The technical advances in molecular biology have endowed us with a wealth of knowledge, which has allowed us to identify the cause of diseases not only at a single gene level but at a greater magnitude, where a substitution or deletion of a single base pair can be identified. Our present task is to establish a clear link between phenotype and nucleotide sequence. Obviously, a gene is no longer an imaginary entity. Recent discoveries in a number of bewildering traits, whose inheritance do not follow simple mendelian rules, have caused much amazement. For example, fragile X-syndrome, spine and...

The Genetical Theory of Natural Selection by R.A. Fisher (1930) dictated that sexual dimorphisms may depend upon a single medelian factor. This could be true for some species but his suggestion could not take off the ground as gender in Drosophila is determined by the number of X chromosomes. Technical advances in molecular biology have revived the initial thinking of Fisher and dictate that TDF or SRY genes in humans or Tdy in mice are sex determining genes. The fortuitous findings of XX males and XY female, which are generally termed sex reversal phenomenon, are quite bewildering traits...