Pleiotropy

Pleiotropy is the manifestation of several </span>traits present in an organism, which are caused by a single Gene .

The outcome of these crosses stray from the normal Mendelian ratios. This is due to lethal alleles resulting in some crosses producing a lethal genotype. An example of this is known in Manx cats. Manx cats carry a Dominant mutation which affects spinal formation in cats. This leads to the tail of the cat being shorter, or not developing at all. The ratio of genotypes produced when two manx cats are crossed is 2:1, this is not the expected Mendelian ratio of 3:1. This is because the homozygous dominant genotype is lethal, meaning the embryo does not develop. The phenotypic ratio produced is therfore 1 normal: 2 Manx .

Further examples of pleiotropy can be seen in cats with white fur and blue eyes, the dominant white allele (W) is epistatic to all other coat colour genes, resulting in this phenotype.  The mutant allele, W blocks the production of melanin through melanocytes . In 40% of cases this pattern of pigmentation is associated with deafness, believed to result from the migration of pigment cells to various tissues, including the middle ear- where functionality is essential for hearing . The cochlea of the ear contains a band of melanocytes that regulate ion balance, if this balance is interrupted signal transmission to the brain degenerates and irreversible deafness results.Defective pigments therefore lead to deafness and thus deafness is considered a pleiotropic effect of white coat and blue eye colour - the dominant white locus exhibits pleiotropy, affecting both coat colour and hearing.