A couple with normal vision have two sons, one colour blind and one with normal vision. What are the genotypes of parents and sons. If the couple also have daughters, what proportion of them will have normal vision. Represent it in the form of a cross. 

Dear student,      
Please find below the solution to the asked query         
Colour blindness is an X linked recessive disorder. So, a male is affected when X chromosome has a recessive allele for color blindness. In a male, an allele of color blindness in the hemizygous condition(X c Y) on X chromosome will express the disease. A normal visioned man marries a normal visioned woman. It means that the genotype of the male is (XCY) and female is (X CX) because male is normal and female is normal but their offspring is colour blind. In the cross, it is depicted that one of the male child is affected by the disease. Therefore female is a carrier.
Gametes XC X
XC XCXC (Colour blind daughter) XCX (Normal visioned daughter)
Y XCY (Colour blind son) XY (Normal visioned son)

If the couple also has daughters, then one of them (25% of the total progeny) will have normal vision.

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