How many of the male offspring of parents 1 and 2 were normal?

A person with one recessive allele for colour-blindness ($X^c$) may not be colour-blind if they are female.

Step 1: Females have two X chromosomes (XX).
Step 2: A female can be heterozygous: $X^N X^c$, where $X^N$ is the dominant allele for normal vision.
Step 3: The dominant $X^N$ allele masks the recessive $X^c$ allele, so she has normal colour vision.

Males with a similar allele are colour-blind.

Step 1: Males have one X and one Y chromosome (XY).
Step 2: A male with the recessive allele has genotype $X^c Y$.
Step 3: There is no second X chromosome with a dominant $X^N$ allele to mask $X^c$.
Step 4: The recessive allele is expressed, so he is colour-blind.

The trait is X-linked recessive.