Genes carried on the sex chromosomes are said to be sex - linked. In the case of the heterogametic sex (having \(xx\) or \(xy\) ) there is a portion of the \(X\) chromosome for which there is no homologous region of the Y chromosome. Characteristics determined by genes carried on the non - homologous portion of the \(X\) chrosome therefore appear in males even if they are recessive. This form of linkage explains the inheritance of sex - linked traits or syndrome such as red - green colour blindness, premature balding, haemophilia etc. River blindness (onchocerciasis) is caused by a roundworm known as onchocerciavolvulus, and is transmitted by black fly simulium.