>> Hello, and welcome to the "Penguin Prof Channel."
Today I want to talk about heredity and sex and Mendel,
peas, cats, and so much more.
Probably everybody has looked at their family members, and, oh,
gosh, actually asked a lot of questions,
but wondered why it is that we look like our parents
but not exactly, and where all
of these family resemblances come from
and how they get passed down.
We refer to that as, as heredity,
and that the scientific study of heredity is called genetics,
and questions about this date way, way back.
I'm just going to mention a couple
of the big thinkers on this topic.
Hippocrates.
Of course, he was a big thinker about a lot of topics.
He came to believe that particles came from all parts
of the body, and they were incorporated
into the eggs and sperm.
He called these particles pangenes.
Aristotle was really not much in favor of pangenes.
He really thought it was all in the blood.
He believed that semen actually was a purified form of blood,
and he believed that semen would mix
with the mother's menstrual blood in the uterus,
and that that would give rise to the offspring
and all of its traits.
Now, you might think that's really strange thing
to think about, it's very blood centered, but, actually,
his beliefs persevere in a way, and we use the term bloodlines.
We use them especially when we're talking
about thoroughbreds and breeding dogs and horses and things
like that, but I just, I think that's kind of cool.
In the 19th century, it became popular to believe
that we were a blended mixture of our parents.
You can see where that would have some appeal.
I mean, you kind of do look like a blended version
of your relatives, but it turns out that
that really doesn't make any sense because, I mean,
if you have ever blended paint,
you know that once you blend two colors together,
you can't get those original colors back out,
and that's certainly not the same with inherited traits.
In 1866, a monk by the name of Gregor Mendel
from Austria had been playing with peas in the abbey garden,
and he was a really important person in the history
of genetics, as I'm sure you know,
because he actually applied mathematical principles
to gardening, something that really had not been done before.
He was very observant, and he took really good notes,
and he argued, correctly as it turns out, that parents pass
on discrete, what he called heritable factors.
In other words, they, these things are not blended
from the parents into the offspring,
but they retain their individuality generation
after generation.
These heritable factors later became known as,
interestingly enough, back to Hippocrates, pangenes in 1889
by a botanist, Dutch botanist actually, Hugo de Vries.
The term pangenes was later shortened to genes, which,
of course, is the term that we use today.
Mendel defined a character as a heritable feature
that varies among individuals, and in, in pea plant terms,
he actually studied seven different characters.
Things like flower color, plant height, the,
the skin of the seeds, whether the flower position was axial
or terminal.
The trait is a variant for a character.
So, for example, if you're looking at flower color,
your trait could be either a purple trait or a white trait.
If you're looking at the character of plant height,
the trait could be tall or short.
He also defined true breeding varieties.
Now this is why working with plants is so great
because plants, you can either control the breeding yourself
by taking pollen with a paintbrush,
or you can actually allow many plants to self-fertilize,
and if you do that,
true breeding varieties will always produce offspring
which are identical to the parent.
If you take two different varieties,
and you cross fertilize them, we call that a hybrid.
Finally, true breeding parents,
we're going to call the p generation, p is for parental.
The hybrid offspring are called the f1's.
This is the first filial generation.
This comes from the Latin which, filialus, which means son,
if you cross the f1's, you get f2's,
and then you could cross those and get f3's and so forth.
So what Mendel did was he was able to manipulate
and control the breeding of these pea plants,
it's actually pretty simple to do,
and he could use a paintbrush to do the crosses that he wanted,
or he could simply cover, since these are perfect flowers,
they have both male and female parts,
he could actually prevent cross fertilization.
If you cover these with a material he actually used,
it's like a little bag basically,
they will self-fertilize, and so they're really, really handy to,
to use for genetic experiments.
After fertilization, the peas mature,
and then you can plant them,
and then you can look and see what you get.
Mendel came up with four hypotheses, and I'm going
to describe them using modern terminology.
This, by the way, is the abbey where his work was done.
The first hypothesis states
that alleles are alternative forms of a gene.
So an allele could be, for example, in,
in the plant height character, we can have an allele
for tall plants, and we could have an allele for short plants.
So we can use capital T and lower case t,
respectively, to symbolize that.
Now, for each characteristic, you're going
to inherit one allele or one different form
of the gene from each parent.
So you're going to get two alleles for any given gene,
and those alleles maybe the same,
they may say the same thing.
They may be two big T's or two little t's,
or they could be different.
We call that heterozygous, one of each,
and those terms will be very important to us as we go along.
The third hypothesis says that if the alleles are different.
So you're talking about now only heterozygous individuals,
one is visible, and the other is not.
So we refer to the, the expressed trait
as being dominant, and the unexpressed, it's still there,
but it's hidden, we call that recessive.
So these are the characters that Mendel looked
at with his pea plants, and what you notice is
that for each, there are two traits.
So, for example, plant height, they came tall and short.
If you have a heterozygous individual that has one allele
for tall and one allele for short,
what it will actually express is tall,
but it will carry the allele for short.
In future generations, you'll actually see
that short allele come back out again.
Finally, the fourth hypothesis, which is so important
that we actually refer to it as law, states that a sperm
or egg cell carries only one allele
for each inherited character because alleles separate
from each other during gamete formation.
Now, that's kind of a lot of, of terminology,
but this is actually a very simple idea, but did,
did you catch what that really means?
I want to give a little example here what I mean
by that using something
that most people are just intuitively familiar with,
and that is, well, we're going to talk about sex, of course.
We're actually going to talk
about gender determination in humans.
