for warming.

7. Genotype (Gk. genos - race, typos - type, Johannsen, 1911). It is representation of genetic constitution or gene complement of an individual with respect to one or more characters, e.g., pure tall (TT), hybrid tall (Tt) dwarf (tt). Genotype does not change with age or environment. It is also not influenced by phenotype. Genotype is obtained by an individual in inheritance with some components got from one parent and other components from second parent. All the components of genotype do not express their effects. Only the dominant trait or pure recessive traits produce their effects.

8. Homozygous (Gk, homos - same, zygon - yoke). It is an individual having similar mendelian factors or copies of the same allele. The term was coined by Bateson and Saunders (1902). A homozygous individual is genetically pure. Self breeding produces similar homozygous offspring. Allele present in the homozygous state expresses its effects. Depending upon the nature of the allele, a homozygous can be homozygous dominant (TT) or homozygous recessive (tt).

9. Heterozygous (Gk, heteros - other, zygon - yoke). It is an individual which possesses two contrasting alleles of the same character on its two homologous chromosomes. The term was given by Bateson and Saunders (1902). Heterozygous individuals or heterozygote is also called hybrid for that character. It is not pure since self breeding produces three types of individuals. Heterozygous individuals or hybrid may show than either of the homozygous parents. The phenomenon is called hybrid vigour (Shull, 1914).

10. Hemizygous (Gk, hemi – half, zygon - yoke). It is a condition where a gene is present in only a single copy in a diploid cell. Hemizygous condition occurs for those genes which are found on differential parts of XY chromosomes. Human males are hemizygous for haemophilia and red-green colour blindness. In hemizygous condition, the single recessive allele produces its effect.

11. Dominant Factor. It is a factor or allele of a character which expresses its effect in the phenotype even in the presence of its alternate allele. Dominant allele expresses its effect both in the homozygous and heterozygous condition. It is the property of the specificity of the allele. There is little effect of the parer contributing the allele. The dominant allele is represented by capital letter, e.g., T or D for tallness.

12. Recessive Factor. It is an allele of a character which is unable to express its effects in hybrid bu can do so in the presence of another identical allele or homozygous state. The recessive allele is designat h small letter, e.g., t word for dwarfism.

13. Non-alleles. They are factors present on different gene loci.

14. Gene Locus. It is the particular position of gene on the chromosome. All the alleles of a gen present in the gene pool would occur on the same locus. In a diploid individual there are only two alleles o a gene present on similar gene locus of the two homologous chromosomes.

15. F1 Generation. It is also called first filial generation (L. filus - son, filial - daughter). The term was coined by Bateson (1905). F generation represents the offspring of a cross between two genetically different parents. They are usually hybrids.

16. F2 Generation. It is second filial generation which is formed through selfing or inbreeding of individuals of F1 generation. The term was coined by Bateson (1905). The crossing of heterozygotes of F1 generation amongst themselves is also called intercrossing.

17. Pure Line. It is a strain of true breeding individuals which are genetically pure or homozygous because they have been formed either through repeated self fertilisation of ancestors or breeding between identical homozygous ancestors. Because of homozygosity, pure lines produce similar offspring. Pure lines are maintained for specific traits because of their usefulness in cross-breeding and production of new varieties. The term pure line was coined by Johannsen 1900, 1903

18. Monohybrid Cross. It is a cross made between individuals having contrasting traits in order to study the inheritance of a pair of alleles or factors of a character.

19. Monohybrid Ratio. It is a ratio found in F2 generation of a monohybrid cross where F individuals are selfed. Monohybrid ratio is of two types

(i) Phenotypic Monohybrid Ratio. 3:1

(ii) Genotypic Monohybrid Ratio. 1:2:1.

20. Dihybrid Cross. It is a cross made between individuals having contrasting traits in order to study the inheritance of two pairs of alleles or factors belonging to two different characters.

21. Dihybrid Ratio. It is the ratio found in F2 generation of a dihybrid cross where F2 individuals are selfed.

Dihybrid ratio is of two types: 
(i) Phenotypic Dihybrid Ratio. 9:3:3:1. Here first and last are similar to parental types while the middle ones are recombinants.
(ii) Genotypic Dihybrid Ratio. 1:2:2:4:2:2:1:1:1.

The first and last are genotypically parental types, the rest are recombinants of various types.

22. Trihybrid Cross and Trihybrid Ratio. Trihybrid cross involves inheritance of 3 pairs of unit factors controlling three distinct characters. The ratio obtained in F2 generation of a trihybrid cross is called trihybrid ratio. The phenotypic trihybrid ratio is 27:9:9:9:3:3 :3 : 1. Genotypes are 27 in number 1:2:2:4:2:4:4:8:1:2:2:4:1:2:2:4:1:2:2:4:1:2:1:2:1:2:1.

23. Reciprocal Cross. It is a cross involving two groups of individuals where half of each group are allowed to function as male and other half as female and vice versa. Reciprocal cross is made to eliminate the effect of cytoplasmic traits and to know whether the alleles are present on sex chromosomes or autosomes.

24. Punnett Square. It is a checker board or square divided into smaller squares where all the possible ypes of gametes are shown, male in horizontal row and female in vertical column. They are fused randomly to obtain various types of phenotypes and genotypes. The checker board was first devised by Punnet (1927) and is known after his name as Punnett square.

25. Mendel's Calculations. Mendel used algebraic formula and theory of probability for his calculations,

For example, gametes carrying two different factors when allowed to randomly fuse, produce three types of individuals by the formula (A+ a)2= AA+2Aa+aa. Similarly, the probability of any of the two alternate

events is 50% or1/2.

The probability of joint occurrence of two different events is multiple of theiri individuals probabilities, i.e., 25% or 1/4(1/2*1/2).

26. Back Cross. It is a cross between the hybrid and one of its parents which can be repeated two or more times for obtaining the maximum number of traits of that parent. Back cross is a standard procedure in breeding programmes. The original cross for obtaining hybrid is made to introduce one or more highly useful traits (e.g., disease resistance, pest resistance, high protein content, high vitamin content). This

however dilutes the good characters of original variety. Back crosses are made to reintroduce all the good characters of original variety.

27. Test Cross. It is a special type of back cross which is made between individual with a dominant trait and its recessive parent in order to know whether the individual is homozygous or heterozygous for the trait. Offspring of a test cross will be 100% dominant if the individual was homozygous dominant. The progeny will consist of 50% dominant and 50% recessive for trait if the individual is heterozygous, i.e., ratio is 1 : 1. In case of dihybrid, the test cross will give a ratio of 1:1:1:1.

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