Why is it not always that gene mutations appear in the first generation? What are they caused by?

Genes or “loci” are regions of DNA, but not the DNA sequence in that region.

If a woman, for example, has a certain trait that is on one of her X chromosomes (an “X-linked trait), there is a 50% chance that she will be passed on to either her son or daughter. If a son inherits a trait, he will always express it because he only has one X chromosome. If a daughter inherits a trait, she expresses it or not, based on the normal rules of allelic dominance. This is where generational skipping comes in.