Gregor Mendel developed the model of heredity that now bears his name by experiments on various charactersitics of pea plants: height (tall vs. Short); seed color (yellow vs. Green); seat coat (smooth vs. wrinkled), etc. The following explanation uses the tall/short trait. The other traits Mendel studied can be substituted for tall and short.
Mendel started out with plants that "bred true". That is, when tall plants were self-pollinated (or cross-pollinated with others like them), plants in following generations were all tall; when the short plants were self-pollinated (or cross- pollinated with others like them) the plants in following generations were all short.
Mendel found that if true breeding Tall [T] plants are crossed (bred) with true breeding short [t] plants, all the next generation of plants, called F1, are all tall.
Next, he showed that self-pollinated F1 plants (or cross- pollinated with other F1 plants) produce an F2 generation with 3/4 of the plants tall and 1/4 short.
A. 1/4 of the F2 generation are short plants, which produce only short plants in the F3 generation, if they are self- pollinated (or crossed with other short F2 plants;) these F2 plants breed true.
B, 1/4 of the F2 generation (1/3 of the tall plants) are tall plants that produce only tall plants in the F3 generation, if they are self-pollinated; these tall F2 plants breed true.
C. 1/2 of the F2 generation (2/3 of the tall plants) are tall plants that produce 1/4 short plants and 3/4 tall plants in the next [F3] generation, if they are self-pollinated. This is the same proportion of tall to short that F1 plants produce.
Mendel created a model that accounted for these and other data he got from his breeding experiments. The following summarizes the model's first basic feature.
Mendel's model for the F1 generation is summarized in the table at the right. The model states that each trait is controlled by a pair of hereditary packets we now call genes. One packet comes from each parent. The alleles (= forms) of the gene for height are the same in true breeding plants ( T T and t t parent plants ). Cross breeding T T with t t plants produces T t plants in the first or F1 generation. The F1 plants receive a T allele from the tall parent and a t allele from the short parent. The F1 plants are tall because the T allele is expressed and "cover up" the t allele. So the T (tall) allele is called dominant and t (short) allele is called recessive.
The diagram at the right shows how Mendel's model explains the 3:1 ratio of tall to short plants in the F2 generation. In the F1 generation each plant had one T and one t allele of the gene controlling height. Plants in the F2 generation had a 50:50 chance of getting a T or a t from each parent plant. The diagram shows that this results in 1 out of 4 plants getting only t genes and 3 plants getting at least one T gene (which makes the plant tall, because T is dominant over t)
The diagram also shows that the F2 generation actually has three kinds of plants. 1/4 are t t plants, which are short and produce only short plants in following generations in self pollinated. Of the remaining 3/4 tall plants, 1/4 are T T, which are tall and produce only tall plants in following generations if self pollinated. The remaining 2/4 get a T from one parent and at from the other. When self pollinated, they produce a pattern exactly like the F1 generation: 1 short plant for every 3 tall plants. These plants are exactly like the F1 generation.
From these and similar breeding experiments, Mendel deduced (figured out, proved logically) how traits are transmitted from generation to generation. These deductions have held up very well and form the basis of modern genetics, even though many traits and many species do not show the specific patterns of inheritance that Mendel observed.
Transmission of each trait from one generation to the next is carried out by discrete units, which we now call genes. (Some people believe that Mendel already had this idea before he did the actual breeding experiments.)
The gene for each trait comes in pairs, one from each parent. This deduction was confirmed by studying chromosomes, which are visible under the microscope during cell division. Chromosomes come in pairs and carry the genes, which themselves are invisible under the microscope.
