Every human has 46 chromosomes. Other species have different numbers. Each corresponding chromosome ( DNA (Deoxyribonucleic Acid) strand) is exactly the same length, measured in base pairs, in all humans. Each slot along each chromosome has the exact same purpose in all humans. E.g. where eye colour is encoded in the DNA is in the same place in all humans. Other species have different schemes, but similar animals like chimps, are similar to ours (they have 2 extra chromosomes). That addressing scheme is what defines a species.

~ Roedy (1948-02-04 age:70)