Video Transcript
In this video, we’ll learn about
Mendel’s laws of inheritance and the experiments that led to their formulation. Then, we’ll investigate how
probability can be used to predict the results of genetic crosses. And we’ll review the use of the
Punnett square. So, let’s act like an Austrian
scientist investigating heredity and get started.
Classical genetics is the study of
inheritance based only on visible results, and it’s widely accepted that classical
genetics began in earnest with the work of Gregor Mendel. Eventually, the observations of
classical geneticists were explained at the molecular level, which gave rise to the
modern study of genetics as we know it today. Gregor Mendel was an Austrian monk,
famous for his experiments using pea plants. These experiments led to the
development of three rules, which we refer to as Mendel’s laws. Mendel’s laws were not written by
Mendel himself, but rather by the scientists who studied his work.
The first law is also referred to
as the fundamental theory of heredity. It states that inheritance involves
the passing of discrete units of inheritance from parents to offspring. The second law is also referred to
as the principle of segregation. It states that during reproduction
the inherited factors that determine traits are separated into reproductive cells
and randomly reunite during fertilization. And the third law is referred to as
the principle of independent assortment. And it states that each of these
factors will be inherited independently of each other. With some modification, these laws
still form the foundation of modern genetic theory. Remember that this research had
been done before scientists knew what a gene was or DNA had even been
discovered. So, let’s take a look at the
experiments that led to these conclusions.
Mendel worked with what he called
pure-breeding pea plants. These plants have flowers that will
self-pollinate before the bud is even open. He observed these plants for
generations and chose plants that always had offspring with the same traits, for
example, a plant with purple flowers whose offspring always had purple flowers or a
plant with white flowers whose offspring also always had white flowers. Then, he cross-pollinated these two
plants together. All of the offspring from this
pairing inherited one trait and not the other. They all had purple flowers. He called these offspring the F one
generation.
This itself is an interesting
finding, but Mendel didn’t stop there. He then allowed the plants from the
F one generation to self-pollinate like before. These offspring he called the F two
generation. And while most of them had purple
flowers, some of them had white flowers. In fact, out of 929 F two
offspring, 705 had purple flowers, while just 224 had white flowers, a ratio which
works out to almost exactly three to one. This experiment is referred to as a
monohybrid cross. And traits that follow this pattern
of inheritance are called Mendelian traits.
This experiment showed that one
trait was dominant over the other, since only purple flowers showed up in the F one
generation, but that the factor responsible for the recessive trait, white flowers,
must still have been present in those offspring, since when the F one generation
self-pollinated without introducing factors from any other plants, even though they
all had purple flowers, some of their offspring had white flowers, a trait which
only could have come from two generations before. And the fact that these traits
always occurred in the same pattern and the same ratio meant that it was highly
unlikely it was random.
This brings us back to Mendel’s
laws. Now that we know where they come
from, we can see how with some modifications they represent genetics as we
understand it today. Now we know that these discrete
units of inheritance are in fact chromosomes made of DNA. Now we call the inherited factors
that determine traits genes. We refer to reproductive cells as
gametes and the process of their formation as meiosis. The third law needs a little more
help. So, let’s take a look at an
illustration.
So, here, I’ve drawn pea plants,
and I represented four of its chromosomes. This segment on the first pair of
chromosomes represents the gene for flower color. Our pea plant has the dominant
purple characteristic, and we’ll go ahead and say that it’s homozygous. Now this segment on the second pair
of chromosomes represents the gene for seed color. Again, our pea plant expresses the
dominant trait of having green seeds. But this time, we’ll say it’s
heterozygous. As you can see, homozygous means
that two alleles match, and heterozygous means that they’re different.
