Specific topics include gene structure, gene expression, gene regulation, chromosome structure, epigenetics, repair & recombination, genomic interactions and molecular evolution. There are several branches in genetics such as microbial, eukaryotic, animal, plant and human genetics. Genetics and molecular biology has great impact on health, medicine, agriculture, fisheries and even law. Genetic Diseases are caused by a mutation in one gene, or by mutations in multiple genes or by a combination of gene mutations and environmental factors, or by damage to chromosomes. All genetic disorders are present before birth, and some genetic disorders produce birth defects, but many birth defects are developmental rather than hereditary.