Cytogenetics arose as a fusion of 19th-century cytology and 20th-century genetics, which came into being in 1903 with the articulation of the chromosome theory of inheritance. The developing field concerned itself with detailing the behavior of chromosomes and their functional subunits, the genes, during reproduction, and with relating that behavior statistically to characteristics of the resulting cells or animals. Modern molecular cytogenetics involves the microscopic study of chromosomes that have been fixed in mitosis and stained with various agents to delineate characteristic bands. DNA probes can be applied to locate specific gene sequences. Karyotyping is the arrangement of photographs of stained chromosomes in a standard format. Cytogenetic techniques are used to test for inborn errors of metabolism and genomic aberrations such as Down syndrome and to determine sex in cases where anatomy is inconclusive.