The poor immune response was linked to albinism. The mosaic albino condition is found in horses but is uncommon in cats. In an experiment done with mouse embryos, cancerous mouse cells were injected into the embryo. Mosaicism can take many forms, from one cell having an abnormal amount of genetic material to whole groups of cells being different from the original fertilized egg. Very occasionally a mutation occurs in the cells which go on to produce sperm and the stud cat sires kittens which are 'genetically impossible' according to his own coloration or type.
Mosaicism can occur in many ways and take many forms.
The one that leads to male tortoiseshell cats is simply one we can see. It probably happens more frequently than we even know about, but is usually masked by the cat's genetically inherited coat pattern.