So a great example of the law of segregation is the production
of eggs and sperm in the determination of gender.
Most people know that males have the sex chromosomes x and y
and females are x, x. So when a male produces sperm,
fifty percent of his sperm, approximately,
carry the x chromosome and those would fertilize with the egg
to make a girl, and fifty percent carry the y chromosome.
Now, for females, half of the eggs carry one x and half
of the eggs carry the other.
Of course, they end up looking the same, but the point is
that an xy male only sends forward either the x or the y
to the next generation in his sperm.
Females send forward only of the two x's, not both.
OK. So across between two individuals,
differing in a single character we call a monohybrid cross,
which should make sense to you because mono means one,
but I don't know about you.
I'm a little sick of peas.
I'd rather talk about cats because, you know,
you've got to have cute cats in order
to have a popular YouTube video.
So here we go.
So if you take an individual that is true breeding
for brown fur, we're going to call it big B, big B,
and you cross it with a cat that is true breeding for white fur,
we're going to call it little b, little b,
all of the kittens will have brown fur,
and they will be heterozygous.
They will have one big B allele and one little b allele.
And the monohybrid cross describes what you get
when you cross two hybrid cats.
So two cats, each big B, little b. So the way to do this is
with a [inaudible] square, your friend the [inaudible] square.
So the [inaudible] square helps you to keep all
of your alleles straight.
So, for example, in this square, the two big B's,
you get one big B from this cat and one big B from this cat.
This individual, you get the big b from this guy
and the little b from that guy.
Hopefully, you, you can see how that works.
And what that looks like in terms of actual color fur,
you get one homozygous dominant brown fur cat, you're going
to get two heterozygous, still brown fur cats,
but that white color is going to come out,
the white that you didn't see in the f1's, you didn't see it
in either of these guys, comes out in the f2's.
So you get the white back
when you have a homozygous recessive individual.
And you get a fairly famous ratio here.
The phenotype is the trait as it is expressed that you can see,
and that ratio we call three to one in a monohybrid cross.
In other words, you have three individuals expressing the
dominant phenotype, in this case, brown fur,
and one individual expressing the recessive, in this case,
the white fur phenotype.
The ratio of genotypes is one to two to one.
One homozygous dominant, two heterozygous individuals,
one homozygous recessive individual.
OK. Dihybrid crosses, when you take two varieties, and you try
and handle them at the same time.
So it can get a little messy.
Mendel wanted to know if the inheritance
of one trait influenced the inheritance of another.
So, for example, does being green instead
of yellow give rise to more round seeds instead of wrinkled.
Are they linked in some way?
The answer is no.
The gene responsible for seed color is independent
to the gene that's responsible for seed shape.
So these observations gave rise to Mendel's second law,
which is the law of independent assortment.
It states that the inheritance of one character has no effect
on the inheritance of another.
Basically it says that a dihybrid cross is equivalent
to two monohybrid crosses.
It's like saying flipping a coin
and getting heads does not affect rolling a die
and getting the number two.
They're, they're totally independent events.
So we're going to look at cats again, and we're going
to look at two traits now.
This little kitty is true breeding for white fur
and a short tail, and this kitty is true breeding for brown fur
and a long tail, and we're going to cross those together,
and you're going to get individuals
that are all heterozygous for both traits.
So all of these kitties have brown fur and short tails,
and in a dihybrid cross, we're going to cross two
of these f1's together to get an f2,
and because you have two traits that you're worried about,
you're going to end up with a fairly exhaustive looking table
like this, and my students always ask me can you do the
dihybrid cross for us, and, sure, why not, but we'll,
we'll speed it up a little bit.
[ Music ]
OK. This music is not available for download on iTunes.
What you see in the dihybrid cross is the classic nine
to three to three to one ratio.
What you're going to get are nine cats
with brown fur and the short tail.
The, these are the individuals that inherit both
of the dominant traits.
You're going to get three cats that have the white fur
and the short tail, you're going to get three cats
that have brown fur and the long tail,
and only one individual is going to have both recessive traits -
white fur and long tail.
So there you go.
So, unfortunately, a lot
of Mendelian genetics is taught incorrectly by professors,
and you still see incorrect stuff in textbooks.
We're very beholden to these great Mendelian trait stories
because there are so many of them that are really fun,
and you can do them in class.
Unfortunately, most common visible human traits are not
inherited in Mendelian fashion.
These are myths.
And I have to admit that I was taught that some
of these were true, and I, myself,
when I started teaching would do some of these in class.
They are fun.
Unfortunately, none of these are inherited in Mendelian fashion.
I know, I know.
The cleft chin, the hair color, the, the toe length,
the widow's peak, this refers to the, the hairline.
This refers to the expression of pigments when you eat beets.
Oh, I should probably explain this one.
This has to do with the excretion
of a really foul-smelling material
by individuals who eat asparagus.
None of these are inherited in Mendelian fashion.
Usually, the, the trait doesn't even fall
into two distinct categories
like the hitchhiker's thumb is taught as a, a Mendelian trait,
but, actually, the range of, of the angle
of the thumb is, is continuous.
Most thumbs are actually somewhere in the middle.
Some of these traits aren't even genetic
at all, like arm crossing.
An extensive study was done in 1932, but no one wants
to believe it because it's really fun to do in class.
Tongue rolling is one that I believed in.
It is a character, not a trait.
So, in other words, there are many genes that are responsible.
The truth is out there.
You can get current information
about Mendelian inheritance in humans.
This is put out by John Hopkins University.
If you just Google OMIM, you will get the information there.
As always, I hope this information was helpful.
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