By Robert Hazen, George Mason University
Gregor Mendel’s famous genetic research on pea plants was conducted between 1856 and 1863. He studied pea plants, and focused his attention on seven distinctive traits of pea plants. His findings were revolutionary and govern the study of biology and genetics today.
Mendel’s Laws of Classical Genetics
Mendel performed more than 28,000 individual experiments to determine the behavior in plants. From this vast accumulation of data, he derived four key ideas, which might be called the four laws of classical genetics.
The first law states that there exist what Mendel called “atoms of inheritance”—what we now call genes. These genes carry traits: in peas, there’s an atom of inheritance for tallness, there’s an atom of inheritance for shortness, and so forth. Mendel didn’t know what these atoms were. He knew that they must be physical structures of some kind, but he didn’t have any idea what they were. He knew that they had to be very small, because they fit into cells, but other than that, who knew?
Each Parent Contributes Equally
The second law is that each parent contributes half of an offspring’s genes. Each individual, therefore, carries two genes for each trait; one from the father, and one from the mother. This was a dramatic demonstration of a concept which had been much debated: whether the mother contributed genetic information more than the father, or vice versa. Mendel said, it’s equal—half from each.
The third law is that genes come in different forms, what we now call different alleles. Some alleles are dominant, some are recessive. Tall is dominant, short is recessive. When both alleles are present, the dominant one is the one that’s always expressed.
Mendel’s fourth law, which actually holds true only in special cases, is that these different alleles are expressed independently, randomly, of each other. All combinations of alleles are equally likely.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Understanding Mendel’s Laws in a Square Diagram
These principles of classical genetics can be illustrated in a square diagram. Two genes of one parent are listed on one side of the square, the top side, for example; and two genes of the other parent are listed on an adjacent side, for example on the vertical side of the square. If you crossbreed a pure-bred tall plant, it’s designated with two T’s on the top, while a pure-bred short plant is designated with two t’s on the vertical.
Then you see what the cross is, and it turns out that when you have two T’s on the top side and two t’s on the vertical side, every combination is a Tt, a combination of the tall and the short allele; and therefore the tall is always expressed. Every offspring is a hybrid, and is designated by Tt, indicating the tall and the short allele together.
When you crossbreed two of these hybrids, on the top you have a T and a t in the square. On the vertical, you also have a T and a t, and this leads to a different combination of letters. On average, one out of every four offspring is TT, one out of every four is tt, and two out of every four are Tt; that is, the mixed, hybrid type of arrangement. That’s why you see one short, three tall, on average, when you crossbreed the hybrids.
Different Combinations of the Square
A more complicated square occurs when you try to match up two traits. You’ll see you have a 4 x 4 square, and this leads to 16 different combinations. As we said, you have, for example, smooth traits for seeds and wrinkled traits; yellow seeds, green seeds, and so forth, and you see those different combinations that are possible.
Ultimately, if all combinations of four alleles are equally likely, 9 plants out of 16, on average, will have smooth yellow seeds; 3 will have wrinkled yellow seeds; 3 on average will have smooth green seeds; and, finally, an average of 1 out of every 16 combinations will be wrinkled green seeds, with the two recessive traits.
Mendel’s Laws Ignored?
Mendel’s results were nothing short of revolutionary, and one might think that they would have transformed biology right then and there, but it’s not what happened. There are a number of reasons for that. First of all, Mendel announced his findings at a meeting of the Natural Science Society in Czechoslovakia, in March of 1865. Probably, this was a meeting with lots of people discussing, and most people weren’t thinking about genetics at that point, so his results really didn’t resonate with anyone in the audience, at least in any major historical way.
His paper was published the following year in the Society’s widely distributed, but seldom-read, journal, and that of course also led to its being ignored for a long time. Indeed, it was completely unappreciated in Mendel’s time, and his great discoveries in genetics were not fully rediscovered until about 1900, when three different European scientists independently reproduced his results.
Common Questions about Gregor Mendel’s Laws of Classical Genetics
Gregor Mendel’s first law of classical genetics states that there exist “atoms of inheritance”—what we call genes now. These genes carry traits: in peas, there’s an atom of inheritance for tallness, there’s an atom of inheritance for shortness, and so forth. Mendel didn’t know what these atoms were. He knew that they must be physical structures of some kind, but he didn’t have any idea what they were.
Gregor Mendel’s third law states that genes come in different forms, what we now call different alleles. Some alleles are dominant, some are recessive. Tall is dominant, short is recessive. When both alleles are present, the dominant one is the one that’s always expressed.
Gregor Mendel’s genetic research on pea plants was conducted between 1856 and 1863.
