Answer:
X chromosome is dominant.
Explanation:
Bob has X chromosome ( along with the genes ) and he passes them further to his female children, while the Y chromosomes are passed to his sons. As genes act in pairs, each female child gets each gene from both of their parents. That's why female children get genes from their fathers and sons get their genes from their mothers. In this case, Bob's X chromosome was more dominant and since it's a chromosome for formation of a female child, he passed it on to his female children, not male.