BIOLOGY OF HEREDITARY AND VARIATION (GENETICS)
DEFINITIONS OF GENETIC TERMS
Genetics is the scientific study of heredity and variation in all living things. The word genetics was coined by Dilliam Bateson (1906).
Hereditary/Inheritance is the transmission and expression of characters or traits from parents to offspring. It accounts for the usual resemblance between a parent and its offspring.
Variation isthe differences which exist between parents and offspring and among the offspring.
Genes are hereditary or basic units of inheritance located in chromosomes and responsible for the transmission of characters from parents to offspring. The hereditary units were named genes by Johannsen in 1909.
Chromosomes are rod or thread like bodies found in the nucleus of a cell which house the genes.
Characters are inheritable features possessed by an organism e.g height, complexion, colour etc.
Gamete is a mature sex cell which takes part in sexual reproduction. If is of two types male and female gamete e.g pollen grains and ovules in plants, sperms and ova in man. Gamete is usually haploid.
Zygote is a single cell formed as a result of the union of male and female gamete. It is diploid.
Allelomorphs (alleles) are pairs of genes on a particular position of chromosomes. They control contrasting characters.
Locus is the site or location of a gene in the chromosome.
Genotype is the sum total of the genes or the genetic make up of an individual inherited from both parents. It includes both dominant and recessive traits.
Phenotype is the sum total of all observable features of an organism. It includes the physical, physiological and behavioural traits e.g. height, skin colour etc.
Dominant trait is expressed in an offspring when two individuals with contrasting characters are crossed. Dominant trait is controlled by dominant gene e.g Tt
Recessive trait is masked, with no effect in the presence of a dominant character. This trait is controlled by a recessive gene
Homozygous is when an individual has two similar genes for the same character i.e. the individual has two identical alleles at a locus as a pair of chromosomes e.g. TT, tt.
Heterozygous is when an individual has different or contrasting alleles located on the locus as a pair of chromosomes e.g. Tt.
Filial generation is made up of offspring of parents. The generations are represented as F1, F2, F3, e.t.c one giving rise to the other.
Hybrid is an offspring from a cross between parents that are genetically different but of the same specie.
Hybridization is the crossing of plants with contrasting characters. This can be monohybridization (i.e. two pure traits crossed) or dihybridization (two pairs of contrasting characters crossed).
Haploid is when an organism has one set of chromosomes in the gamete (23 in number). It is represented by small letter n.
Diploid is when an organism has two sets of chromosomes in the body cell (46 in number). It is represented by 2n.
Mutation is the change in the genetic makeup of an organism resulting in a new inheritable characteristic.