Color blindness is the inability or decreased ability to see color, or perceive color differences, under normal lighting conditions. The most common such defect is an inability to distinguish red from green. There is no actual blindness, but there is a deficiency of color vision. The most usual cause is a fault in the development of one or more sets of retinal cones that perceive color in light and transmit that information to the optic nerve. This type of color blindness is usually a sex-linked condition. The genes that produce photopigments (the green, and red-receptor proteins) are carried on the X chromosome. The gene encoding the blue receptor protein is located on chromosome-7.
In order to analyze the pattern of inheritance of this defect, we will examine some crosses by considering only one gene, which is responsible for causing defects in the green receptor protein, as a single sex-linked recessive allele. It must be remembered that Y chromosome does not contain the gene controlling color vision. Now, observe the following five possible crosses involving inheritance of color blindness through reciprocal mating of affected and unaffected parents.
Five possible crosses involving inheritance of sex-linked recessive trait partial colour blindness: (a) normal male x colour blind female. (b) colour blind male x normal female. (c) colour blind male x normal female who is a carrier. (d) normal male x normal female who transverse colour blind female x colour blind male.
From the above crosses, we found that the normal male and colorblind (recessive, homozygous) female produce normal but heterozygous daughters and all sons with the disease (cross a). According to the cross b, a color blind male (hemizygous) and a normal (homozygous) female produces no affected offspring, but the daughters are carriers (i.e. criss-cross inheritance). In cross c, the color blind male with a carrier female results in 50% colorblind offspring. In cross d, a normal male with a carrier female, produces all normal female offspring and 50% affected male offspring. In cross e, we had observed that the mating of two colorblind individuals result in all color blind offspring, provided they have the same colorblindness