They each have a gene that codes for short hair (inherited from its mother or father) and a gene that codes for long hair (inherited from the other parent). In combination, this gene pair for fur length results in medium length hair.
Now when these two dogs are crossed, what new combinations of the genes are possible in the resulting offspring? The second row of Figure 1 shows this:The dog at the far left has inherited its father’s short-hair gene and its mother’s short-hair gene. Result: short hair.
The two dogs in the middle have each inherited a long-hair gene from one parent and a short-hair gene from the other parent. Result: medium-length hair (just like the mother and father).
The dog at the far right has inherited its mother’s long-hair gene and its father’s long-hair gene. Result: long hair.
A casual observer, looking only at the outward appearance, i.e. unaware of what is happening at the genetic level, might think: “There were no long-hair dogs in the parents’ generation. This long hair is a new characteristic—evolution is true!”