According to the Reptile Database, the majority of snakes have 36 pairs of chromosomes. Twenty of the chromosomes are considered microchromosomes, while the rest are called macrochromosomes. However, the term "snakes" refers to over 3,400 different species and many have not had their karyotype studied, so some variation may exist.
In contrast to many reptiles, such as crocodiles, turtles and some geckos, the gender of snakes is determined by their chromosomes, rather than the environmental conditions that surround the developing eggs. However, snakes exhibit the reverse pattern than that which is demonstrated by humans. While human males are the heterogametic sex, featuring an X and a Y chromosome, female snakes are the heterogametic sex and feature a Z and W chromosome.