Mendel's Study of Peas: A Big Step Toward Understanding Heredity

Mendel chose the pea plant for his research because they had traits that had two different possibilities. For example, he could study whether the flowers were red or white, whether the seeds were yellow or green, and whether the plant was tall or short.

To start his research, Mendel first had to get pure plants. For this, he bred plants with red flowers until the seeds reproduced only plants with red flowers. If there were any plants with white flowers, he set them aside. He crossed plants with red flowers only with plants with red flowers again and again, so he could be sure the plants were pure.

Pure red-flowered plants are labeled RR. There are two "R"s because the offspring inherited one "R" from the male pollen and the other "R" from the female egg. It is easy to see how traits can be passed down by using a square table called a Punnett square.

	....R....	....R....
....R....	....	....
....R....	....	....

In a Punnett square, the traits of one parent are written across the top and the other down one side.

	....R....	....R....
....R....	....R....	....R....
....R....	....R....	....R....

Then, the traits for the parent that is written across the top are copied under the columns.

	....R....	....R....
....R ....	...RR...	...RR...
....R ....	...RR...	...RR...

Next, the traits for the parent that is written down the side are copied across the rows.

The completed Punnett square shows all the possible trait combinations for the offspring of two parents.

Now Mendel decided to cross the pure red-flowered plants with pure white-flowered plants. When he crossed them together the offspring all had red flowers. None of the flowers were white and none of them were pink, a mixture of the two colors.

What happened when he crossed the pure red-flowered plants with pure white-flowered plants? The red-flowered offspring had inherited the trait for red flowers from one parent and the trait for white flowers from the other parent, but all the flowers were red. The trait for white flowers was hidden by the trait for red flowers. This hidden trait is called a recessive trait. The trait for red flowers covered up the trait for white flowers, therefore it is a dominant trait.

	....R....	....R....
....r....	...Rr...	...Rr...
....r....	...Rr...	...Rr...

Recessive traits are designated by a small letter. So, the trait for white flowers can be designated by an "r". This Punnett square shows the crossing of pure red-flowered plants with pure white-flowered plants.

Because the trait for red flowers is dominant, in order for a plant to have white flowers, a plant must inherit a gene for white flowers from each parent.

The gene combination Rr is a hybrid. A hybrid has two different genes for a trait. The hybrid red-flowered pea plants looked like their pure red-flowered pea plants, but they had a hidden gene for white flowers.

When Mendel crossed his hybrid plants, he found that most of the offspring had red flowers, but some did have white flowers.

Here is the Punnett square that shows the crossing of two hybrid red-flowered plants.

	....R....	....r....
....R....	...RR...	...rR...
....r....	...Rr...	...rr...

Three out of every four offspring had red flowers, as the Punnett square shows. One of the plants was a pure offspring with red flowers and two of them were hybrid red-flowered. One of the offspring was a pure white-flowered plant.

This experiment shows that traits do not disappear. They are just covered up by other traits and they can be passed to future generations. Mendel's experiments show that genes are responsible for an organism's traits.