Colour blindness is a recessive threatif,a couple with normal vision has two sons, one coloured blind and another one with normal vision. If the couple also has daughters ,what proportion of them will have normal vision?

First of all color blindness X-linked recessive disorder.
1: X linked the defective gene is present on the X chromosome.
2: It is expressive only in its recessive condition
i) X is a Sex chromosome and it is present in both male and female in a different proportion, in the male it is 1, and female it is 2.

If the male get’s the defective gene then he will be expressing color blindness, but where as in the female they have 2 X chromosome and until otherwise both of their X chromosomes are defective. If only one X of a female is found defective, then the other could compensate.

Now coming to your question:
1) Father has normal vision So no faulty X
2) Mother provides the X chromosome for the male fetus and one of the sons is colorblind and the other is not so the mother should be a carrier with one Faulty X (X)
3) Daughter needs two X chromosomes and father has one normal which will suffice for the female child to have normal vision.
It will be 100%

Approved

Colour blindness is a recessive sex linked disorder. Usually it is transferred from mother to son . Since it is related woth x chromosome and a male recieves x chromosome from mother only. The above situation shows that the mother is carrier i.e. xx^ is her genotype .here^ shows that one x chromosome is of colourblindness . Fathet is normal i.e.xy .diseased son have genotupe x^y whereas normal son have genotype xy. No daughter will be colourblind since for a female to be colourblind the genotype must be x^x^. I.e. the mothet should be either carrier or colourbind and father should be colourblind.

Approved