Gregor Mendel (1822–1884) is often called the “father of heredity.” Mendel was a monk and a high school physics, mathematics, and Greek teacher, but he was also one of the first genetics researchers. Most of Mendel’s research was carried out in the Czechoslovakian monastery where he lived. He experimented with the way traits are passed from generation to generation in pea plants by researching seven main characteristics: flower color, flower position, stem length, pod shape, pod color, seed shape, and seed color.

Mendel picked the common pea plant, Pisum sativum, for his research because it is easy to grow in large numbers and its reproduction is easy to control. Pea plants have both stamens (the male reproductive organ in plants) and a pistil (the female reproductive organ). Under normal circumstances, these plants can self-pollinate, but they can also be cross-pollinated with other pea plants. Mendel allowed the plants to self-pollinate for several generations. During this time, he controlled access to other pea plants by covering the plants so pollen could not be transferred from one plant to another. In this way, Mendel was certain of the parentage of each plant. Eventually, he produced a plant that was purebred for whichever trait he was considering in his experiments at the time. The purebreds had two identical alleles for that particular trait, meaning that they were homozygous. For example, one of the traits that interested Mendel was the pea plant’s seed shape. The pea plant seeds (peas) came in two shapes, round or wrinkled. Therefore, he created purebred plants with round seeds (RR) and purebred plants with wrinkled seeds (rr).

Gregor Mendel focused on seven main characteristics
during his study of how traits are passed down from generation to
generation in pea plants.

Once he had purebreds for each trait, Mendel was ready to start his experiments. This time, he planned to crossbreed the purebreds. To do this, Mendel eliminated the possibility of self-pollination by carefully removing a plant’s stamens before they were mature enough to pollinate the plant. Then, using a paint brush to apply pollen from a selected plant onto another plant, he was able to control the cross-pollination. Once again, this allowed Mendel to pinpoint the exact parentage of each pea plant.

To continue his study into the pea plants’ seed shape, he painted pollen from a plant that had wrinkled peas (rr) onto a plant that had round peas (RR). He also did the reverse, painting pollen from a plant with round seeds onto a plant that had wrinkled seeds. Mendel found out that it did not matter which plant the pollen came from—the offspring were identical. However, it was possible that the origin of the pollen could have made a difference. By trying the experiment twice, using the pollen from one parent plant in one trial and the pollen from the other parent in the second, Mendel was following the proper scientific method.

The plants being cross-pollinated are called the parental generation, or the P generation. The offspring of this mating is called the first filial generation. Scientists abbreviate the first filial generation as F1. Mendel discovered that the F1 plants that resulted from a cross of a purebred parent plant with round peas, as well as one that had wrinkled peas, all had round seeds.

This observation led Mendel to define the trait that he could observe, in this case round peas, as the dominant trait. Mendel then allowed the plants in the F1 generation to self-pollinate and produce an F2, or second filial generation. To his surprise, some of the plants in the F2 generation had wrinkled seeds. The wrinkled pea trait had reappeared.

Mendel conducted enough experiments to convince himself that this did, indeed, happen every time he crossed plants with round seeds with those that had wrinkled seeds. He also figured out that every time he did that same cross, 25% of the offspring in the F2 generation had wrinkled seeds. Mendel decided to call the trait that disappeared in the F1 generation and then reappeared in the F2 generation the recessive trait.

Again, Mendel allowed the F2 plants to self-pollinate. He discovered that the F2 plants that had wrinkled seeds produced only plants with wrinkled seeds. One-third of the round-seeded plants also produced only round-seeded plants when they self-pollinated. The other two-thirds of the round-seeded plants produced both round-seeded and wrinkled-seeded plants in the same ratio that the F1 generation produced.

Many years after Mendel’s experiments, an English geneticist named Reginald Punnett (1875–1967) developed a way to visualize genetic crosses like the ones done by Mendel. The diagram is called a Punnett square, and it helps biologists determine the probability that an offspring of a genetic cross will inherit a particular trait.

