Heredity: Crash Course Biology #9


So, I have this brother, John. You may have
heard of him. JOHN: Hi there! HANK: As it happens, John and I have the exact
same parents. JOHN: Yes, Mom and Dad Green. HANK: And since we have the same parents,
it’s to be expected that John and I would have similar physical
characteristics because the source of our DNA is exactly the
same. JOHN: Hank and I share some genes, but nobody
knew anything about chromosomes or DNA until the middle of the 20th century. And people
have been noticing that brothers tend to look alike since like, people started noticing
stuff or whatever. HANK: That was very scientific, John. JOHN: I will remind you that I am doing you a favor. Heredity: it’s basically just the passing
on of genetic traits from parents to offspring. Like John said, the study of heredity is ancient,
although the first ideas about how the goods are passed on from parents to kids
were really really really really really really wrong. For instance, the concept that people were
working with for nearly 2,000 years came from Aristotle, who suggested that:
We’re each a mixture of our parents’ traits, with the father kind of supplying the life
force to the new human and the mother supplying the building blocks to put it all
together. Aristotle also thought that semen was like
highly-purified menstrual blood, which is why we still refer
to “bloodlines” when we’re talking about heredity. Anyway, since nobody had a better idea, and
since nobody really wanted to tangle with Aristotle, for
hundreds of years everybody just assumed that our parents’
traits just sort of blended together in us: like if a black squirrel and a white squirrel
fell in love and decided to start a family together, their offspring would be gray. The first person to really start studying
and thinking about heredity in a modern way was this Austrian
monk named Gregor Mendel and Mendel demonstrated that inheritance followed particular patterns. In the mid-1800s, Mendel spent sort
of an unhealthy amount of time grubbing around in his garden with a bunch of pea plants,
and through a series of experiments, crossing the pea plants and seeing which traits got
passed on and which didn’t–he came up with a framework for understanding how traits actually
get passed from one generation to another. So, to talk about Classical Genetics, which
includes Mendel’s ideas about how traits get passed along from
parents to children, we kind of have to simplify the crap out of
genetics. I hope you don’t mind. So we’ve all got chromosomes, which are
the form that our DNA takes in order to get passed on from parent
to child. Human cells have 23 pairs of chromosomes.
Now a gene is a section of DNA in a specific location on a
chromosome that contains information that determines a trait. Of course, the vast majority of the time,
a physical trait is a reflection of a bunch of different genes working
together, which makes this all very confusing, and when this happens it’s called a polygenic
trait. Polygenic: many genes. And then again, sometimes a single gene can
influence how multiple traits are going to be expressed;
these genes are called pleiotropic. However, some very few, but some single traits are decided by a single gene.
Like the color of pea flowers for example, which is
what Mendel studied when he discovered all of this stuff, and when that happens,
in Mendel’s honor, we call it a Mendelian trait. There are a couple of examples of Mendelian
traits in humans, one of them being the relative wetness or dryness of your ear wax. So, there is just one gene that determines
the consistency of your earwax, and that gene is located at the very
same spot on each person’s chromosome. Right here! Chromosome 16. However, there’s one version of this gene,
or allele, that says the wax is going to be wet, and there’s another
allele that says the wax is going to be dry. You may be asking yourself what the difference
is between these two things and I’m glad you asked because we actually know the answer
to that question. Among the many amino acids that make up this
particular gene sequence, there is one exact slot where
they’re different. If the amino acid is glycine in that slot, you’re
gonna have wet ear wax. But if it’s arginine, it’s dry. Now comes the question of how you get what
you get from your parents. In most animals, basically any cell
in the body that isn’t a sperm or an egg — these are called somatic cells — are diploid,
meaning there are two sets of chromosomes, one inherited from
each of your parents. So you get one earwax-determining allele from your mom and one from your dad. I should mention that the reason for this
is that gametes, or sex cells–Senor Sperm and Madame Egg–are haploid cells, meaning
they only have one set of chromosomes. Again, for emphasis, non-sex cells are called
somatic cells and they are diploid. Sex cells are gametes and they are haploid. This makes a lot of sense because a sperm
or an egg has a very specific motivation: they’re seriously hoping to score, and if
they do, they plan to join with a complementary haploid cell that has the other pair of chromosomes
they’re going to need to make a new human, or buffalo or squid or whatever. Also, just so you know, some plants have polyploid
cells, which means they have more than two sets of chromosomes in each
cell, which isn’t better or anything–it’s just how they do. But
anyway, the point of all that is that we inherit one version of the
earwax gene from each of our parents. So, back to earwax! So, let’s just say your mom gives you a
wet earwax allele and your dad gives you a dry earwax allele. Good Lord, your dad has horribly ugly ears! Anyway, since your parents have two alleles,
each for one gene inherited from each of their parents, the one passed along to you is entirely
random. So, a lot of what Mendel discovered is that
