Polyploidy
If the entire number of chromosomes is multiply present, one speaks of polyploidy.
If a triple number of chromosomes (69 chromosomes) is present, one calls this a triploidy. This happens when either two sperm cells fertilize the oocyte or when a diploid oocyte is fertilized. This results in various anomalies.
A doubling of the whole set of chromosomes (4n) leads to a tetraploidy. This arises mostly through a disturbance during a mitotic division. Physiologically, tri- and tetraploidies occur in liver and bone marrow cells. Children with a triploid or tetraploid chromosome set can not survive.
Mosaic
In a chromosomal mosaic two or several cell populations with differing karyotypes exist in one body. Physiologically, all female mammals are mosaics regarding the inactivation pattern of the X chromosomes. A pathologic mosaic can also arise in that a disturbance during the first mitotic division occurs. It either affects only the embryonic part, only the extraembryonic part or both. The mosaic can take place as well as by the autosomes as also in the gonosomes. Relatively frequently one observes a gonosomal mosaic in that in an early embryonic stage an embryonic cell with an XX or XY constellation divides unevenly. On the one side, daughter cells with XX or XY arise and, on the other side, cells with X0 are produced (Turner's syndrome).
(Details)
The Lyon hypothesis is as follows:
- In female cells one of the two X chromosomes is always inactivated.
- The inactivation takes place during early embryogenesis 5c-6
- The inactive X chromosome in various cells of the same individual can have either a paternal or maternal origin.
- In all daughter cells the same X chromosome is inactivated, just as in the stem cells from which they come. One also calls this a clonal inactivation.