Video Transcript
In this video, we’ll learn how to
define and explain key terms in genetics. And this knowledge will help you to
understand and communicate important concepts of genetic inheritance.
What is genetic inheritance? Well, traits like the texture of
our hair or the shape of a bird’s beak usually have at least some basis in our
DNA. And DNA stores information about
our traits in sections called genes. So, genetic inheritance is all
about how we inherit our particular DNA and where it came from. Here’s a simplified cell from our
character Dan that we’ll use to investigate the origins of his curly hair. And here’s an extra big nucleus so
we can see his DNA a little better.
DNA coils around special proteins
to form structures called chromosomes. Each chromosome contains only one
DNA molecule or two identical DNA molecules. And Dan, like most humans, has 46
chromosomes. But so far, I only have 23
chromosomes in this nucleus because these are the 23 that Dan inherited from his
mother. Dan also inherited 23 similar but
not completely identical chromosomes from his father, shown here in green. And let’s say that the gene that
controls hair texture is located on Dan’s chromosome pair number one. Even though hair texture is a lot
more complicated than just being a single gene trait, for simplicity, let’s imagine
it is for now. The funny thing is that both Dan’s
parents have straight hair, while Dan has wildly curly hair. What if Dan has children? Will any of them get Dan’s curly
hair? Let’s find out.
When Dan produces sperm, each sperm
will contain only one chromosome from each of his 23 pairs. And you can see that Dan will pass
on some of the yellow chromosomes he inherited from his mom and some of the green
chromosomes from his dad. Here’s Dan’s wife, Chloe. She has straight hair, and so do
her parents. This is one of Chloe’s normal body
cells. Again, it shows the chromosomes
from her parents. In this case, the blue are from her
father, and the pink are from her mother. The location of genes on
chromosomes is the same for members of the same species. So, Chloe’s hair texture gene is at
the same location as Dan’s. And just like Dan, when Chloe
produces eggs, each receives only one randomly selected chromosome from each pair
again, some from her mother and some from her father.
Now, if this sperm fertilizes this
egg, we can see where the resulting baby’s chromosomes come from. Some come from Dan’s mother, some
from Dan’s father, some from Chloe’s mother, and the rest from Chloe’s father. In other words, the new baby here,
Destiny, who will be growing straight hair is like the rest of us, the expression of
merged genetic inheritance from four grandparents. And they were the product of
countless generations that continuously shuffled and merged DNA for billions of
years.
So, how does that help us
understand how Dan ended up with curly hair? Well, Destiny’s chromosome pair
number one both came from her grandmothers, and both her grandmothers’ hair were
straight. So, should we draw out all the
chromosomes of Destiny’s grandparents next? No, definitely not because it’s
easier and less cumbersome to use some vocabulary terms to represent all those
merging and shuffling and divvying up of chromosomes. So, next, we’ll continue with our
investigation, but we’re gonna use more vocabulary and fewer diagrams.
We already know that a gene is a
section of DNA that generally codes for a protein and can affect our traits. We also know that that DNA coils up
into structures called chromosomes and that according to our scenario, Dan’s hair is
curly because of the hair texture gene on chromosome number one. We also know that the genetic
information in the chromosomes of Dan’s parents or anybody’s parents is similar but
not identical, since parents are not identical to each other. So, the hair texture gene can vary
allowing for different textures of hair such as straight or curly.
Different versions of the same gene
are called alleles, and letters are used to represent them. Let’s let the letter capital S
represent the straight hair allele, and we’ll let the letter lowercase s represent
curly hair. These two terms, gene and allele,
parallel the next two terms, characteristic and trait. A characteristic is a common
feature among the members of a species, and our example here is hair texture. So, in our example, hair texture is
a characteristic, and hair texture is a gene. And traits since they’re versions
of a characteristic, like the straight or curly hair in our example, are based on
the combination of alleles an individual inherits from their parents. So, genes are to characteristics as
alleles are to traits.
Next, let’s take a look at dominant
and recessive alleles. Here, we have chromosome pair
number one with the hair texture gene we’ve been talking about in our scenario and
the two alleles for straight or curly hair texture and the letters capital S and
lowercase s that we’re using to represent them. If a person inherits two capital S
alleles, they’ll have straight hair. And their hair will still be
straight even if they only inherit just one capital S allele. But inheriting a lowercase s allele
from each parent results in having the trait of curly hair. It’s easy to see that having two
capital S alleles is gonna lead to straight hair and that two lowercase s alleles is
gonna result in curly hair.
But why would a person with a
capital S and a lowercase s have straight hair? Although the details can vary, the
expression of some alleles can mask or cover up the expression of other alleles. In this case here, a straight hair
allele will cover up a curly hair allele. So, we say the straight hair allele
is dominant over the curly hair recessive allele. And we use capital letters to
represent dominant alleles and lowercase letters to represent recessive ones.
Next, we’ll take a look at another
set of terms, genotype and phenotype. The term phenotype refers to the
physical characteristics of an organism. And in our example, these
characteristics can be either straight hair or it can be curly hair, while the term
genotype refers to the genetic makeup of an organism and often consists of a pair of
letters that represent the alleles of a gene. In our example, the genotypes that
result in straight hair are capital S capital S or capital S lowercase s. And the only genotype that results
in curly hair are two lowercase s’s.
