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These days scientists know

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how you inherit characteristics
from your parents.

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They're able to calculate probabilities
of having a specific trait

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or getting a genetic disease

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according to the information
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
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
he was growing in the monastery's garden,

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he discovered the principles
that rule heredity.

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In one of most classic examples,

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Mendel combined
a purebred yellow-seeded plant

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with a purebred 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,
he got both yellow and green seeds,

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which meant the green trait
had been hidden 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
Mendel found in each seed,

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we can have what we call a homozygous pea,
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 amongst descendants,

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we can a diagram
called the Punnett square.

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You place the different
alleles on both axes

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and then 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 dominant yellow allele
as an uppercase "Y"

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and the recessive green allele
as a lowercase "y."

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The uppercase Y always
overpowers his lowercase friend,

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so the only time you get green babies

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is if you have lowercase Y's.

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In Mendel's first generation,
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
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
being yellow or green,

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peas may be round or wrinkled.

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So we could have all
these possible combinations:

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round yellow peas, round green peas,

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wrinkled yellow peas, wrinkled green peas.

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To calculate the proportions
for each genotype and phenotype,

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we can use a Punnett 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
about genetics and heredity.

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And there are many other ways in which
some characteristics are inherited.

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But, it all started
with Mendel and his peas.