Science Puzzle
How Many of the Pea Plants Will Be Short?
In the 1850s and 1860s Gregor Mendel, a monk in Brno, grew thousands of pea plants in his monastery garden. Some of his peas always bred tall plants and some always bred short ones. When he crossed a tall plant with a short one, every one of the children grew tall. The shortness seemed to have vanished.
Then he let those tall children pollinate themselves and grew their seeds. Out of all the grandchildren, how many were short?
The Answer
About one in four. Mendel counted 787 tall plants and 277 short ones, very close to three tall for every one short.
Mendel's explanation still stands. Each plant carries two copies of an instruction for height, one from each parent. Today we call these instructions genes. Call the tall version T and the short version t. The tall version is dominant: a plant with even one T grows tall. A plant only grows short if both of its copies are t.
The tall parent was TT and the short parent was tt, so every child got one of each and was Tt. They all grew tall, but each one was secretly carrying a t. When a Tt plant makes pollen or egg cells, each one gets just one of its two copies, picked at random. So a grandchild can be TT, Tt, tT or tt, all equally likely. Only one of those four combinations, tt, grows short.
Mendel published his results in 1866 and almost nobody noticed. In 1900, sixteen years after his death, three other scientists found the same patterns and dug out his paper. He's now known as the father of genetics.
The principle: Dominant and recessive genes. Each parent passes on one of its two copies of a gene at random, so a hidden recessive trait reappears in about one in four of the offspring of two carriers.