The nature is introducing us with its fascinating and amazing life forms. The tortoise shell cat and calico cat are one such example for beauty of nature. Have you ever wonder why these cats have Different colours such as black orange white and mixture of all these?.
Answer for the question is hidden in their gene’s and its expression during the development of cat. Usually we have seen cats with only black, only orange only white or with the patches of all these colours. The gene responsible for these coat coloration lie in the X-chromosome with allele R responsible for orange coat colour and allele B responsible for black coat colour. Homozygous condition of these allele will results into black coat colour (XBXB) or orange coat colour (XRXR) but the heterozygous (XBXR) condition results into mixture of black and orange coat colour, which is called as calico cat. One of the amazing fact is all the cat with mixed colouration (calico cat or tortoiseshell cat) are female.
Don’t you surprised how it is and what affects to result into particular condition and sex? The process known as Lyonization or X-inactivation responsible for such condition, is discovered by the english genetist Mary Lyon in the year 1961. During development of fertilised egg or zygote into blastula, zygotic cell undergoes continuous division to form mass of cell. Each cell will have a pair of X chromosome with its allele responsible for coat colour (for example if we have 16 cells, then all the cells are with either XBXB, XRXR or XBXR allele on their X chromosome). During gastrulation one of the X chromosome in each cell get inactivated randomly irrespective of their origin and forms barr bodies (used to identify the sex of the foetus). This explains the variation in coat colour in female cat, in case of cats with black and orange coat colour, cells of developing embryos holds homozygotic alleles B and R respectively in their X chromosome. Inactivation of one X- chromosome will not affect on their coat colour whereas in case of the developing embryo with heterozygous (XBXR) condition, random inactivation of X-chromosome in different cells of gastrula results into patches or mixture of orange, black and white coat colour. The presence of male calico or tortoiseshell cat are rare and is formed only during non-disjunction of chromosomes during meiosis results into allosomal trisomy (XRXBY).
Next time when you see a cat with mixed coat colour then remember its a magic of nature not the curse
Pradeep D’ Cunha
Co-ordinator Competative Examination Cell and Head of the Dept. Biology