If chromosomes change in structure and/or number without losing or gaining any genetic material, they are called balanced chromosome changes. They do not influence cells and therefore are not rated as illnesses. During the development of mature eggs or sperm unbalanced eggs or sperm can be generated alongside balanced ones. If an unbalanced germ cell is fertilised, then a germinitive predisposition is created which will carry an unbalanced chromosome set. After fertilisation this can result in either the absence of pregnancy because the resulting embryo does not have the ability to attach to the uterine mucosa, or to the birth of a deformed child.

There are also unbalanced chromosome changes:

45.X-Karyotype (Turner syndrome):
Women with a karyotype 45.X are smaller than their peers and have some particular external characteristics. Their ovaries, from childhood, reshape connective tissue traces so that eggs cannot mature. Therefore these women cannot have children.

47.XXX-Karyotype:
Women with this chromosome set are healthy and fertile but they reach the menopause prematurely. They can transmit a surplus X chromosome to their children. Their sons will then carry the karyotype 47.XXY and their daughters will have the same chromosome set as their mother.