When you look in the mirror, what do you see?  Do you have your mother's nose or your father's eyes?  Has someone said that you smile like your grandmother?  Maybe you have been told that you have hair just like your grandfather did when he was young.  In each of these cases, you got some of the traits for how you look and act from family members who came before you: parents, grandparents, and so on.  Half of the directions for your traits come from your mother's side and half from your father's side.

Gregor Mendel was a person who was able to figure out a lot about how directions are passed down from parents to children.  He was a scientist and priest who lived over one hundred years ago, during the 1800s.  We think of him as the father of modern genetics.

Gregor Mendel loved peas, doesn't everyone?

Mendel studied simple pea plants because they grow quickly and it's easy to see changes in their stem length, seed color, and flower color.  He started with two plants and kept track of all the plants that came from them.  He decided when the plants would have new plants and which plants would have them.  This way he knew what directions were passed along to the new plants.  He compared the directions he knew the parent plants would pass on to what the new plants looked like.  He began to see patterns in how new plants looked.

Mendel got two very important ideas from this work.  First he realized that each parent gives half of its directions to the children.  If Mendel put together a pea plant with pure yellow peas, or one that only has directions for being yellow, with a pea plant with pure green peas, the next set of plants would each get both yellow and green directions.  At the time, no one knew the word "genes," so he called these directions "materials."  A material is what something is made from.  The material for a shirt might be cotton, but for a book will most likely be paper.

Mendel thought that the parent pea plants passed down directions to the next set of plants.  That's what made the flowers purple or white and the seeds yellow or green.  As more and more plants came from the parents, he discovered he could make a good guess as to what new plants would look like.  Some seed colors, such as yellow, were more likely to show up than other seed colors, like green.  Still, he could figure out how likely it was for those colors to show up in new plants.

The other thing Mendel noticed was that the each of the traits he studied was passed on to the new plants without being tied to the other traits.  If a plant had a white flower, that did not mean it was more likely to have a certain stem size.  The stem might be long or it might be short.  Neither length was affected by the color of the flower.  The flowers of the parent plants could be white, pink, or purple, but they would still pass down the same stem length.

Not all purple flowered peas have long stems you know.

However, today we know that some traits can be tied together in people.   If people have one trait that is tied to another, there is a high chance that they will also have the other one.  You may have noticed that a lot of people with yellow hair have blue eyes.  These traits can be tied together; so having yellow hair means you have a good chance of also having blue eyes.  Genes are tied together in people, but Mendel did not make a mistake with his plants.  The traits he looked at were not tied together.

Mendel also saw that plants with the same trait, like long stems, did not always come from the same kind of parent plant.  Even though the directions inside the plants' cells were different, they looked the same.  Today we can make a useful chart called a Punnett square that shows which directions each parent has and what the children could look like.

An example of a Punnett square.

This Punnett square is shaped like a window with four panes of glass.  Above each column, you write one parent's directions.  In this case, a pure tall parent has a "T" at the top of both columns to show it is "Tall."  The capital "T" means that it is a dominant gene.  The pure small parent has a "t" next to each row.  The lower-case or small letter means that it is a recessive gene.  You put the letter for the parent from that row and column inside each pane.  Then you will see all the possible combinations for the children.  In this case, each box will have "Tt" in it. 

Many of our modern genetic techniques have been developed from Mendel's work.  He studied peas for years, and what he discovered was finally published in 1866.  He stopped his plant work two years later.  While he was alive, no one paid much attention to his work.  It was not until 1900 that scientists who were trying to figure out how directions are passed down from parents to children took a closer look at Mendel's ideas.  It took a while for people to notice Mendel, but we are still discovering new things by using his work.