It is a well known fact that like begets like. It means that the living things tend to produce offspring's that resemble them. Human beings resemble each other as they are produced by human beings only. Why children resemble to each other? It is mainly due to inheritance . or heredity.
Inheritance or Heredity
It is defined as the transmission of characters from one generation to successive generations. Though children resemble their parents, they are not identical. No two human beings are identical except identical twins. Why individuals differ each other? It is due to variations.
The variations are defined as the differences between living organisms. These are raw materials for evolution. Variations are two types. They are acquired and hereditary variations. Acquired variations are developed in the life time due to changes in the environmental factors and food. These are never transmitted to the children. The variations which are formed by birth are called germinal or hereditary variations. The variations are formed only in sexually reproducing organisms. In asexually reproducing organisms, the parent genetic material is transmitted to the offspring’s without change. Hence variations are absent.
The study of heredity (similarities) and variations (dissimilarities) together called genetics.
‘ It is a part of D.N.A that controls the protein synthesis. Mendel called character controlling unit as a factor. But the word gene was first used by Johannsen.
Physical appearance of an individual is called phenotype.
Examples :YeIlow seed colour, green seed colour in pisum plant.
Normally one character contains two phenotypes. The seed colour in pisum plant is character. The yellow and green seeded individuals are the phenotypes of character seed colour.
Allelomorphs - The contrasting forms of a character are called allelomorphs.
i. The tall and dwarf plants in pisum are allelomorphs of character height.
ii. The Black and White colours of Guinea pig are the allelomorphs of character skin colour.
The two genes that lie on same locii of two homologous chromosomes are called alleles. Normally alleles are represented as english alphabets. The dominant allele (gene) is represented as capital letter of the first letter of dominant contrasting form. The recessive allele is represented as small letter of the first letter of dominant contrasting form.
i. The tallness controlling allele in Pisum plant is represented as ‘T’ and dwarfness controlling allele is represented as ‘t’.
ii. The black colour controlling allele in Guinea pig is represented as ‘B’ and white colour controlling gene is represented as ‘b’. Normally every individual contains two alleles for a given character. Where as the gamete always contains one allele for a given character.
Irrespective of the phenotype, the genetic representation of an individual for a given character is called genotype.
Genotypes are two types for a given character.
In this the two alleles of a given character are identical. The homozygous individual receives identical genes from both the parents. The homozygous individuals are two types. They are homozygous dominant and homozygous recessive.
Examples : i. Homozygous tall (TT) pisum plant. ii. Homozygous recessive dwarf (if).
In this the two alleles of a given character are different. The heterozygous individual receives different genes from the parents.
Examples: i. Heterozygous tall pisum plant (Tt)
ii. Heterozygous black Guinea pig (Bb)