Why did Mendel use Pea plants and his experiments

Answer 1

You can read Mendel's own answer in his original work (translacted) here:

http://www.esp.org/foundations/genetics/classical/gm-65.pdf

(look on page 2)

Briefly, he knew he was taking on a huge problem that had stumped lots of folks. He wanted to be able to count his results carefully, so he wanted traits where he could clearly tell what the result was (i.e. he didn't want something like height in humans, that vary over a wide range). So he looked for things that were obviously/objectively either/or.

He also wanted to know where the kids came from, so he looked for an organism where accidental pollination was unlikely (peas protect their sex parts inside petals) and where he could hand pollinate readily (stamens could be cut off to prevent accidental self-pollination and also easily brushed over stigmas to pollinate chosen recipients).

He doesn't mention it, but it's useful that the plants reproduce relatively quickly (oak trees would not have been a good choice :-).

Answer 2

Mendel did hybridization experiments and used pea plants because was easy to see the traits (seven), easy to grow in large number, and manipulate reproduction. He used around 29,000 plants and established two conditions: 1) possess differentiated characteristics; 2) hybrid these plants and protect from foreign pollen.

The seven traits were: flower color (purple or white), pod shape (inflated or constricted), pod color (yellow or green), flower position (axil or terminal), stem length (long or short), seed shape (round or wrinkled),and seed color (yellow or green).

In the first generation of hybrids the ratio was 3:1, three plants showed dominant trait and 1 recessive.

In the second generation the ratio 2:1:1, two hybrid traits, one dominant, and one recessive. There was no offspring with intermediate traits.

From these experiments he stated two laws that were the beginning of genetics:

1. law of segregation, for one particular trait, the pair of parental alleles segregate and pass from one parent to the offspring;
2. law of independent assortment, different pair of alleles are passed to the offspring independently from each other.