In Mendel’s original experiment with round versus wrinkled pea plants, he crossed plants that always produced round seeds with those that always produced wrinkled seeds. These plants were homozygous for either round or wrinkled seeds. Mendel found out that round seeds was the dominant trait (because when he crossed round-seeded plants with wrinkled-seeded plants he always got round-seeded plants). To show that a trait is dominant in a Punnett square, geneticists use an uppercase letter. Recessive traits are often given the same letter, but it is lowercase. So, for example, round seeds would be designated by an “R,” while wrinkled peas would be designated with an “r.” Remember that each plant would have two alleles for each trait—one from each parent (even if it self-pollinated, it would get one allele from the male part of the plant and one allele from the female part). If the plant is homozygous for the dominant round peas, both alleles would carry the “R” designation. If the plant is homozygous for the recessive wrinkled peas, both alleles would be “r.”

A cross where only one trait is being considered is called a monohybrid cross. A Punnett square for a monohybrid cross has four boxes like the one shown below.

The alleles for one parent are written on the side of the Punnett square, while the other parent’s alleles are written across the top (which parent goes on top and which one goes on the side is unimportant).

To find out the possible genotypes of the offspring, bring down the “R” that belongs to the round-seeded parent from the top of the Punnett square, and bring the “r” that belongs to the wrinkled-seeded parent from the left, and place them into the upper left-hand corner box. By convention, geneticists write the capital letter first.

Now do the same thing for the remaining boxes.

As the Punnett square shows, when a homozygous round-seeded plant is crossed with a homozygous wrinkled-seeded plant, the genotype of the offspring will always be “Rr.” Because the alleles are different (one is “R” while the other is “r”), the offspring will be heterozygous. And because the allele for round seeds is dominant, the offspring will have round seeds. This represents Mendel’s F1 experimental data.

Now try the same thing for the F2 generation. Remember that in this generation, Mendel allowed the F1 offspring to self-pollinate. Because all of the offspring have a genotype of “Rr,” Mendel was crossing two “Rr” plants (even if he did not know that at the time). Because these plants are heterozygous, they make two different types of gametes (eggs and sperm, or pollen). One type of gamete contains the allele for round seeds (R) and the other one contains the allele for wrinkled seeds (r). The set-up for the Punnett square would look like this:

Fill in the Punnett square like the one above by bringing down the allele belonging to one parent from the top and bringing the other allele over from the left to fill in the boxes.

As this Punnett square shows, an offspring produced by crossing two heterozygous plants can have three possible genotypes: RR, Rr, or rr. Because round seeds are dominant, these three genotypes can result in two different phenotypes: round peas (RR or Rr) or wrinkled peas (rr). This Punnett square can help explain Mendel’s results from his F2 crosses. It shows the probability (chance) of an offspring possessing a particular genotype. There is a one in four, or 25%, chance that an offspring of this cross will have the genotype “RR.” There is a 50% chance that the offspring will have the genotype “Rr.” And there is a 25% chance that the offspring will be homozygous for the wrinkledseeded trait (their genotype is “rr”) and show wrinkled seeds.

Therefore, when Mendel allowed the wrinkled-seeded plants (rr) to self-pollinate, only plants with wrinkled seeds would be the result.

Of the plants that exhibited the round-seeded phenotype, one-third has the genotype “RR.” If these plants are allowed to self-pollinate, all of their offspring will be homozygous for the dominant roundseeded trait.

The other two-thirds of the round-seeded plants have the heterogeneous genotype “Rr,” just like the F1 generation crossed in this example. Just like the F1 generation, when these plants self-pollinate, they will produce both round-seeded and wrinkled-seeded plants in the same ratio that the F1 generation produced them in.

It took Mendel seven years to cross, observe, and record the results of thousands of plants in order to prove his ideas of how traits were passed from one generation to another. His research helped him disprove the idea of “blending,” which was popular at the time. The blending theory stated that the parents’ traits were “blended” to give the offspring an average of their traits. In other words, if one parent is tall and one is short, proponents of the blending theory would expect all the offspring of these parents to be mediumheight.

Through his research, however, Mendel was able to show that a cross between a tall pea plant (TT or Tt) and a dwarf pea plant (tt) would produce generations of plants that are either tall or dwarf. But none of the plants produced by any of these crosses were mediumtall and, therefore, were not blended.

