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Mendel was right,
but he still had to prove it.

How did Mendel prove his theory?

Mendel began by studying several characteristics of each pea plants.  He studied the plant's height, color of flowers it produced, and the color and shape of seeds.  Mendel used letters to represent characteristics of the plants.  He used a small "y" to represent a green trait in a plant and he used a big "Y" to represent a yellow trait in the plant.

As we said before, yellow and green are simple traits which means that when two parents are combined together they will produce either yellow plants or green plants, not a combination of the two.

Mendel cross-pollinated or combined two plants that had two yellow traits or "YY."  Mendel found that the yellow plants remained yellow when they were combined with other yellow plants.  From this he decided that the plants must have contained something that made them yellow and that it was inherited by their offspring.  He gave the children of these yellow plants the name YY.

He made a graph called a punnent square to show how two "YY" plants produce four "YY"children.  This is what the punnent square looked like:

Mendel found that when two pure "YYs" are combined their offspring is 100 percent "YY."  That means that all four of the offspring were "YY."

The big Y symbolizes the dominant or most powerful characteristic.  A good way to think about dominant characteristics in simple characteristics is to know that no matter where a big Y is, it overpowers the little y.  The big "Ys" on the outside of the box are the parents, the four pairs of big "Ys" on the inside of the box.  Because a big "Y" represents the trait of yellow color, the children of two big "YYs" are always yellow.  They have no little "ys" to make them green.

Mendel also tried to combine two recessive traits or little "yys."  A good way to think about recessive traits in simple characteristics is to think that they are always overpowered by the big "Y" unless they are with each other and then the plant expresses the recessive trait, in this case green.  Recessive traits need two small letters for the recessive trait to be passed on.  A green plant would look like this: "yy."

Mendel made a punnent square of this too.  Here's what happens when two recessive parents combine together:

Just like the two pure "YYs" that were cross-pollinated, the two pure "yys" produce 100 percent or four out of four "yy" children.

When two green parents combine together their children always have the recessive trait.  In this case, all the children of the two green plants are also green.

What would happen if a little "yy," or green parent and a big "YY," or yellow parent were combined?

· The dominant trait, in this case yellow, is passed on.

· The recessive trait, in this case green, is passed on.

· They can't be cross-pollinated.
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