Another set of terms can help
replace all the repetitive talk about capital letter this and lowercase letter
that. And these terms are homozygous and
heterozygous. Homozygous means having two of the
same alleles for a gene. So, you can have two capital
letters for a genotype and be homozygous dominant. Or you can have two lowercase
letters at a gene and be homozygous recessive. The term heterozygous, on the other
hand, means having two different alleles for a gene such as the capital S lowercase
s in our example.
Well, okay, we’ve got our key
genetics terms that we were gonna cover all covered. So, let’s put them to use and
finally figure out how Dan inherited his curly hair.
Here’s Dan’s family. And they all have straight hair
except for Dan. So, let’s use what we’ve learned to
make sense of this. Here’s chromosome pair number one
and the location of the hair texture gene, which determines a person’s hair
texture. This gene comes in two different
versions or alleles. One allele results in straight hair
and the other one in curly hair. Dominant alleles are represented by
a capital letter, and recessive alleles are represented by lowercase letters. Since each of the family members
will inherit two of these alleles, one from their mom and one from their dad, there
are three possible combinations: capital S capital S, capital S lowercase s, and
lowercase s lowercase s.
And these allele combinations are
called genotypes. Genotypes with two of the same
alleles are called homozygous. If both the alleles are dominant,
we can say that the genotype is homozygous dominant. And if there are two recessive
alleles, the genotype can be said to be homozygous recessive. And a genotype with both a dominant
and recessive gene is called heterozygous. Finally, phenotype is the physical
expression of the genotype. So, as long as there’s at least one
dominant allele, the phenotype will be straight hair. But if both alleles are recessive,
the hair will be curly.
Now, we can use that information to
add a capital S to the genotype of every family member that has straight hair since
you only need one capital S to have the straight hair trait. We can also fill in Dan’s genotype
with two lowercase s’s since that’s the only way he can have naturally curly
hair. Where else can we put lowercase
s’s? If you guessed one for each of
Dan’s mother and Dan’s father, you’re right. They had to each have a lowercase
or recessive curly hair allele to give to Dan in order for Dan to have curly
hair.
We still have one lowercase s that
we can place on one of the four remaining blanks. Where should it go? Yes, to Destiny. Her father is only able to pass on
lowercase s alleles to his daughter, where her mother, Chloe, gave Destiny a
dominant straight hair allele. And we know that for sure because
Destiny’s phenotype is straight hair. Well, we did it! We used our new vocabulary and
understanding of genetic inheritance to determine how Dan’s able to have this curly
hair in a family full of straight-haired people.
Let’s work a genetic inheritance
practice problem next.
A pea plant has the following two
alleles: capital P capital P, for purple flowers. Which of the following terms can be
used to describe this pea plant? (A) Homozygous recessive, (B)
homozygous dominant, (C) heterozygous recessive, (D) heterozygous dominant.
Key knowledge required to select
the correct option here is an understanding of genetic inheritance and the terms
used to discuss such things, especially alleles. So, let’s start reviewing by
connecting the term alleles with a pea plant. To get to the alleles, we need to
take a look inside one of the pea plant cells, more specifically, inside the
nucleus, where the DNA-containing chromosomes are located. Pea plants have 14 chromosomes, but
our cell here only contains seven blue chromosomes right now to show that half its
chromosomes come from one parent and this green half from the other parent. But to see the alleles, we should
take a closer look at chromosome pair number one here, where the flower color gene
is located.
Recall that genes are sections of
DNA that usually code for proteins and affect an organism’s traits, like color. And this is also where we find the
two alleles. Since pea plants, humans, and other
sexually reproducing species receive one chromosome of each pair from each parent,
the genetic information on the chromosomes is highly similar but often not
completely identical. So, alleles then are different
versions of the same gene and lead to varying traits such as pea plants having more
than one possible flower color.
The two alleles from the question,
though, are identical and cause purple flower color. And each allele is represented by
the capital letter P. But why a capital rather than a lowercase p? Well, capital letters represent
dominant alleles. And dominant is an important term
in our question, along with the term recessive. Recessive alleles are represented
by lowercase letters. And that brings us to the term
genotype. Since there’s only two possible
alleles for this gene, capital P and lowercase p, we’re gonna have three possible
genotypes. We can have capital P capital P as
in the question, we can have capital P lowercase p, or we could have lowercase p
lowercase p.
The term homozygous can be used for
genotypes that have identical alleles like capital P capital P or lowercase p
lowercase p. We can extend these terms by adding
dominant and recessive. Capital P capital P genotype can
also be called homozygous dominant, and the lowercase p lowercase p genotype can
also be called homozygous recessive. A genotype that contains both a
capital and a lowercase allele is called heterozygous. And we’re ready to answer our
question. Which of the following terms can be
used to describe a pea plant with two capital P alleles? And the answer is homozygous
dominant.
Here’s some of the key points from
the video. Many of our characteristics are
determined by sections of DNA that are called genes. And DNA coils into structures
called chromosomes. We inherit half our chromosomes
from each parent, forming chromosome pairs. A gene can have more than one form
called alleles. And we get one allele from each
parent for most of our genes. Dominant alleles are expressed when
one or two such alleles are present, and they’re represented by capital letters. For a recessive allele to be
expressed, two copies must be present, and they’re represented by lowercase
letters.
Genotype refers to the genetic
makeup of an organism, and it’s usually written as pairs of letters for a particular
gene. So, from our diagram, we could have
combinations such as capital B capital B, capital B lowercase b, or lowercase b
lowercase b. We can also use the terms
homozygous dominant, heterozygous, or homozygous recessive to describe these
genotypes. And finally, the term phenotype is
the physical expression of a trait.