Leah here from leah4sci.com and in this MCAT video we're going to look at Mendelian genetics
specifically as it applies to punnett squares.
Say you have a family of plants where some of the plants are tall and other ones are
short.
Some of the flowers are blue, while other ones are yellow and say these plants produce
pods where some of them have green seeds and some of them have purple.
If these plants are related and every generation that gets fertilized gives rise to another
generation, how can we understand the height of the plant, the color of the flowers, the
color of the seeds?
This is something we look at with Mendelian genetics brought to us by Gregor Mendel who
studied these exact features specifically in pea plants.
Before we go into the specifics, let's look at some vocabulary.
All of the traits that we see on a living organism is coded for in their genes.
Our genes come from two parents, the gene codes for a specific trait, but we can have
at least two alleles, if not more for any given trait.
Take a look at the two plants here.
Each plant has a gene that codes for flower color, but the first plant has an allele that
makes it present a blue color.
The second plant has an allele that makes it present a yellow color.
The alleles give us specificity, but the gene codes for that specific thing.
In this case flower color, the colors that I've drawn here tell us the phenotype of each
plant.
The phenotype is the visible expression of the gene.
The first one has a blue flower phenotype.
The second one has a yellow flower.
The first one is tall.
The second is short.
Those are phenotypes, but we don't necessarily know what the specific alleles are just by
looking at what we see because the genotype can vary and still give us the same outward
expression.
How can I have two phenotypically blue flowers but have different genotypes?
With Mendelian genetics?
We're looking at very simple characteristics.
Gregor Mendel discovered these traits before DNA was discovered or understood, so even
though he didn't call it genes, he implied that one gene is going to give us one trait.
Now we know there are different traits.
For example, how tall you are.
That depends on more than one gene, but for the sake of our example and the Mendelian
genetics, we're going to keep it very simple.
Remember, flowers also undergo fertilization where they need the genetic information from
two different parents.
We'll assume for the sake of this example that tall plants with the letter T is the
dominant trait and short plants, which will show as a little t is the recessive traits.
We'll assume that blue flowers with the letter B is a dominant trait and yellow flowers with
little b is a recessive trait.
Finally, let's assume that green seeds with the letter G is the dominant traits and purple
seeds with a lowercase g is recessive.
In simple Mendelian genetics.
When you have one allele from each parent, one of the alleles is going to be dominant.
I like to think of this as the loud allele, the one that will be heard no matter what
else is present.
The recessive allele is the quiet one.
The recessive allele is not shouting or looking to dominate over the other alleles and in
the presence of something else, something louder.
The recessive allele is hidden.
It's lost.
But if there's nothing allowed to shout over the recessive, allele, only then will you
be able to hear it.
Wait, what?
What does all that mean?
Let's take a step back.
Going back to our made up plant example.
If we have blue flowers, we know that we have some dominant alleles present, but we don't
know how many.
For example, we can have some blue flowers that have the genotype Big B, Big B.
This means a dominant allele from one parent.
A dominant allele from the other parent.
Dominant is loud, but in this case it doesn't matter which one we're listening to because
when the Genotype is big B, big B, both of them coding from blue, the phenotype is going
to be blue.
There's nothing else present.
But there could be a scenario where a blue flower has the genotype big B, little b.
Okay?
It got the blue allele from one parent, the yellow allele from the other parent.
But remember the dominant is the loud one and if the dominant is present, it doesn't
matter what else is there.
The loud one will be heard and the phenotype once again will be blue without telling us
if it's homozygous or heterozygous.
That's what we looked at here.
Homozygous and heterozygous have the ending zygous which should make you think of fertilization
and offspring.
Homo means the same and hetero means different.
In the case of zygosity, what we're looking at here, if something is homozygous, that
means it has two have the same allele.
Big B big B is homozygous dominant telling us that it's homozygous two of the same dominant
of the Lauder allele.
In the case of big B, little B, we have one of each making it heterozygous.
Something to recognize here is that the phenotype of a homozygous dominant and a heterozygote
are impossible to distinguish because they both appear blue on the outside.
The yellow flowers are the recessive color and the only way they are able to show is
when there's nothing loud present.
And that means the yellow flowers must have a genotype of little b, little b.
Little b.
Being homozygous for the recessive trait is the only way for the recessive trait to be
visible because we have a quiet allele and a quiet allele.
In the presence of nothing loud to drown them out.
That's when the quiet allele can be heard and that's when you see the recessive color
show through.
Now that we understand the vocabulary, let's see how to predict the percentages of heredity
based on different parents and offspring.
We'll learn how to do this with a Punnett square.
You can follow along with the punnett square cheat sheet on my website leah4sci.com/genetics.
Now here's the thing about the Punnett square.
Learn what they are, understand how to do it, draw some in practice, but never draw
one on the MCAT.
It takes too long and it's a waste of time.
So after I teach you how to do the Punnett squares, I'll show you a shortcut for calculating
ratios without ever having to draw another one under pressure and time constraints of
your MCAT.
We'll start with a simple Punnett square where we're looking at just one trait being passed
on.
Simple way to do it, Draw a box, cut it in half one way, cut it in half the other way.
Let's start with the most common example.
A monohybrid cross.
Mono tells us we're looking at one gene, one trait hybrid tells us we're looking at a mix
and in this case we're going to look at two heterozygotes and cross, meaning we cross
them parent A, parent B. Let's see what happens.
