Peas in a Pod

Basic genetics:  In the 19th century, a Czech monk, Gregor Mendel, experimented with pea plants.  He noticed that plants were either tall or short, but when he crossbred a tall plant with a short one he didn't get a medium sized plant - he got a tall one.  However, when he crossbred these new tall plants, sometimes it would result in a short one.  He concluded that 'tall' was dominant and 'short' recessive.

Experiments with true-bred green and yellow pea plants led him to conclude that each parent contributed either a G or a Y characteristic (gene), and that G is dominant over Y, so that GG, GY, and YG plants all appear green, and only YY plants appear yellow.  He concluded that crossing a GG plant with a YY plant would result in offspring which all appeared to be green.  However in the following generation, a GY or YG plant could give its offspring a Y gene rather than a G one.  If the GY or YG plant was cross-bred with a YY plant, not all of the offspring would be green.