Video Transcript
In this video, we’ll learn how to
use genetic diagrams to make predictions about which traits may show up in
individuals from one generation to the next. These predictions are generally
based on the genetic makeup of family members, which can sometimes be inferred from
their physical traits or phenotype. And we’ll also apply our
understanding of genetic inheritance to predict an individual’s sex.
What do you need to make an
organism, like this sloth? Clearly, any sloth needs matter and
energy, but there must be some sort of information that organizes the matter and
energy into a sloth. And to make one particular sloth,
you need the particular information for that sloth. Well, you’ve probably already
learned that this information is genetic information. And it’s based on molecules called
DNA that coil up into structures called chromosomes.
Chromosomes from sexually
reproducing organisms come in pairs, since one comes from each parent. Only one chromosome of each pair is
sorted into an individual’s gametes, which can be either eggs or sperm, that combine
in the process of fertilization, forming the baby sloth’s first cell. And the traits of this new
individual will be based on the genetic information in the chromosomes from the
parents.
Next, let’s review how genetic
information is stored in chromosomes. We’ll use these hedgehogs to
connect the genetic information in their chromosome to their physical traits, such
as prickliness. Each of these chromosomes is made
out of one very long and very coiled molecule of DNA. But diagrams of chromosomes are
often X-shaped, and you may already know the reason for this. X-shaped chromosomes have been
replicated, which means they contain two identical molecules of DNA. And like other sexually reproducing
organisms, hedgehog chromosomes come in pairs, since one of each pair comes from
each parent.
Hedgehogs have 24 pairs of
chromosomes, but we’ll only look at one pair from each hedgehog to keep things
simple. Genes are sections of DNA that code
for proteins that affect physical characteristics. And we’ll look at four
characteristics of hedgehogs here: prickliness, face color, forehead furrow, and
quill color. Here are the locations of four
genes that determine these characteristics. Even though these traits are more
complicated than single gene traits, for simplicity let’s imagine it is for now.
Then alleles, alleles are versions
of a gene. So the difference between alleles
of the same gene causes differences in characteristics that we call traits. So the gene that codes or contains
genetic information for how prickly a hedgehog will be has two alleles here. One code is for very prickly
hedgehogs, and it’s represented by capital P. The other code is for just prickly
hedgehogs, and it’s represented by lowercase p.
Now let’s find out which alleles
our hedgehogs have for this gene. We can keep track of the hedgehogs
by their names: Barb and Lance. Lance inherited the very prickly,
or capital P, allele from his mother and the prickly, or lowercase p, allele from
his father. So which letters go in the two
remaining prickliness gene locations on Lance’s chromosomes? Since the chromosomes of each
parent contain two identical DNA molecules, their alleles are also identical. Barb inherited a copy of the
prickly, or lowercase p, allele from each parent. So her normal body cells with
replicated chromosomes contain four copies of the prickly allele.
This information gives us the
genotype for each hedgehog. The alleles from each parent are
written as pairs of letters. And we just write one allele from
each chromosome, since they have identical alleles.
Let’s compare the hedgehogs to
their genotype. The physical expression of a trait
is called its phenotype. So Lance’s is very prickly, while
Barb’s is prickly. Why the difference? Because the very prickly allele is
what we call dominant over the prickly allele, meaning that the physical expression
of the very prickly allele shows up or is expressed in Lance because he has at least
one of these alleles. But Barb has two copies of the
recessive, or prickly, allele, so she expresses the prickly trait. Capital letters indicate dominant
alleles, while lowercase letters identify recessive ones.
All the possible traits and their
letter symbol are listed at the bottom of the screen now, as well as the alleles
present on each chromosome. Take a minute to determine the
genotypes that belong on the pairs of blanks labeled one through six for each of the
three remaining characteristics: face color, forehead droop, and quill color.
Not sure? Well, recall that each genotype
should contain two letters, one for each of the alleles inherited from the
parents. So Lance’s genotype needs to
contain a capital F from his mother and a lowercase f from his father. So Lance’s genotype is capital F
lowercase f. Go ahead and figure out the
rest. The answers will be up in a
second.
Now that we’ve listed all the
genotypes of the hedgehogs, let’s determine their phenotype. Phenotype is the physical
expression of a trait, and it’s determined by an organism’s genotype. We’d already said that Lance’s
phenotype is very prickly because he has at least one dominant allele. And that’s all you need for its
expression. And we had noted that Barb’s
phenotype was prickly because she has two copies of the recessive allele that codes
for prickly.
Let’s determine Lance’s phenotype
for face color. Capital F stands for the dominant
allele dark face, while lowercase f stands for the recessive allele of light
face. Lance has one dominant allele and
one recessive allele. But you only need one copy of a
dominant allele in order for it to be expressed. So Lance’s phenotype for face color
is dark face, and we can see that on Lance as well.
You may wanna pause the video for a
minute and see if you can figure out the rest of the phenotypes for blanks seven
through 11, but we’ll list the rest of them up here in just a second. Lance has at least one dominant
allele for each of these characteristics. So Lance expresses all of the
dominant characteristics, which are very prickly, dark face, forehead furrow has a
big droop, and he’s a black-brown color. Barb has two recessive alleles for
each characteristic. So she expresses the recessive
traits of being prickly, having a light face, a forehead furrow with a normal droop,
and she’s a cinnacot color.
