DEFINITION OF SOME GENETIC TERMS

In order to understand the Mendelian Laws and its usefulness in plant breeding, it is important to know some technical words which we shall come across in crop improvement.

1. Genes: Genes are hereditary units or basic units of inheritance. They are located in chromosomes and are responsible for the transmission of characters from parents to offspring.
2. Chromosomes: Chromosomes are rod or thread-like bodies found in the nucleus of a cell. The chromosomes house or contain the genes.
3. Characters or traits: These are the inheritable attributes or features possessed by an organism, e.g. seed colour, seed size, plant height, etc in plants.
4. Gamete: Gamete is a matured sex cell which takes part in sexual reproduction. There are two types: male gamete or spermatozoon (in animals) and pollen grains (in plants) and female gamete, egg or ovum (in animals) and ovules (in plants). Gametes are usually haploid.
5. Zygote: Zygote is a single cell formed as a result of the union of a male gamete with a female gamete. Gamete is usually diploid.
6. Allelomorphs: Allelomorphs are pairs of genes on the position of a chromosome (i.e. locus) that control contrasting characters. A pair of allelomorphs are called allelic pair while each member of the pair is the allele of the other.
7. Phenotype: Phenotype is the sum total of all observable features of an organism, i.e. the physical, physiological and behavioural traits, e.g. height, weight and skin colour.
8. Genotype: This term is used to describe those traits or sum total of the genes inherited from both parents. In other words, the genotype of an individual is his genetic make-up or constitution. Genotype includes both the dominant and the recessive traits that form the genetic make-up of an individual.
9. Dominant character: This is a trait or character that is expressed in an offspring when two individuals with contrasting characters or traits are crossed. Dominant genes on the other hand are genes which control dominant characters. For instance, in a very tall plant, there may be the gene for shortness but the gene has no influence on the gene for tallness.
10. Recessive character: This is the character or trait from one parent which is masked or does not produce the effect in the presence of dominant character. From the illustration above, shortness is the recessive character while tallness is the dominant character. Recessive genes on the other hand are genes which control recessive characters.
11. Homozygous: An individual is said to be homozygous if it has two similar genes for the same character, i.e ., it has two identical alleles at the same position on a pair of chromosomes; the pair of genes controlling a given pair of contrasting characteristics are identical, e.g. (TT) for tallness or (tt) for shortness.
12. Heterozygous: An individual is said to be heterozygous if the two members of a pair of genes controlling a pair of contrasting characters are different, i.e ., it has two different or contrasting alleles located on the same position on a pair of chromosomes, e.g. (Tt) for tallness or a plant with Rr genetic composition has a heterozygous red flower plant.
13. Filial generation: The offspring of parents make up the filial generation. The first, second and third generations of offspring are known as the first, second and third filial generation respectively, and are denoted by the symbols F1, F2, and F3, respectively, too. F1, generation givesrise to F2, generation.
14. Hybrid: Hybrid is an offspring from a cross between parents that are genetically different but of the same species.
15. Hybridization: Hybridization is the crossing of plants with contrasting characters. Monohybridization involves the crossing of two pure traits while Dihybridization involves the crossing of plants with two pairs of contrasting characters.
16. Locus: Locus is the site for location of a gene in a chromosome.
17. Haploid: Haploid is when an organism has one set of chromosomes in the gamete. Gametes at certain stages in the life cycle of plants are haploid. It is represented by small letter “n”.
18. Diploid: Diploid is when an organism has two sets of chromosomes in the body cell. The bodies of animals and plants are diploids. Diploid number is double the haploid number of chromosomes and is represented by “2n”.
19. Mutation: Mutation is a change in the genetic make-up of an organism resulting in a new characteristic that is inheritable.
20. Back cross: Back cross is the crossing of an organism with the homozygous recessive organisms from the original parental generation.
21. Test cross: Test cross is the crossing of an organism with the homozygous recessive organism. Back cross and test cross are used to determine the genotype of organisms showing dominant phenotype.