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These days scientists know how you inherit

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characteristics from your parents.

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They are able to calculate probabilities

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of having a specific trait

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or getting a genetic disease

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according to the information they have

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from the parents and the family history.

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But how is this possible?

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To understand how traits pass

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from one living being to its descendants,

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we need to go back in time to the 19th century

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and a man named Gregor Mendel.

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Mendel was an Austrian monk and biologist

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who loved to work with plants.

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By breeding the pea plants

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he was growing in the monastery's garden,

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he discovered the principals that rule heredity.

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In one of most classic examples,

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Mendel combined a pure-bred, yellow-seeded plant

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with a pure-bred, green-seeded plant,

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and he got only yellow seeds.

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He called the yellow-colored trait the dominant one

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because it was expressed in all the new seeds.

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Then he let the new yellow-seeded hybrid plants self-fertilize.

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And in this second generation,

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he got both yellow and green seeds,

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which meant that the green trait had been hidden

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by the dominant yellow.

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He called this hidden trait the recessive trait.

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From those results, Mendel inferred

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that each trait depends on a pair of factors,

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one of them coming from the mother

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and the other from the father.

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Now we know that these factors are called alleles

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and represent the different variations of a gene.

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Depending on which type of allele

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Mendel found in each seed,

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we can have what we call a homozygous pea,

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where both alleles are identical,

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and what we call a heterozygous pea,

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when the two alleles are different.

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This combination of alleles is known as genotype

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and its result, being yellow or green,

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is called phenotype.

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To clearly visualize how alleles are distributed

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among its descendants,

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we can use a diagram called the Punnett Square.

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You just place the different alleles on both axes

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and then you figure out the possible combinations.

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Let's look at Mendel's peas, for example.

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Let's write the dominate yellow allele as an upper-case "Y"

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and the recessive green allele as a lower-case "y".

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The upper-case Y always overpowers his lower-case friend,

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so the only time you get green babies

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is if you have lower-case y's.

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In Mendel's first generation,

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the yellow, homozygous pea mom

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will give each pea kid a yellow, dominant allele,

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and the green, homozygous pea dad

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will give a green, recessive allele.

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So, all the pea kids will be yellow, heterozygous.

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Then, in the second generation,

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where the two heterozygous kids marry,

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their babies could have any of the three possible genotypes,

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showing the two possible phenotypes

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in a three-to-one proportion.

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But even peas have a lot of characteristics.

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For example, besides for being yellow or green,

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peas can be round or wrinkled,

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so we could have all these possible combinations:

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round yellow peas,

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round green peas,

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wrinkled yellow peas,

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and wrinkled green peas.

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To calculate the proportions of each genotype and phenotype,

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you can use a Pennett Square too.

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Of course, this will make it a little more complex.

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And lots of things are more complicated than peas,

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like, say, people.

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These days scientists know a lot more

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about genetics and heredity.

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And, there are many other ways

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in which some characteristics are inherited.

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But, it all started with Mendel and his peas.