In asexually reproducing organisms variation occurs due to inaccuracies in DNA copying at the time of nuclear division.
For example, one bacterium divides, it will give rise to two bacteria. These daughter bacteria would be similar in body design, but with subtle differences.[See Fig.]
Fig. Appearance of variations over succeeding generations
The resultant bacteria divide again, and each bacterium will give rise to two bacteria in the next generation i.e., there would be four individuals as shown in Fig. (iii). In the bottom row i.e., Fig.(iii) the four individuals will be different from each other.
They may have differences of two types (i) some of the unique type of differences and (ii) differences inherited from, their respective parents, who were different from each other.
Mendel's Laws of inheritance are-:
i. Law of dominance: When two homozygous individuals with one or more sets of contrasting characters are crossed the characters that appear in the F1 hybrids are dominant characters.
ii. Law of segregation: Contrasting characters brought together in a hybrid remain together without being contaminated and when gametes are formed from the hybrid the two separate from each other and only one enters each gamete.
iii. Law of independent assortment: In inheritance of more than one pair of contrasting characters simultaneously, the factors for each pair of characters assort independently of other pairs.