Page 5 - Lesson Notes- Principle of inheritance & Variation

Mendel called the ‘factors’ that passes through gametes from one generation to next
generation. Now a day it is called as genes (unit of inheritance).

Genes that code for a pair of contrasting traits are known as alleles.

Alphabetical symbols are used to represent each gene, capital letter (TT) for gene
expressed in F1 generation and small letter (tt) for other gene.

Mendel also proposed that in true breeding tall and dwarf variety allelic pair of genes for

height is homozygous (TT or tt). TT, Tt or tt are called genotype and tall and
dwarf are called phenotype.

The hybrids which contain alleles which express contrasting traits are called

heterozygous (Tt).

The monohybrid ratio of F2 hybrid is 3:1(phenotypic) and 1:2:1(genotypic).

Test cross is the cross between an individual with dominant trait and a recessive organism in

order to know whether the dominant trait is homozygous or heterozygous.

Principle or Law of Inheritance

Based on observations of monohybrid cross, Mendel proposed two law of inheritance-

1. Law of dominance- states that –

a. Characters are controlled by discrete units called factors.

b. Factors always occur in pair.

c. In a dissimilar pair of factors one member of pair dominate the other.

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