characteristics from your parents.

They are able to calculate probabilities

of having a specific trait

or getting a genetic disease

according to the information they have

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 principals that rule heredity.

In one of most classic examples,

Mendel combined a pure-bred, yellow-seeded plant

with a pure-bred, 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 that 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

among its descendants,

we can use a diagram called the Punnett Square.

You just place the different alleles on both axes

and then you figure out the possible combinations.

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

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

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

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

so the only time you get green babies

is if you have lower-case 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 for being yellow or green,

peas can be round or wrinkled,

so we could have all these possible combinations:

round yellow peas,

round green peas,

wrinkled yellow peas,

and wrinkled green peas.

To calculate the proportions of each genotype and phenotype,

you can use a Pennett 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.