Genetic disorders can be classified in three major groups: (a) Chromosomal, (b) Single gene, and (c) Polygenic. Single gene disorders are the disorders which follow the Mendelian laws of inheritance. The classical patterns of inheritance are autosomal recessive, autosomal dominant, X-linked recessive and X-linked dominant. These disorders are caused by mutation(s) in the genes which carry out variety of cellular functions. Amongst the various hematological disorders the most common genetic disorder which require prenatal diagnosis are beta thalassemia and hemophilia A and B. In some genetic disorders, due to the large size of the gene and its heterogeneity, mutation detection in every family becomes very expensive and impractical. In such cases genetic testing based on linkage analysis can be offered. Genetic linkage occurs when particular genetic loci or alleles for genes are inherited jointly. Genetic loci on the same chromosome are physically connected and tend to stay together during meiosis, and are thus genetically linked. Alleles for genes on different chromosomes are usually not linked, due to independent assortment of chromosomes during meiosis. Naturally, occurring variations in DNA sequence can be detected by restriction endonuclease analysis. These restriction fragment length polymorphisms (RFLPs) can be used in informative families for carrier detection and prenatal diagnosis by analysis of their linkage to the mutant gene. The most useful RFLPs are the highly variable regions of DNA composed of tandemly repeated short DNA sequences known as variable number of tandem repeats.