Adaptive genes are selected and passed from generation to generation.
According to Darwin's theory of natural selection, natural selection selects for adaptive genes in organisms and deselects non-adaptive genes.
Thus, adaptive genes are passed from generation to generation.
In the case of the giraffes. It is plausible that their ancestors kept on stretching their necks in order to feed as the trees they fed on kept growing taller. The taller the tree grows, the longer the ancestors of giraffes stretched their necks to reach it. Otherwise, they just might have to die of starvation.
This adaptation activated the long-neck genes in the ancestors of giraffes. Natural selection selects this trait and it was passed down across generations.
More on natural selection can be found here: https://brainly.com/question/9830102
#SPJ1