Re: is cat eye syndrome recessive

Hi Michael,

Strictly speaking, cat eye syndrome would have to be called dominant, but it often does not act quite the same way as other genetic traits that are called dominant.

Cat eye syndrome results from an abnormal extra copy of chromosome 22. It is smaller than the normal chromosome, and may contain duplicated regions. So the affected individual has two normal copies of chromosome 22 along with an abnormal copy. Because this condition results from an extra chromosome, it does not fit neatly into standard Mendelian genetics and the usual uses of the terms �dominant� and �recessive�.

This condition can be inherited from either parent, and usually arises �de novo�, which means that the parent does not have the disorder, and the extra copy arose during meiosis (during formation of the sperm or egg). In other words, the cells of the parent do not carry the extra copy of chromosome 22. In these cases, since the parent does not show signs of the disorder but the child does, it *appears* to be a recessive trait, but it is not recessive in the usual sense.

Individuals with cat eye syndrome are able to have children, and each child has about a 50% chance of inheriting the extra chromosome. Since one extra copy is sufficient to cause the syndrome, the child can inherit the syndrome even if the other parent is completely normal. In this case, the disease behaves as a dominant trait.

Further complicating matters is the fact that an individual can be �mosaic� for the extra chromosome. That means some of their cells carry the extra chromosome, and some do not. Depending on which cells are affected, the person may or may not show symptoms. However, if the germline cells (the cells that give rise to sperm or eggs) are affected, then there is a high likelihood that their children will get the disease. On the other hand, an individual may be mosaic in such a way that they show symptoms, but their germ line is unaffected. If this is the case, none of their children will get the disease, and it will appear to be a recessive trait, but again, it is not recessive in the usual sense.

You can read more about cat eye syndrome here:
http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=115470

Dr. Alex Brands