A colour blind girl is rare because she will be born only

1. her mother and maternal grand father were colour blind
2. her father and maternal grand father were colour blind
3. her mother is colour blind and father has normal vision
4. parents have normal vision but grand parents were colour blind.

The genotype of the mother was to be either X^C X^C or X^CX and that of father X^CY so that the daughter becomes colour blind.