A quick reminder that in meiosis we create haploid gametes and the haploid gametes, meaning
the allele that is passed on to the next generation can be one of the two carried by each parent.
In this case, we're going to look at plant height where we take two phenotypically identical
heterozygotes so that the mother is big T little t and the father is also big T little
t.
Trick for remembering that this is male., just think of it as having an erection.
The female gets drawn to the left, the male gets drawn at the top.
If you mix it up, it's not the end of the world because the results will be exactly
the same.
What we do is write the gametes down one at a time, so remember if this is the mother's
genotype, it's carrying two different alleles, one for tall, one for short, but the gametes
are haploid.
They only have one copy, one allele, and so we break them up into the two gametes where
one is big T and one is little t.
same thing for the father.
This is the genotype.
It's got two copies.
We break them up, one on the left and one on the right.
The way to figure out what happens as a result of this cross is to take one allele at a time
and draw it into all the appropriate boxes as follows.
If we take big T, we draw it into all the boxes, we'll take little t and draw them into
the boxes.
Now we'll do the same thing for the top big T, draw them down into the appropriate boxes
and same thing for the little t.
Let's break down the results.
In the top left we have big T big T. that means we have one offspring that is going
to be a homozygous dominant.
For short, I'm gonna write homo down.
Top right and bottom left even though the code is drawn backwards, technically you're
supposed to put the capital letter first, but it doesn't matter.
It still tells us that each one has one upper case and one lower case.
This means two offspring are heterozygous and on the bottom right we have little t,
little t telling us that one of the offspring is homozygous recessive, which I'll right,
as homo rec, does this mean that every set of pea plants will only have four offspring?
No, no, no.
This is just a ratio.
The ratio for the genotypes in a monohybrid cross is one to two to one.
We get 50% that are heterozygous and 50% that are homozygous, but the homozygous are half
homozygous dominant and half homozygous recessive.
Genotypes can only be identified through testing either through mono hybrid, cross breeding,
or through a genetic evaluation.
What we can see with the eye is the phenotype and these are the ratios that you absolutely
have to memorize.
The phenotypes regardless of being dominant or recessive, tells us that three out of four
offspring are going to have a dominant trait.
In this case, 75% of the offspring will be tall plants and 25% of the offspring meaning
one out of four is homozygous recessive, which we can see as a short plant.
This is the ratio to remember three to one, three phenotypically dominant, one phenotypically
recessive, and if you look at them as fractions because the MCAT loves to test you in fractions
and decimals, we have three out of four dominant, which is 75% and one out of four recessive,
which is 25% . Knowing how to draw them, knowing how to work through it is great and I absolutely
want you to do this and practice for different types of parental genotypes, but on the MCAT,
you're not gonna to draw any of this on the MCAT I want you to focus only on the math
so that you can calculate results without having to waste your time drawing Punnett
squares.
So let's work through some math.
Say you have two of these plants that are crossed.
Say you're told that in a certain experiment, the scientists decided to cross a plant that
is heterozygous for the green seed color, which would be big G, little g with, and just
put an X to show what you're crossing it with.
Another plant that is homozygous recessive for the purple seed color.
Giving us little g little g and we want to know the percentage of the offspring that
will have purple seeds.
Again, we could do this with a Punnett square and why don't we do the Punnett square after
just to prove our numbers, but let's do it mathematically first.
It doesn't matter which parent we take first.
What we wanna do here is look at each parent one at a time and ask ourselves what is the
percent chance that we're going to have a certain outcome.
In this case, the percent chance that the offspring will be purple and specifically
purple is a little g. looking at the first parent, we have big G and little g. that means
that we have a one in two chance of getting a little g passed on because we have one of
the two alleles as the little g and we cross that with, we multiply it by the percent chance
that the second parent will give us a little g.
The second parent is all little g, so there's a 100% chance and remember 100% is simply
equal to one.
So we multiply that by one one over two times one or times one over one gives us one over
two which is a half or 50% now if you're looking at this and you're saying, wait, but we have
two little g out of two options, you could have done that as well because if we did one
over two for the first parent and two over two for the second parent, one times two is
two, two times two and the denominator is for two over four can be reduced by two on
each side, which actually breaks down to one over two and again, 50%.
Let's quickly prove this with a Punnett square, not because I want you to rely on it, but
rather because I want you to trust that no matter how we did the math, it was accurate
and you can trust the math moving forward.
We'll carry the big G and the little g across.
We'll carry the purple little g's down in both columns and let's take a look.
Two of the four results gave us big G, little g, which is heterozygous, but the dominant
trait, the green seeds, but the other two out of four gave us little g, little g meaning
homozygous recessive, and the two out of four is 50% as we predicted.
Let's make this even harder.
What if I asked you to run a heterozygous cross, but this time between a plant that
is heterozygous for the height gene and heterozygous for the flower gene, how would you draw that
dye hybrid cross?
And more importantly, how would you do this quickly and efficiently if you're asked about
it on your MCAT?
And that is exactly what I'll be covering in the next video.
First, if you found this video helpful, give it a thumbs up, make sure to subscribe so
you never miss out on any new videos.
And then go to my website for the genetic series along with the Punnett square cheat
sheet leah4sci.com/genetics.