We know that when this pea plant
produces gametes, only one chromosome from each pair will be passed on. So, this is how the gene for flower
color and the gene for seed color are inherited independently. Now, let’s imagine that a third
gene, this one for plant height, was also present on the second chromosome pair and
that our pea plant is also heterozygous for this trait. Well, if a gamete inherited the
chromosome with the dominant allele for seed color, they would also end up
inheriting the recessive allele for plant height. And if a gamete inherited the
chromosome with the recessive allele for seed color, in this particular case they
would also inherit the dominant allele for plant height. Since they share a chromosome, the
traits for seed color and for plant height are likely to be inherited together. So, we can see that Mendel’s third
law is not always true, since each of our chromosomes carries more than one
gene.
Let’s look at another example. We often associate the Punnett
square with Gregor Mendel and his work. But it was actually invented about
50 years later by a scientist named Reginald C. Punnett who developed the Punnett
square as a visual representation of the experiments that Mendel had carried
out. So, we’ll try the monohybrid cross
experiment again, this time accompanied by Punnett squares. First, we’ve crossed the plant
that’s pure-bred for yellow seed pods with one that’s pure-bred for green
seedpods. And our F one generation all has
green seedpods. That tells us that green is the
dominant trait and yellow is the recessive trait. And since our parent plants are
pure-bred, they’re both homozygous. And as expected, we can see that
all of the offspring are green.
In order to produce the F two
generation, Mendel allowed the plants from the F one generation to
self-pollinate. All of the plants from the F one
generation are green and heterozygous. And since the plant is
self-pollinating, both sides of the Punnett square should be the same. Now when Gregor Mendel conducted
this experiment, out of 580 F two plants, 428 had green seedpods, while 152 had
yellow seedpods, a ratio that’s once again almost exactly three to one.
What about our Punnett square? Well, when we look at the results,
we see that one, two, three of the predicted offspring are green and one is
yellow. This three-to-one figure is what we
call the phenotypic ratio because it tells us about the traits that the offspring
possess. The F two generation can also be
described with a typical genotypic ratio, which would show that there’s one
homozygous dominant offspring, two heterozygous offspring, and one homozygous
recessive, likely out of four.
Let’s use what we’ve learned in one
more example. Let’s say that we take some seeds
from the F two generation of Mendel’s experiment and cross them. But we don’t know the genotypes of
the plants, just that one has green seedpods and the other has yellow. We crossed them and get 47
offspring, 22 of which have green seedpods and 25 of which grow up to have yellow
seedpods. With this information, can we
determine the genotypes of the parents from the F two generation? Well, one of our parents has yellow
seedpods. And the only way to have yellow
seedpods is to have the homozygous recessive genotype.
So, we know the genotype of one
parent is lowercase g lowercase g. But for the green plant in our F
two generation, we have homozygous and heterozygous parents with green pods. So which genotype is the parent in
our experiment? Well, the phenotypic ratio of their
offspring was approximately one to one. And we know that at least one
allele in our plant with green pods must be dominant. If we fill in the part of the
Punnett square that we’re sure about, so far, it only yields green offspring. So, we’d expect these last two
cells to have the genotype for yellow offspring, which is necessary to achieve the
one-to-one ratio. And we’re already aware that the
only genotype that will give you yellow seed pods is lowercase g lowercase g. So now we figured out that the
genotype of the parent plant with green seedpods must be uppercase G lowercase g, or
heterozygous.
Now that we’ve learned about
Mendel’s laws and the experiments that they’re based on and how to use Punnett
squares and ratios to determine the genotypes and phenotypes of parents and
offspring, let’s take a moment to review what we’ve learned.
In this video, we learned about
Mendel’s laws. The fundamental theory of
inheritance, which states that inheritance involves the passing of discrete units of
inheritance from parent to offspring. The law of segregation, which
states that during reproduction, the inherited factors that determine traits are
separated into reproductive cells and randomly reunite during fertilization. The law of independent assortment,
which states that each of these factors will be inherited independently from each
other. And we also learned about Mendel’s
experiments, which led to these laws. We used Punnett squares to
illustrate these experiments, and we learned how to determine and use phenotypic
ratios.
