Inheritance

In spite of the great enthusias evinced in hybrids and hybridisation of orchis , our knowledge of the mode of inheritance of characters in this group os plants is still very inadequate . In this field ,Hurst's (1925) pioneering work on Cattleya flower colour still remains very much our reference record . The apparently erratic expression of flower colours noticed during the breeding experiments with white and coloured Cattleyas piqued the curiosity of Hurst and led him on to the first experiments in orchid genetics and formulation of his theories on colour inheritance . Coloured piqued the curiosity of Hurst and led him on to the first experiments in orchid generics and formulation of his theories on colour inheritance. Coloured Cattleyas bred to coloured forms sometimes produced coloured progeny . Hurst suggested that two genes C and R are responsible for the exression of colours in Cattleya flower . The colour is expressed only when these two factors are present in dominant condition -either heterozygous or homozygous ie . either CcRr or CCRR. When either or both the genes are in the recessive state , no colour is expressed . ccrr represents the pure alba form to all outward appearances ,but on crossing with each other give all coloured progeny -all having the constitution of Cc Rr . When the parent plants are alba forms but heterozygous for the colour factors , ie Ccrr or cc Rr , one fourth of the progeny is coloured and the rest white. Ccrr produces gametes of the constitution Cr and cr. Similarly ccRr produces gametes cR and cr. When these two forms are brought together , the progeny will be of the following constitution:

Similarly when two coloured forms with CcRr constitution are crosed with each other , some of the progeny would be white and others coloured .

The result would be 9: 7 of coloured : white

Besides the pure white and coloured forms, there is also a third form of flower in Cattleya , namely , the white flower with a coloured lip, or semialbas as they are popularly called. Mehlquist (1958) proposed a genetic theory to account for their occurrence . According to him a gene P is responsible for the method of distribution of colours in orchids. This gene , when present in homozygous dominant or heterozygous condition along with the genes C and R, makes the whole flower coloured . On the other hand , when this gene is present in the recessive state , with the genes C and R in dominant condition , the colour is expressed in the lip alone. When genese C and R are in the recessive state , the gene P has no action and the result is a pure white form. Thus CCRRPP or CcRrPp are both fully coloured ; CCRRpp and CcRrpp are both white with coloured lips or semi albas and ccrrPP, ccrrPp and ccrrpp are pure white or albas.

Storey (1950, 1958) conduct edstudies on the breeding behaviour of Spathoglottis plicata,S. plicata var. alba and S. grandifolia . He found out that the purple colour in both S. plicata and S. grandifolia is controlled by two genes -P, which is the dominant gene for pink colour and red spotting on the calli, its recessive allele being p, which produces neither colour nor spotting and T, which is the dominant gene for pale pink colour , producing tinging of sepals and petals , its recessive allele being t, which produces no colour . When these two genes are present together , they produce deep purple colour . When either one is the dominant state, the other being recessive, then the respective pink or tinged condition results.