Genetics is the study, which deals with the science of genes, heredity and its variation in living organisms. Mendelian disorders/single gene or monogenic disorders, chromosomal aberrations, and Complex/multifactorial multigenic/polygenic disorders are three classifications of genetic disorders. Autosomal dominant pattern of inheritance, autosomal recessive pattern of inheritance, X-linked dominant pattern of inheritance, and X-linked recessive pattern of inheritance are four different types of Mendelian disorders, which are given in this chapter. Numerical chromosomal aberrations and structural chromosomal aberrations are classification of chromosomal aberrations. Trisomy 21 (Down syndrome), Klinefelter syndrome, and Turner syndrome are three different types of polygenic disorders, which are provided in this chapter. Four well-leveled figures related to the types of chromosomal rearrangements, clinical features of Down syndrome, features of Klinefelter syndrome, and clinical features of Turner syndrome are also presented in this chapter.