(i) Addition or deletion in the number of chromosomes in a cell. This is called ploidy. It is of two types Euploidy and Aneuploidy.
(ii) Euploidy is the condition in which the individual bears more than the usual number of diploid (2n) chromosomes. If an individual has three haploid sets of chromosomes, the condition is called triploidy (3n). Triploid plants and animals are typically sterile. If it has four haploid sets of chromosomes, the condition is called tetraploidy (4n). Tetraploid plants are advantageous as they often result in addition or deletion in the number of chromosomes in a cell. This is called ploidy. It is of two types Euploidy and Aneuploidy. Euploidy is the condition in which the individual bears more than the usual number of diploid (2n) chromosomes. If an individual has three haploid sets of chromosomes, the condition is called triploidy (3n). Triploid plants and animals are typically sterile. If it has four haploid sets of chromosomes, the condition is called tetraploidy (4n). Tetraploid plants are advantageous as they often result in increased fruit and flower size reased fruit and flower size.

During Replication of DNA:
(i) In one strand, the daughter strand is synthesized as a continuous strand which is called leading strand.
(ii) In the other strand, short segments of DNA are synthesized. This strand is called lagging strand. The short segment of DNA are called Okazaki fragments.

The law of segregation states that "When a pair of contrasting factors or genes or allelomorphs are brought together in a heterozygote or hybrid, the two members of the allelic pair remain together without mixing and when gametes are formed, the two separate out, so that only one enters each gamete".

(i) Some chromosomes possess secondary constriction at any point of the chromosome.
(ii) They are known as the nuclear zone or nucleolar organizer (formation of nucleolus in the nucleus).