Mendelian Genetics: Genotypes, Phenotypes and Hybrids
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Mendelian Genetics: Genotypes, Phenotypes and Hybrids


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!

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