Explain why pedigrees are needed to identify the carriers of a recessive trait in a family

A pedigree displays the hereditary outcome of a certain species that has not crossbred with another type e.g. dogs--> a pedigree poodle is the offspring of 2 poodles and there has never been any other type of dog cross bred in the earlier generation of that poodle so its a purebreed. OR think of it like harry potter and purebloods who don't have a drop of muggle blood in they're veins compared to hermione who's parents are muggles)
anyway there are dominant and recessive traits that pass down from generation to generation and they can be seen in the offspring. but you probably already know that the dominant gene shows up a lot more. Say for example that you had two purebred poodles (sorry i couldn't think of another thing) and they were both white but when they had babies, 3 of them were white but one of them was brown. the white colour would be Dominant and the brown would be recessive (but we can only determine this if the poodles are from a line of purebreeds,)
if one of the poodle parents was another breed of dog, then we wouldn't be able to tell if the brown poodle was brown due to a recessive trait from the purebred poodle parent or if it was inherited from the other breed parent.

TA DA i hope this helps and wasn't confusing

By water evaporation from the leaves is created a negative pressure at the top of the tree that promotes the ascension  of the water through channels that are called xylem. there are also two molecule forces that are involved: the intra-water molecules cohesion (water molecules stick to each other) and the adhesion when the water molecules stick to the channels they are running in. this is what enables water to go from the roots to the top of a tall tree.