The branch of biology concerned with the study of heredity and variation. Classical genetics is based on the work of Gregor Mendel (see mendelism). During the 20th century genetics expanded to overlap with the fields of ecology and animal behaviour (see behavioural genetics; population genetics), and important advances in biochemistry and microbiology led to clarification of the chemical nature of genes and the ways in which they can replicate and be transmitted, creating the field of molecular genetics. Since the 1980s, automated DNA sequencing techniques coupled with advances in computerized data handling have transformed genetics, enabling rapid determination of the nucleotide sequences of entire genomes. The bioinformatics revolution has allowed evolutionary relationships between organisms to be traced at the genome level and gene function to be analysed at the cellular level. Genetics today is thus the starting point for unravelling the complexities of every aspect of metabolism, function, and development. See Chronology. See also genetic engineering.