How Mendel's pea plants helped us understand genetics - Hortensia Jiménez Díaz

View full lesson: http://ed.ted.com/lessons/how-mendel-s-pea-plants-helped-us-understand-genetics-hortensia-jimenez-diaz

Each father and mother pass down traits to their children, who inherit combinations of their dominant or recessive alleles. But how do we know so much about genetics today? Hortensia Jiménez Díaz explains how studying pea plants revealed why you may have blue eyes.

Lesson by Hortensia Jiménez Díaz, animation by Cinematic Sweden.

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.