These days scientists know how you inherit characteristics from your parents. They're able to calculate probabilities of having a specific trait or getting a genetic disease according to the information from the parents and the family history. But how is this possible? To understand how traits pass from one living being to its descendants, we need to go back in time to the 19th century and a man named Gregor Mendel. Mendel was an Austrian monk and biologist who loved to work with plants. By breeding the pea plants he was growing in the monastery's garden, he discovered the principles that rule heredity. In one of most classic examples, Mendel combined a purebred yellow-seeded plant with a purebred green-seeded plant, and he got only yellow seeds. He called the yellow-colored trait the dominant one, because it was expressed in all the new seeds. Then he let the new yellow-seeded hybrid plants self-fertilize. And in this second generation, he got both yellow and green seeds, which meant the green trait had been hidden by the dominant yellow. He called this hidden trait the recessive trait. From those results, Mendel inferred that each trait depends on a pair of factors, one of them coming from the mother and the other from the father. Now we know that these factors are called alleles and represent the different variations of a gene. Depending on which type of allele Mendel found in each seed, we can have what we call a homozygous pea, where both alleles are identical, and what we call a heterozygous pea, when the two alleles are different. This combination of alleles is known as genotype and its result, being yellow or green, is called phenotype. To clearly visualize how alleles are distributed amongst descendants, we can a diagram called the Punnett square. You place the different alleles on both axes and then figure out the possible combinations. Let's look at Mendel's peas, for example. Let's write the dominant yellow allele as an uppercase "Y" and the recessive green allele as a lowercase "y." The uppercase Y always overpowers his lowercase friend, so the only time you get green babies is if you have lowercase Y's. In Mendel's first generation, the yellow homozygous pea mom will give each pea kid a yellow-dominant allele, and the green homozygous pea dad will give a green-recessive allele. So all the pea kids will be yellow heterozygous. Then, in the second generation, where the two heterozygous kids marry, their babies could have any of the three possible genotypes, showing the two possible phenotypes in a three-to-one proportion. But even peas have a lot of characteristics. For example, besides being yellow or green, peas may be round or wrinkled. So we could have all these possible combinations: round yellow peas, round green peas, wrinkled yellow peas, wrinkled green peas. To calculate the proportions for each genotype and phenotype, we can use a Punnett square too. Of course, this will make it a little more complex. And lots of things are more complicated than peas, like, say, people. These days, scientists know a lot more about genetics and heredity. And there are many other ways in which some characteristics are inherited. But, it all started with Mendel and his peas.