In 1856 Mendel began to study the mechanisms of inheritance, working with varieties of garden peas from the genus Pisum. (18) In the course of his experiments his garden flowered, as did his understanding of heredity. Mendel discovered several generalities from his experiments that remain the foundation of twentieth‐century genetics. Any student of biology knows Mendel’s work. Known as Mendel’s laws, these basic tenets describe heredity in two simple mechanisms: the law of independent assortment and the law of segregation.
Figure 1.3 Although it took decades for Gregor Mendel’s work on pea plants to revolutionize hereditary theory, his impact is today still felt in the biological sciences.
Credit: American Museum of Natural History
Mendel began an experiment with purebred peas. One breed had yellow seeds, the other green seeds. When purebred yellow‐seeded peas were bred with each other, their offspring through the generations would have yellow seeds. Under the same circumstances, the green‐seeded peas would always have green‐seeded progeny. However, when he bred the purebred pea with yellow seeds to a purebred pea with green seeds, the offspring, or the first generation of this breeding cross, always had yellow seeds. The green seed trait seemed to be gone. Mendel called traits like the yellow‐seed trait dominating (now called dominant) because in first‐generation crosses they would always appear. (19) Traits like the green‐seed trait were called recessive—although they disappeared completely in the first generation, they reappeared in the second. Thus, when Mendel took the yellow seeds from the first generation and either self‐pollinated them or pollinated them with pollen from other yellow peas from the same first‐generation breed, he discovered that some of these offspring, the second generation, again had the green seed trait. The plants, Mendel concluded, retained the ability to produce green seeds—of the second‐generation seeds, 6022 were yellow and 2001 were green. Likewise, when he used six other traits, he found the same pattern in the second generation—traits that had disappeared in the first generation reappeared in the second. (20) The chart below shows the relationship between dominant and recessive traits in second‐generation pea plants in the seven traits Mendel experimented with.
| Dominant trait | Recessive trait | ||
| Round seeds | 5474 | Wrinkled seeds | 1850 |
| Yellow seeds | 6022 | Green seeds | 2001 |
| Gray seed coats | 705 | White seed coats | 224 |
| Green pods | 428 | Yellow pods | 152 |
| Inflated pods | 882 | Constricted pods | 299 |
| Long stems | 787 | Short stems | 277 |
| Axial flowers | 651 | Terminal flowers | 207 |
(21)
From these experimental data, Mendel made several conclusions that are at the heart of his revolutionary contribution to hereditary theory. From the 3:1—dominant to recessive—ratio in the second generation, Mendel concluded that the traits he studied came in two different forms and that these forms existed in pairs in the plant. Mendel called these forms factors. Today we call them genes. During the process of making reproductive cells, Mendel deduced, these genes segregate from each other—that is, the two copies of a gene that you get from each parent segregate, and in the subsequent reproductive cells, only one half of the pair is passed on to offspring. At fertilization, a gene from each parent reconstitutes the pair. How else could Mendel explain how two yellow‐seeded pea plants could produce offspring with green seeds? In this case, the green‐seed trait was as much a part of the pea plant as the yellow‐seed trait despite sometimes being hidden. Mendel also concluded that the factors that were dominant (in the left‐hand column above) somehow overcame the factors that were recessive (in the right‐hand column) when they were combined in offspring from crosses. When all first‐generation plants were crossed, they had both kinds of factors. Mendel’s calculations allowed him to predict a 3:1 ratio if the two factors were segregated. This is Mendel’s first law, the law of segregation, which states that the factors specifying different alleles are separate or segregated, that only one may be carried by a gamete (an egg or sperm), and that gametes combine randomly. Therefore, a child has the same chance of inheriting allele A as it does allele B. (22)
Without the assistance of a calculator or computer, Mendel counted thousands and thousands of plants. Even more remarkable, he constructed lineages that had all possible combinations of two of the seven traits together. For example, he crossed a line of pea plants with round yellow seeds with a line whose seeds were wrinkled and green. This cross gave rise to first‐generation plants with seeds that were all yellow and smooth. But when he crossed these first‐generation plants to each other (a self‐cross), an amazingly regular ratio in the offspring arose—the seeds of nine were yellow and round, three yellow and wrinkled, three green and round, and one green and wrinkled. Mendel reasoned that mating these first‐generation plants was like taking the two possible types of each trait (e.g., seed texture and seed color) and throwing them into a hat. Nature then randomly chose from the hat how to combine the genes. Although the choice is random, the outcome is a remarkably regular ratio of 9:3:3:1. (23).
Figure 1.4 Mendel’s first law of segregation says that alleles will segregate randomly between generations. Mendel’s second law, the law of independent assortment, represented in the figure above, says that pairs of alleles will segregate independently between generations. (P = parents, F1 = first generation, F2 = second generation).
Credit: Wiley
These observations are now known as Mendel’s second law, the law of independent assortment—if two traits (genes) are being controlled with different controllers (alleles), offspring will be produced by random combinations of the controllers (alleles). (24) In other words, a trait is independently and randomly distributed among offspring.
Mendel was either very lucky or very perceptive: it turns out that seven is the number of chromosomes of Pisum. For all seven of the traits he examined to show true independent assortment with respect to one another, none of them can be linked—that is, none of them can be on the same chromosome (or in the case of one of the traits he examined, they have to be very far apart on the same chromosome). (25) Mendel must have watched his peas very closely. Perhaps he recognized the pattern of segregation as he was weeding his garden and thus performed his experiment with an expectation based on his knowledge as a pea