In some cats, only some of their cels have an extra X chromosome. This condition is know as chromosomal mosaicism. If the cells of the reproductive system have the normal XY genotype, these cats will be fertile. Another mechanism by which a fertile sire may be produced is known as chimerism. In these cats, two zygotes merge into a single embryo, and this may create a kitten with some cels of the body that are XX and other cells that are XY, or with only XY cells, but W chromosomes that come from two different individuals. If the cells that comprise the reproductive system are XY, but the cells tha produce the melanocytes are XX, a fertile tortoiseshell male may result. Chimerism can create cats with even stranger color variations. For instance, if one of the fertilized eggs had the genotype of a dilute color, but the other fertilized egg had the genotype of a dominant type color, a cat that is both red and blue may be produced. Another mechanism is when an error occurs during cell division and the orange gene itself is damaged in a portion of the cat's cells. This can cause anything from a small patch of black on an otherwise orange colored cat or a full bodied tortoiseshell appearance. Based on how the affected cells develop and when the gene damage occurs, fertile tortoiseshell males may exist that produce both sperm with orange allele and sperm with the non-orange allele. Recent studies of tortoiseshell males indicate that Klinefelter's syndrome is comparatively rare in cats, while the other mechanisms, especially chimerism, is more common. Thus, fertility in tortoiseshell and calico sire may actually more common than infertility. I have often wondered about this myself. The reason it took so long to answer this question was to make sure I had all my facts straight. This particular study, like what I believe you are asking about, is done on calico cats. Hope this helps and answers your question thoroughly!