Briefly mention the contribution of T.H. Morgan in genetics.
T. H. Morgan (1866-1945) was an American genetist who was awarded Nobel Prize in 1933 for his work. (i) Morgan found fruit fly Drosophila melanogaster to be better material for experiments on genetics as it was easy to rear and multiply it throughout the year. (ii) He established the presence of genes over the chromosomes. (iii) Morgan established the principle of linkage and crossing over. (iv) He discovered sex linkage and criss-cross inheritance. (v) He developed the technique of chromosome mapping. (vi) He observed mutations. (vii) In 1926, Morgan wrote a book "The theory of Gene."