Chromosomal conditions
When cells divide to make egg and sperm, each new cell usually ends up with 23 chromosomes, one from each pair. Sometimes, however, part or all of a chromosome is missing, rearranged or duplicated. Most fertilised eggs that have more or fewer than 46 chromosomes will not survive. In some cases, however, the embryo will survive, but the chromosomal alterations may affect the way it grows and develops.
