Terminology in genetics
1.Gene:
- Gene is the shortest segment of DNA responsible for the expression of its specific character.
- Gene is also called as the basic unit of heredity. It is small DNA segment that determines a biological character of an organism. Gene is situated on chromosomes.
- Mendel used gene as the term ‘elementi’ for the genes. The term ‘factor’ was actually given by Carl correns.
2. Genome:
- It is the total genetic composition of an organism.
3. Alleles (Allelomorph):
- An Allele is one of a particular form of a gene. Allele is short form allelomorph. The term was coined by Bateson and Saunders (1902) for characters which are contrasting or differing from one another.
- Allele is an alternate form of the same gene in which one is parental and other is maternal. In case of plant height, tallness and dwarfness are the two alleles of a gene.
- A pair of alternative corresponding genes that are occupying the same position or locus of the homologous chromosomes are called allele or allelomorph.
4. Locus:
- The point on a chromosomes where an allele is located is called locus.
5. Dominant allele:
- Out of two alleles, the one that is capable of expressing itself by hiding or suppressing its contrasting allele is known as dominant allele.
- Dominant allele is an allele that expresses itself in the same form whether present in the homozygous or in the heterozygous condition.
- It is represented by capital letter ‘T’ is the dominant allele for tallness.
6. Recessive allele:
- Out of two alleles, the one that is being suppressed by its alternative allele is called as recessive allele.
- Recessive is an allele that does not express itself when present with the dominant allele (in heterozygous form).
- It expresses itself only in the homozygous form. It is represented by small letter as small ‘t’ is the recessive allele for dwarfness.
7. Homozygous:
- A diploid individual carrying two identical alleles is known as homozygous.
- It is pure for a trait or character
- Capital T and T are the two identical alleles for tallness. Similarly, small t and t are the two identical alleles for dwarfness.
- The homozygous organisms when self-pollinated or inbred always gives rise to true breeding homozygous forms.
8. Heterozygous:
- A diploid individual carrying two different alleles is known as heterozygous or hybrid.
- It is impure for a trait (T t).
- This term was proposed by Bateson and Saunders in 1902 for a zygote or a diploid individual developed from it (zygote) which carries both factors of a pair of alleles.
9. Hemizygous:
- When a genotype is determined by a single gene/factor.
10. Genotype:
- A genotype is a genetic expression of an organism. The term was proposed by Johannason 1909 for hereditary or genetic constitution of an individual.
- Genotype is the genetic constitution of an organism.
- For plant height, TT, Tt and tt are the different genotypes.
- The genotype ratio of F2 in monohybrid cross is 1:2:3. It is observable phenomenon.
11. Phenotype (Trait):
- It is the physical or the observable expression of an organism.
- The term was coined by Johannason 1909 for the visible characters of external appearance of an organism with respect to particular character or a group of characters.
- It is the observable characteristic of an individual.
- Though, the phenotype is determined by the genotype, but certain genes do not express for number of reasons. So genotype and phenotype of an organism may not be the same.
12. Character:
- It refers to a general feature of an organism such as eye color, plant height etc.
13. Dominant character:
- The character which express itself in all the members of first filial generation from a cross between two pure breeding parents differing in respect of this character is called dominant character
14. Recessive character:
- The suppressed character which does not appear in first filial generation is called recessive.
15. Parental generation:
- The plants used as parents in a cross are said to represent parental generation, designed by ‘P’.
16. Hybrid:
- The product of a cross between two genetically different individual belonging to the same genus or species or between two different genera is called hybrid.
- It is product of cross in between two dissimilar parents having contrasting characters.
17. Hybridization:
- It is a phenomenon which speaks about the mixing of two unlike genetic constitutions.
18. Monohybrid cross:
- A cross between the two parents that are differing in one pair of alternating (contrasting) character is called monohybrid cross.
- In this type, single character is used.
19. Dihybrid cross:
- A cross between the two parents that are differing in two pairs of alternating character is called dihybrid cross.
- In this type, two contrasting characters are used. For example, seed shape and seed color (round yellow versus wrinkled green).
20. Polyhybrid cross:
- This is a cross between two parents differing in more than two sets of alternating characters.
21. Filial generation:
- The progeny obtained as a result of crossing between parents is a hybrid progeny and is called First filial generation, represented by F1 (filial=offsprings).
- The progeny obtained as a result of self-fertilization among F1 plants represents second filial generation represented as F2.
22. Pure line:
- It is the variety (line) which is homozygous in condition. It is called as true breeding varieties.
23. Reciprocal crosses:
- A set of two reciprocal crosses means that the same two parents are used in two experiments in such a way that in one cross one individual is used as female parent and other as male parent and in second cross of same genotype sexes are reversed. For example, cross between a tall male and dwarf female is reciprocal of a cross between a tall female and dwarf male.
- The purpose of the reciprocal crosses is to find out whether both parents are making equal contribution.
24. Back cross:
- It is a cross between F1 hybrid with one of its parents (P1 or P2)
- Backcross is often used in genetics for the analysis of genetic constitution of the offsprings.
- It is quick way to make desirable homozygous individuals.
25. Test cross:
- In this cross, the individual of an unknown genotype is crossed with homozygous recessive parents.
- This cross is made in order to determine the genotype of the unknown individual.
- A test cross is conducted to know whether an individual is homozygous or heterozygous for a dominant character.
- The off springs will be 100% dominant, if the individual is homozygous dominant. But the ration will be 50% dominant and 50% recessive in case of hybrid or heterozygous individual.