Dr. ANJU BATTA SEHGAL
Few interesting deviations from classical Mendelian genetics are the Coat colour patterns in Dogs. For dog lovers it’s very interesting to note how different patterns are formed in dogs. Two most important phenomenon involved are:
- Recessive Epistasis.
- Complementary genes.
Both the processes cannot be explained on the basis of laws of heredity given by Mendel. According to Mendelian inheritance each dominant allele is going to express phenotypically in the next generation, making the progeny expressing that gene externally. According to Mendel each character is controlled by a pair of genes called as alleles. For example TT for tallness, tt for dwarfness and Tt for hybrid tall. Both genes are located on the same locus on the homologous chromosomes (similar chromosomes one of paternal origin and other of maternal origin). There can be multiple genes controlling the same character. For example different colours of bell pepper. Sometimes more than two genes contribute for same trait, working together. Also called as gene interaction.
Recessive Epistasis is the interaction when one gene masks or changes the effect of another gene. In other words it does not allow the other gene to express itself fully. When this effect is because of recessive genes it’s called as recessive epistasis. This recessive epistasis is responsible for the coat colour formation in dogs. Coat colour in Labradors is determined by two genes – BB and EE. B gene is responsible for black colouration. Gene B is dominant over b. Whereas gene E determines how well the colour is deposited on the hair of the dog. Gene E is dominant over e. Whenever the ee genes are present in homozygous condition the colour pigments are not deposited. Cellular machinery to pump the colour pigment into the hair does not work. Leading to Colourations BBEE—Black, BbEe—Black, bbEE—Brown, bbEe—Brown, bbee—Yellow, BBee—Yellow. Gene E controls the expression of B gene. It seems that B gene is begging E gene for expressing its black colour, whereas gene E decides whether it will allow or not.
Coat colour in dogs is also controlled by complementary genes, multiple genes interactions. A complex form of interactions decides the colour patterns in different variety of dogs. Four important genes at different loci decide the coat colours. (A) Agouti- determines the depth and distribution of colour in the hair of dog,(B) Black- determines whether black colour can be formed or not, (E)-determines where on the dogs body A allele will be expressed. (S) gene determines spotting patterns. Mix and match of all these four genes give most dogs their coat colour and pattern.
Many allelic forms of these four genes in different hierarchy of dominance act together for forming final look of the dog. As is solid black, aw Agouti white grey, ay yellow, as saddle markings (dark colour on the back with extreme tan markings in the head and legs, at dark colour over most of the body with tan markings on the feet and eye brows, aw Agouti wolf like (salt & pepper) appearance produced by band of yellow pigments on the black hair, ay– yellow look. Em– Black mask with tan coat. E- Allows A locus to express as solid black. ebr Brindle in which black and yellow are in layers. e no black on the body but nose and eyes can be black. B- allows black pigment to be produced, b black pigment cann’t be produced, dogs then become chocolate, liver, tan or red. S -no spots, si -Irish spotting ; numerous white spots, sppiebald spotting; various amounts of white, sw Extreme white piebald; almost all white.
All these allelic forms of four genes when present in different combinations give different manifestations to the beautiful creatures according to the hierarchy of dominance in dogs.
(The Author is ex Principal NSCBM Govt College Hamirpur & former Professor of Botany)