Now we should be ready to apply
this knowledge to predicting the inheritance of alleles from one generation to the
next. So now, using what we already know
about Lance and Barb, we can make predictions about what any of their hoglets will
look like using a diagram called a Punnett square. That doesn’t mean that they’re
necessarily gonna have any hoglets at all though. We’re just gonna look at what’s
possible based on their genetic information.
We already know their genotypes and
phenotypes for the prickliness characteristic. But to predict the possible
hoglets, we also need to know the genotypes of the gametes, or eggs and sperm. Chromosome pairs are split up in
this process, which is called meiosis. So the gametes only receive one
allele for each characteristic, instead of two as in regular body cells. That means that Lance can make
sperm with either a dominant capital P allele or with a lowercase recessive allele,
while Barb can only make eggs with the lowercase, or recessive, allele for the
prickliness trait. So the first step in filling out a
Punnett square is to list the genotypes of the gametes on the outside.
The process of fertilization is
modeled by copying the genotype information into the inner cells, which represent
possible offspring. We’ll begin by dropping the alleles
from the eggs into the cells below. And the alleles from the sperm on
the outer left of the Punnett square are pulled across into the inner cells toward
the right. So these are three simple steps for
completing a Punnett square, but there’s also a couple of things to know that are
not important. You probably already noticed that
you can do step two and step three in whichever order you want; it really doesn’t
matter. And it doesn’t matter which parent
is on the top of the square and which parent is on the side.
Now that the Punnett square is
completed, we can analyze the information to make predictions about the genetic
inheritance in the offspring. Looking inside the Punnett square
at the genotypes of possible offspring, there are two capital P lowercase p
genotypes and two lowercase p lowercase p genotypes.
Before continuing our analysis,
let’s learn some important vocabulary terms that will help us cut down on saying the
letters of the alleles over and over again. The first of these is heterozygous,
which means there’s both a dominant and a recessive allele in the genotype. Our example here would be capital P
lowercase p. While the term homozygous means
having two identical alleles in a genotype. In our example here, a lowercase p
lowercase p would be a homozygous recessive genotype. Or it’s also possible to have a
homozygous dominant genotype.
There are four small inner squares
in the Punnett square. Two out of four of these small
inner squares have a heterozygous genotype, and the other two of the four squares
have a homozygous recessive genotype. We can also write these as
fractions, two over four, which simplify to one-half. We can also write these as
percents. One divided by two equals 0.5 times
100 percent equals 50 percent. This means that any hoglet that
might be born to this couple will have a 50 percent chance of being heterozygous and
very prickly or homozygous recessive and just prickly.
But now let’s say that Barb is very
prickly, and she has the same genotype as Lance, heterozygous. Now she can make eggs that either
have the capital P allele or lowercase. And we can drop this new capital P
into the cells below to show the possible offspring. So there’s a one-out-of-four chance
of being homozygous dominant, a two-out-of-four chance of being heterozygous, and a
one-out-of-four chance of being homozygous recessive. We can write these as fractions and
convert to percents by dividing the numerator by the denominator and multiplying by
100 percent.
So, if this couple has any hoglets,
each one will have a 25 percent chance of being homozygous dominant, a 50 percent
chance of being heterozygous like the parents, and a 25 percent chance of being
homozygous recessive. Since both the homozygous dominant
and the heterozygous genotype lead to the very prickly phenotype, each hoglet will
have a 75 percent chance of being very prickly and a 25 percent chance of being
prickly.
Next, we’ll use a Punnett square to
predict an individual’s sex. We’ll use humans as our example
here. Humans have 23 pairs of
chromosomes. The first 22 are identified by
number, but the 23rd pair are called sex chromosomes because they contain genes that
determine a person’s sex. Recall that each pair of
chromosomes contains one chromosome from each parent. Here, the pink ones are from the
mom and the yellow from the dad. The sex of the owner of these
chromosomes depends on the combination of sex chromosomes they inherit. Everyone needs at least one X
chromosome, and people with two X sex chromosomes are female, while those with both
an X and a smaller Y chromosome are male.
To predict a child’s sex using a
Punnett square, we need to determine which sex chromosomes will be in the parents’
gametes. And we can figure that out from
what we know about the parents’ sex chromosomes. Moms need to have two X chromosomes
to be moms, so they can only put X chromosomes in their eggs, while dads have to
have an X and a Y chromosome, so they can put either an X or a Y chromosome in their
sperm.
Next, we’ll model the process of
fertilization by transferring the sex chromosomes from each parent into the inner
cells of the Punnett square, forming the possible offspring genotypes. So each child born has a
two-out-of-four chance of having two X sex chromosomes, making them female. And each child born will also have
a two-out-of-four chance of having an X and a Y chromosome, making them male. And that’s the same thing as saying
that each child has a one-half or 50 percent chance of being either sex. That’s not to say that every couple
to have four children will have two daughters and two sons. These are predictions based on what
we know about genetic inheritance, but they don’t always play out exactly in real
life.
One more thing, in humans and many
other animals, the sex of the offspring is determined by the father. But in other sexually reproducing
organisms, the sex of the offspring is determined by the female, or even sometimes
by the temperature of where the eggs have been laid.
Key points from this video include
these vocabulary terms and how to predict genetic inheritance using a Punnett
square. Step one, write the gamete
genotypes on the outside of the Punnett square. Alleles from the gamete on the top
of the Punnett square are transferred to the inner cells below. And the alleles from the gamete on
the left are pulled across towards the right. That means for each turtle produced
by this couple, one-half or 50 percent will have the genotype capital G lowercase g
and have the green phenotype, while the other half will have the yellow phenotype
because their genotype will be lowercase g lowercase g.
