Why is it important for Mendel's pea plant to be a purebred variety?
Pure breeds were important in his experiment so he could record accurate results.
In his pea-plant experiment, Mendel was known to use pure breed carrying only genes yellow and round together, and only green and wrinkly together.
Note: Y = Yellow Gene, R = Round Gene, G = Green Gene, W = Wrinkle Gene. Any combination in an "equation" is refering to a plant with those genes, there will be four letters because peas have 2 of each gene type in his examples. Y and G affect colours, while R and W affect shape.
(IF YOU ALREADY UNDERSTAND WHAT MENDEL DID SKIP THE NEXT TWO PARAGRAPHS)
What actually happened in a result of these experiments is that when plants crossed with a plant with the same characteristics, the offspring had he same characteristics. (eg. YYRR + YYRR -> YYRR and GGWW + GGWW -> GGWW) This allowed him to conclude that genes were passed on through the parents to their offspring.
He then tested YYRR + GGWW to see which parent would pass traits and it resulted with a plant that appeared to be YYRR. However, when that plant was bred with another GGWW it had an offspring that was GGWW. This didn't make sense until further inspection when he discovered that the first offspring which appeared to be YYRR was actually YGRW. Resulting in his discovering that plants with Dominant Genes that show can still have recessive traits within that can still be passed on.
If he wasn't assured pure breeds in his pea plants, he may have multiple pea plants where it had a similar genetic make up to the YGRW example where he didn't know it had traits hidden within. If these type of plants were the originals, and he conducted the experiments they way he did, each result would be different rather than the consistent results he got from the first round like he did in reality.
If the impure breeds would have led the experiment, overall it would have set science as we know it so much further back in the terms of medication, and much of what we know about the functions of life may have not made their way into the knowledge within the scientific world, as much of what we know today if off of the knowledge Mendel shared.