Show how two phenotypically normal parents can produce a male child with color-blindness.

The female (XcX) is carrier for color blindness while the male (XY) is normal. The carrier female transmits its one X chromosome with gene for color blindness to one of her sons making him color blind.

Explanation:

Color blindness is X linked recessive disorder and followed the criss-cross inheritance wherein the color blind sons get the gene from their carrier/affected mother while the color blind father would transmit the gene to his daughters along with X chromosome.

Since both the parents are normal, the mother (XcX) should be carrier for the disease to get the color blind son. A cross between carrier mother (XcX) and normal father (XY) would make 50% of their sons color blind.