SS3 First Term Agricultural Science Lesson Note – Genetic Terms and Mendel’s Laws of Inheritance

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SUB TOPIC 2: GENETIC TERMS AND MENDEL’S LAWS OF INHERITANCE

GENETIC TERMS

CHARACTERS/TRAITS: These are heritable attributes which the plant breeders select eg color, seed size, plant height etc

CHROMOSOMES: It is a structural unit of the nucleus which carries the gene in a linear order

CHROMATIN: It is one of the two thread-like structures formed by the longitudinal division of chromosome during meiotic division

GENES: they are hereditary unit of inheritance responsible for transmission of traits from parent to the offspring

GAMETE: It is a matured sex cell which takes part in sexual reproduction and is usually in haploid condition. Gametes are of two types: male gamete or spermatozoa in animals but pollen grains in plants and female gamete or egg or ovum in animals but ovary in plants

ZYGOTE: It is a single cell formed as a result of union of male and female gametes. It is usually diploid.

PHYNOTYPE: They are physically and physiologically expressed characters of an individual eg height

GENOTYPE: It is the genetic make-up or constitution of an individual. This involves both dominant and recessive characters

DOMINANT CHARACTER: It is a character shown in an individual without any significant influence of contrasting character present in the same individual eg  Tt, Dd, Rr

RECESSIVE CHARACTER: It is the unexpressed character in the presence of a dominant character eg Tt. Dd. Rr

ALLELOMORPHS: they are pairs of genes on the same position of chromosomes that control contrasting characters. A pair of allelomorphs is called allelic pair while each member of the pair is allele

HOMOZYGOUS: A plant is said to be homozygous if the two members of a pair of genes controlling a given pair of contrasting character are identical eg TT, tt, CC, cc etc

HETEROZYGOUS: A plant is said to be heterozygous if the two members of a pair of genes controlling a given pair of contrasting are different eg Tt, Cc, Rr, Dd

HYBRID: It is the offspring got from cross breeding two pure varieties of any species which is intermediate between the parents in appearance.

DIPLOID: It is an organism with two chromosomes of each kind. The bodies of animals and plants are diploids. Diploid number is double the haploid number of chromosome and it is represented as 2n.

HAPLOID: It is a cell or organism with half the total number of chromosomes. This is found in sex or gametal cell

FILIA GENERATION: The offspring of parents make up the filia generation. The first, second, third etc generation of offspring are known as first, second, third etc filia generation denoted by F₁, F₂, F₃ etc respectively

POPULATION: It is a community of individuals which shows a common gene pool of the same species.

MENDELIAN LAWS OF INHERITANCE

Law of Segregation of Genes: This law states that genes are responsible for the development of the individual and that they are independently transmitted from one generation to another without undergoing any alteration. This law can be explained by a particular character such as a cross between the tall and the short plant in F₁. When F₁ plant is selfed to produce F₂, it will segregate into tall and short plant. This can be explained as follows:

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