Genetics is the study of heredity, a process by which children inherit certain character (traits) from family and from their parents. These traits pass from parents to children by genetic code situated in the nucleus of the cell. The code is formed by deoxyribonucleic acid (DNA molecule). The functional unit of DNA is called gene. The total genetic information in a cell is known as genome. Human possess diploid number of chromosomes (46). They are best seen when they are maximally coiled. Karyotyping is a process by which chromosomes are classified and numbered on the basis of decreasing length and position of centromeres. When both the alleles of a pair are fully expressed in the heterozygote, the genes are known as codominant. Sex chromosomes are pair of chromosomes that are responsible for sex determination. They differ in males and females. The two sex chromosomes of males are different from one another and are designated as XY. Philadelphia chromosome is a chromosomal disease affecting autosome. Abnormalities due to alteration of chromosomal morphology are also discussed in this chapter.