Welcome back to BOGObiology! This screencast is going to be about the
concepts of genetics and heredity. For the purpose of this screencast, we will
be using paint cans as a metaphor for an organism. Pay careful attention to the
label on the can as well as the color of the paint because this will help you to
understand some of the key terms. To begin, we're going to define two terms
that many people find confusing; genotype and phenotype. The genotype is the set of
genes that an organism carries in its DNA. These two paint dots depict what
components are found within each can. The phenotype of an organism is its
observable characteristics; it's what we would see when we opened the can of
paint. Once you open the can of paint, you can see its phenotype, i.e. what color the
paint is. The first two cans are pretty straightforward;
the can with the blue-blue genotype has a blue phenotype, and the can with the
yellow-yellow genotype has a yellow phenotype. But what about the can on the
right whose genotype contains both blue and yellow components? This can is called
a hybrid. Its phenotype is blue because the blue paint overpowers the yellow
paint. Even though both paint types are present in the can, i.e. the organism's DNA,
only the blue paint shows up in the phenotype. Only one unit of blue paint is
needed in order to determine the color of the paint in the can. This makes blue
the dominant color. Dominant genes overpower others in order to determine
the organism's phenotype, recessive genes require two copies to determine the
organism's phenotype. For the paint to be yellow, the can needs two yellow
components because yellow is the recessive color. In short, if any blue is
present in the can, it will overpower the yellow.
There are multiple variations ofeach gene and we call these varieties alleles.
These variations arise via mutation or mistakes made during the reproduction process.
In order to easily describe an organism's genotype we use two terms;
homozygous and heterozygous. When an organism is homozygous it has two identical alleles for a given trait.
fFor our purposes, the trait in question is color. Notice that an organism can be
either homozygous dominant or homozygous recessive. When an organism is
heterozygous, it has two different alleles for a given trait. A heterozygous
organism can also be called a hybrid.
A major reason to study genetics is to be able to predict the probability that two organisms will produce offspring that
have certain characteristics. Provided that you know the genotype of both
parents, it's possible to predict the probability that their offspring will
display certain traits. For our first example, we'll be breeding a homozygous
blue paint can with a homozygous yellow paint can. We will put one parent's
genotype here and the other parent's genotype over here. Now your Punnett
square should look something like this. If we assume that these are monogamous
paint buckets, the only possible genes their offspring
can have are those from their parents. Each parent will contribute one allele
to their offspring. We can use this diagram to figure out the possible
combinations of alleles that their offspring could have in this scenario.
Because one parent has only blue alleles and the other parent has only yellow
alleles, all of their offspring will have one blue and one yellow copy of the
paint color gene. When we fill them in, these are the possible genotypes for the
offspring. We have zero homozygous dominant offspring, four heterozygous
offspring, and zero homozygous recessive. Now let's take a look at the phenotypes.
I find color coding my Punnett squares to be helpful, so I'm going to shade in the
phenotypes for both parents first. After shading it in, the Punnett square will
look like this. When we've shaded in the phenotypes for the offspring, we can see
that all of the offspring will have the same phenotype as well. All four
offspring will have the dominant phenotype, and none will have the
recessive phenotype. All of the offspring from the first pairing have the blue
phenotype, but each of them is carrying a copy of the
recessive yellow allele. In addition to calling heterozygous organisms hybrids,
we also sometimes call them carriers. These organisms may not even be aware
that they are carriers for the yellow coloring because their phenotype is blue.
However, the yellow possibility is still there in their DNA and may come out when
they reproduce. Before I fill it in see if you can figure out the possible
genotypes and phenotypes for the offspring of two heterozygous paint cans.
These parents could have offspring with genotypes that were homozygous dominant,
heterozygous, or homozygous recessive. They have a 25% chance for offspring
with a homozygous dominant genotype, a 50% chance of heterozygous offspring, and a
25% chance of homozygous recessive offspring. Instead of having all
offspring with the blue phenotype, there is now a 25% chance that they could have
offspring with the yellow phenotype. It's worth noting that the probability "starts
over" every time they reproduce. Having one blue offspring doesn't increase the
likelihood of getting the yellow offspring the next time around. To finish
up, we're going to tackle a very common problem which is how to determine the
genotype of an organism if you don't already know what it is. The first step
is to determine the phenotype of the unknown organism. Obviously, it's going to
be useful to do a little research to figure out what the different phenotypes
look like. If the organism has the recessive phenotype, you can stop there
because you know that it's going to have the homozygous recessive genotype. If it
has the dominant phenotype, you have two options. The genotype may be homozygous dominant
or it may be heterozygous. If the organism has an intermediate or a
blend of phenotypes, unfortunately that means that you probably have a case of
codominance or incomplete dominance, and traditional
Mendelian genetics rules don't apply. We're going to proceed as if one of the
first two scenarios is the case. Next you're going to perform a test cross.
You're going to breed the unknown organism with a homozygous recessive
organism. Then you're going to analyze the percentages of offspring phenotypes.
Based on the offspring phenotypes you can work backwards to take an educated
guess at what the genotype of the parent is. The more offspring you can produce,
the more accurate your percentages are going to be. The smaller the pool of
offspring, the more likely your data is going to be skewed. This technique is also much
more efficient if you're using it to analyze organisms that reach maturity
and reproduce very rapidly. In Example 1, the unknown organism has the blue
phenotype. To determine its genotype, first ask yourself the question "what are
the genotypes of all the offspring". We know that every one of the offspring is
going to have at least one yellow allele from the yellow parent. However because
all of the offspring also display the blue phenotype, they must have a blue
allele as well. This means that all of the offspring are heterozygous. Now try
and work backwards to answer the second question and work out what the genotype of the blue organism is.
If you worked out that the unknown organism was
homozygous for the blue allele, you're correct. Now see if you can solve this
problem on your own. If you're following along, this is what you should have
written down. Because these two offspring have the yellow phenotype, they must have
inherited a yellow allele from each of their two parents. For this unknown
organism to have contributed a yellow allele but still be blue itself, it must
be heterozygous.
A few final things to keep in mind. This screencast is a simplified overview.
Mendelian genetics are complex. Mendelian genetics are also not
a foolproof system. There are very few one-gene-to-one-trait match-ups.
Most traits are determined by the interaction of several genes, and there are also
cases where one gene determines several different traits. Some traits are also
tied to the sex of the organism, and there are also environmental factors to
take into account. Additionally, in many hybrid organisms,
they display either a blend of dominant and recessive traits, or they show
aspects of both traits simultaneously.
If you're interested in a future video
about epistasis, pleiotropy, codominance incomplete dominance, and related
topics, please let me know by leaving a comment in the field below. Thanks for
watching, and don't forget to subscribe!