The Punnett square shows that the ratio of offspring in a cross between a heterozygous tall plant (Tt) and a homozygous dwarf plant (tt) is 50% tall (Tt and Tt) and 50% dwarf (tt and tt).

A cross between a homozygous tall plant (TT) and a dwarf plant (tt) would lead to all tall plants with a heterozygous genotype (Tt).

But there are still no medium-sized plants—only tall and dwarf. Mendel hypothesized that rather than blending, alleles (which he called “units” or “factors”) are randomly selected to be passed on to the next generation. This hypothesis is now called Mendel’s first law, or the law of segregation.

From his research, Mendel concluded that physical traits were passed from parent to offspring through units or factors that are now called alleles. He also determined that each parent had two factors per trait. After studying the results of his experiments, Mendel observed some patterns. From these patterns, Mendel developed his basic rules of heredity:

• Traits are inherited independently of other traits.
• Some alleles are dominant, while others are recessive.
• An offspring inherits half of its alleles from each parent.
• Different offspring of the same parents get different combinations of alleles.

Punnett squares can also be used to predict the inheritance ratio of offspring that contain two different traits at the same time. This type of cross is called a dihybrid cross. Take the flowers of the pea plant, for example. Mendel discovered that purple flowers (P) are dominant over white flowers (p) and axial flowers (A), which grow in the middle of a stem, are dominant over terminal flowers (a), which grow at the end of the stem (Figure below). If Mendel crossed a pea plant that was homozygous for white terminal flowers with a pea plant that was homozygous for purple axial flowers, what would the Punnett square look like?

In one of his experiments, Mendel cross-pollinated two
true-breeding fl owers, one purple and one white. The first generation’s
offspring all displayed purple petals, but the second generation
produced some purple flowers, and some white. This led Mendel to
conclude that some traits, like the purple flower, are dominant, and
some, like the white flower, are recessive.

The genotype for the plant that is homozygous for white terminal flowers must be “ppaa,” and the genotype for the plant that is homozygous for purple axial flowers would have to be “PPAA.” The plant that produces white terminal flowers can make gametes that contain only the alleles “p” and “a,” while the one that produces the purple axial flowers can make only gametes that contain the alleles “P” and “A.”

As the Punnett square for this parent generation shows, all of the offspring of this cross would appear to have purple, axial flowers. But all of the offspring also carry the recessive white, terminal flower alleles that cannot be seen in this F1 generation. The entire F1 generation is heterozygous for the color and the flower position alleles. If the F1 generation is allowed to self-pollinate, then things get interesting. During Mendel’s experiments, he determined that two different traits were always inherited independently of one another. This idea is called Mendel’s second law, or the law of independent assortment. In this case, each plant can produce four different gametes—PA, Pa, pA, and pa. These are all of the possible combinations of the two alleles.

The Punnett square shows the following: 9 of the 16 offspring of these F2 plants will have purple, axial flowers (genotypes: PPAA, PPAa, PpAA, PpAa); 3 offspring will have purple, terminal flowers (genotypes: PPaa and Ppaa); 3 offspring will have white, axial flowers (genotypes: ppAA and ppAa); and 1 offspring will have white, terminal flowers (genotype: ppaa). Mendel realized that this ratio, 9:3:3:1, happened every time he crossed plants that were heterozygous for two independently assorting traits.

Mendel’s research was published in 1866, but was largely ignored at the time. Then, in 1868, he was promoted to abbot of the monastery where he worked. Consumed with church business, Mendel dropped his scientific pursuits. He died of a chronic kidney disease in 1884.

Thirty-four years after the publication of Mendel’s results and 16 years after his death, his research was rediscovered independently by Hugo de Vries of the Netherlands, Erich von Tschermak of Austria, and Carl Correns of Germany. Each of these men was working on different hybrid plants and determined their own laws of how traits are inherited from generation to generation. Before publishing their results, however, each man searched through the scientific literature, which is a part of the scientific publishing process. During this process, de Vries, von Tschermak, and Correns all came across Mendel’s 1866 publication stating his laws of inheritance. When each of the three men published the results of their experiments, they re-announced Mendel’s work to the scientific world and described how their work confirmed Mendel’s results.