when there are two alleles that decide the outcome of a specific
trait, one of these alleles could be dominant and the other one
recessive. Dominance is the relationship between alleles
in which one allele masks or totally suppresses the expression
of another allele. So, back to earwax, because I know we all
love talking about it so much. It turns out that Mom’s wet earwax allele
is dominant, which is why she gets a BIG W, and Dad’s dry earwax allele is recessive,
which is why he has to be a little w. JOHN: Go, Mom! HANK: Oh, you’re back! JOHN: Yeah! You sound surprised. HANK: Anyway, Mom’s allele is dominant,
and that settles it, right– we’re gonna have wet earwax? JOHN: Uh, something about the way that you
said that tells me it’s not that easy. HANK: Aw, you are so much smarter than you
look. It is indeed not that easy. So, just because an allele is recessive doesn’t
mean it’s less common in all your genetic material than
the dominant allele. Which leads us to the assumption, the CORRECT
assumption, that there’s something else going on here. JOHN: I’m definitely getting that vibe from
you. HANK: So, it has to do with Mom and Dad’s
parents. Because everybody inherits two alleles from their
parents. Mom got one from Nanny and one from Paw Paw. And let’s just say Mom got a little
w from Nanny and a big W allele from Paw Paw. That means Mom’s genotype, or genetic makeup
when it comes to that single trait, is heterozygous, which means she inherited two different versions
of the same gene from each of her parents. Dad, on the other hand is a homozygote. JOHN: Let me guess, that means that he had
two of the same allele, either a little w or a Big W
allele inherited from both Grandma and Grandpa. HANK: Right! And in order for this to all
work out the way that I want it to, let’s just say that both Grandma and Grandpa would
have passed little w’s down to Dad, making his genotype homozygous recessive for this
gene. JOHN: Okay, so I’m keeping score in my head
right now. And according to my brain, Mom is a Big W, little
w and Dad is a little w, little w. HANK: And now we’re going to figure out
what our earwax phenotype is. And phenotype is what’s
expressed physically, or in this case, what you’d see if you looked
into our ears. JOHN: Alright, so are we gonna do a Punnett
Square or anything? This is why I do history, if we’re
going to do Punnett Squares, I’m leaving! HANK: But I was just going to start to talk
about people again. So Reginald C. Punnett, who was a total Gregor Mendel
fanboy, invented the Punnett Square as a way to diagram the
outcome of a particular cross breeding experiment. A really simple one looks like this: So, let’s put Mom on the side here and give
her a Big W and a little w. And let’s put Dad on the top,
and he gets two little w’s. So if you fill this in, it looks like there’s
a 50/50 chance that any child of this mating will be homozygous or heterozygous. And as for our phenotype, it shakes out the
same way: John and I both have a 50% chance of having wet ear
wax and a 50% chance of having dry ear wax. So I just had to go and call John, because
now he’s not participating because he doesn’t like Punnett Sauares, and
it turns out, that he has wet ear wax. I also have wet ear wax. Which, you know, is not
that unlikely, considering that our parents were homozygous and heterozygous. This may explain the odor of our bathroom
growing up because it turns out there’s a correlation between wet ear wax and body odor,
because ear wax and armpit sweat are produced by the same type of gland. Because this one gene has an effect on multiple
traits or phenotypes, it’s an example of a pleiotropic gene, because the gene affects
how wet your ear wax is, and how much you stink. One more thing you might find interesting:
sex-linked inheritance. So we’ve got 23 chromosomes:
22 pairs are autosomes, or non-sex chromosomes, and 1 pair
the 23rd pair, to be exact–is a sex chromosome. At that
23rd pair, women have two full length chromosomes, or “XX,” and men have one X chromosome
(that they inherited from their Mom) and this one little, short, puny, shriveled chromosome
that we call “Y,” which is why men are “XY.” So, certain genetic traits are linked to a
person’s sex and are passed on through the sex chromosomes. Since
dudes don’t have two full chromosomes on pair 23, there may
be recessive alleles on the X that they inherited from their mom
that will get expressed, since there’s not any information on the
Y chromosome to provide the possibility for a dominant allele counteracting
that specific trait. Take, for instance, balding. Women rarely
go bald in their youth like some men do because it is caused by a
recessive allele located in a gene on the X chromosome. So
it’s rare that women get 2 recessive alleles. But men need just
one recessive allele and, Doh! Baldy bald! And that allele is on their X chromosome,
which they got from Mom. But was Mom bald? Probably not. And where did Mom get that allele on her X chromosome? Either from
her Dad or her Mom. So if you’re bald, you can go ahead and blame
it on your maternal grandmother, or your maternal-maternal great-grandfather or your maternal-maternal-maternal
great-great grandfather who probably went bald before he was 30. So, Genetics, you guys. Resistance is futile. Thanks to my brother John for sharing his
personal genetic information with us, and also his face and
voice and all that stuff. That was very nice. Think of us next time you swab out your ears!
Actually they say that you really shouldn’t do that because we have earwax for a reason,
and you might poke your brain or something. Okay, that’s the last time I’m mentioning
earwax. Review! Click on any of these things to go
back to that section of the video. If you have any questions, please ask them in the
comments.

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