Chromosome: An organized package of DNA found in the nucleus of the cell. Humans have 23 pairs of chromosomes. 22 numbered chromosomes, called autosomes and one pair of sex chromosomes X and Y.
The U.S. Federal Census is taken every 10 years on a designated census day by an "enumerator" in a specific area called an enumeration district. The first census was done in 1790; there are no censuses before 1790.
In addition to the census population count, there are a number of special censuses: Slave, Industry & Manufacturing, Agriculture, Mortality, Social Statistics, Union Veteran and Widow, Defective, Dependent and Delinquent.
A census is an official counting of the population living in a given locality on a designated day set at intervals. The census places an ancestor in a specific place at a specific time.
Begin with the latest census available and work backwards. Census records have been taken since 1790. Before 1790 you can use Tax Lists and other local lists that might have been compiled according to the state you are researching in.