In general, chromosomes come in pairs – the cells are diploid. The exceptions to this are the sex cells – eggs and sperm. They contain only one half of the pair, picked at random. Such cells are called haploid, and they can’t divide and grow until the two halves are joined to make pairs again. This happens during conception. When a haploid sperm meets a haploid egg, they fuse together to become a diploid cell, which starts dividing again and eventually (after many divisions and subtle changes) grows into a baby.