Rainbow & "cc", the world's first cloned cat:
Genetics of tortoiseshell & calico cats

    Shown here at 7 weeks, cc  ["copy cat": above, right] was produced by nuclear transplantation of ovarian cumulus cells from Rainbow, a tri-colour calico (orange / black / white) female [above, left], into enucleated cat ova from an (unnamed) donor. The surrogate mother, Allie, is a bi-colour tortoiseshell (orange and black only) [below, with cc]. Analysis of seven polymorphic microsatellite loci confirmed that cc is a genetic clone of Rainbow.

    An interesting aspect of this experiment is the marked difference between the coat patterns of Rainbow and cc.

    The calico pattern results from an interaction between a sex-linked colour locus (C) and an autosomal white-spotting locus (W). The colour locus has two alleles, O and B, for Orange and Black. In heterozygotes, X-chromosome inactivation ("Lyonization") early in development results in melanocyte precursor cells that express only one or the other allele: where an X bearing the O allele is inactivated, the melanocytes are black, and where B is inactivated, the melanocytes are orange. Melanocytes migrate into the epidermis, where they multiply and are seen as differentially-coloured fur patches. The effect of the autosomal White locus is to slow migration of melanocytes: white fur patches result when melanocytes fail to reach the epidermis. The slower the migration, the more extensive the white patches, and the less time there is for the colored patches intermingle. Thus, tri-colour calicos show more discrete colour patches the more white they have, and bi-colour tortoiseshells show a brindled pattern, with orange and black fur more intimately mixed.

    Why doesn't Rainbow look exactly like her clone, cc ? Note first that the proportion and pattern of white fur are roughly the same. Thereafter, the pattern of X-chromosome inactivation in different cells is essentially random, and the movements of melanocytes to the skin cells are also random. Thus, even though Rainbow and cc have identical tortoiseshell genotypes, the phenotypes of their coat patterns are quite different, and their clonal identity is only apparent from genetic tests.