Video Transcript
In this video, we will learn about
Gregor Mendel and his experiments with pea plants. We will understand how he carried
out these experiments and how they improved our understanding of genetics. We will also have a look at the
laws of inheritance that arose from Mendel’s experiments.
You may have heard of Gregor Mendel
before. To many, he is considered the
father of genetics, but he wasn’t always a scientist. Gregor Mendel was actually a monk,
and he was also a keen gardener. Mendel was interested in
inheritance and the traits that organisms pass down to their offspring. He chose to study the inheritance
of specific traits in pea plants.
Mendel chose pea plants to carry
out his experiments for a few different reasons. Firstly, pea plants are commonly
found and easy to grow. They also grow quickly compared to
larger plants like trees. Secondly, pea plants have lots of
contrasting or distinct traits. For example, some pea plants have
green seeds, but others have yellow seeds. Some pea plants have red flowers,
but others have white flowers. Finally, pea plants can produce
offspring in a few different ways. They can either naturally
cross-pollinate with other pea plants, or a single plant can self-pollinate
itself. They can also be artificially
pollinated, which is when a human will transfer pollen to cross-pollinate or
self-pollinate a plant.
Now that we know a little more
about Mendel and his pea plants, let’s take a look at what he was studying,
inheritance. So, what is inheritance? In biology, inheritance refers to
the characteristics or traits that are passed down from parents to their
offspring. For example, these two plants could
reproduce and pass on their trait for red flowers to their offspring. We also see inheritance in humans,
and you may have noticed this yourself. For example, if your biological
mother and father have green eyes, there is a strong chance that you and any
siblings you have will also have green eyes.
Before Mendel’s experiments,
scientists generally believed that an organism’s characteristics was a result of
their parents’ characteristics being blended. An example of this may be that if a
plant with red flowers reproduced with a plant with white flowers, their offspring
would only have pink flowers as a result of the red and white colors of the parents’
flowers blending. Mendel noticed that while this did
sometimes happen, it didn’t happen all the time. So, what was going on? Let’s have a look at the process
Mendel used to study inheritance of traits in pea plants.
Mendel started off by looking at
the color of the seeds of pea plants, the majority of which are found as one of two
colors, green or yellow. First, Mendel self-pollinated the
pea plants. He did this by selecting a pea
plant and transferring the pollen, which contains the male sex cells, to the female
reproductive part of the same flower called the stigma. By self-pollinating the plants,
Mendel could ensure that no reproductive cells from other plants were involved in
the production of offspring from this particular plant. This way, he can make sure that he
had plants that only produced yellow seeds or only produced green seeds.
Next, Mendel cross-pollinated
plants. He would do this with plants that
showed contrasting traits. For example, he would
cross-pollinate a plant that produced green seeds with one that produced yellow
seeds. When Mendel had a plant that was
ready to be cross-pollinated, he would remove the anthers from the flower, which are
the parts that produce pollen, which contains the male reproductive cells. This was to prevent the plant from
self-pollinating. Now, Mendel could look at the
traits that the offspring produced by this cross have. We refer to these offspring as the
F one generation, which stands for first filial generation. Filial is just another word for
offspring. As we can see, the plants of the F
one generation all produced yellow seeds.
Next, Mendel took a plant from the
F one generation and self-pollinated it. As we saw earlier, this involves
transferring the pollen from the male reproductive parts of the plant to the female
reproductive parts of the same plant. The offspring that are produced are
referred to as the F two generation. To Mendel’s surprise, green seeds
started to appear again in the plants of the F two generation. On average, for every three plants
that produced yellow seeds, one plant would produce green seeds. We can put this into a ratio of
yellow to green seeds at three to one.
This is a very interesting thing to
happen. Not only did this disprove the idea
that the traits of offspring are caused by blending of their parents’ traits, but
why did green seeds disappear for a generation and then come back? To understand this, we need to
understand some key terms in genetics and inheritance.
Some traits are called
dominant. If a trait is dominant, it will
always be shown in an organism’s phenotype, which is another word for its physical
characteristics. In the F one generation we just
saw, all the plants produced yellow seeds. We can safely assume from this that
the trait for yellow seeds is dominant. Other traits are called
recessive. Recessive traits are only seen in
an organism’s phenotype if the dominant trait is not present. Recessive traits are more likely to
be seen less commonly than dominant ones. In our example of pea plants, we
can assume that the trait for green seeds is recessive as it only appears once in
the F two generation. The pattern seen in Mendel’s
experiments with seed color can also be seen in the inheritance of other traits in
pea plants.
Flower color is another example of
Mendelian inheritance. If a plant with red flowers is
crossed with a plant with white flowers, the dominant trait of red flowers is most
likely to appear in the majority of the offspring. Mendel’s experiments also
demonstrated that different traits are inherited independently of each other. This means that different traits
for different characteristics are not necessarily inherited together. For example, flower color is one
characteristic, and stem height is another. Flowers may be white or red and
stems may be tall or short. So what happens when we cross these
two plants? In the first generation, all the
offspring show the dominant traits, which are red flowers and tall stems.
Let’s see what happens when we
follow Mendel’s process and self-fertilize a plant from the F one generation. The second generation, or F two
generation, shows a bigger range of characteristics. We can see that plants can have
tall stems and red flowers or tall stems and white flowers. They can also have short stems and
red flowers, or they can have short stems and white flowers. So stem size and flower color are
inherited independently from each other. It is not the case that if a plant
has red flowers, it has to have a tall stem. This idea actually forms one of the
laws of inheritance, which arose from Mendel’s research.
The law of independent assortment
explains that the inheritance of one trait happens independently to the inheritance
of other traits. Let’s take a look at two other laws
of inheritance. The law of dominance explains a
pattern that we’ve already seen. If a trait is dominant, this means
it can mask the appearance of another. Here, we can see that the trait for
red flowers is dominant because in the first generation, it masks the appearance of
white flowers. The final law, the law of
segregation, describes how an organism will inherit two factors for each trait, one
factor from their biological father and one from their biological mother. Mendel and other early geneticists
use the term factors as they didn’t yet know about DNA or how DNA is the unit of
inheritance. Nowadays, we can call these factors
genes.
Now that we know a little more
about Mendel and his experiments, let’s try a practice question.
Which of the following is not a
reason that Mendel chose pea plants for his experiments? (A) Pea plants can self-pollinate
or be artificially pollinated by humans, so Mendel could investigate the effect of
self- and cross-pollination. (B) Pea plants grow quickly, so
Mendel could produce many of them in a short amount of time. (C) Pea plants have pairs of
contrasting traits; for example, their pods can be yellow or they can be green. Or (D) pea plants are very rare, so
Mendel knew no one else was doing these experiments.
Gregor Mendel was a 19th-century
monk who is now widely regarded as the father of modern genetics due to his study on
the inheritance of traits. Gregor Mendel specifically chose
pea plants to carry out his experiments with for a few different reasons. Firstly, pea plants were quite easy
to find and grow. They grow relatively quickly, so
Mendel could produce a large population of them in a short amount of time.
Pea plants can produce offspring in
a few different ways. They can be cross-pollinated, which
means that the pollen which contains the male sex cells from one plant is
transferred to the female reproductive parts of another plant. Alternatively, they can
self-pollinate. This happens when the pollen from
one plant is transferred to the female reproductive parts of the same plant. They can also be artificially
pollinated. This is when a human will transfer
the pollen between plants or between the male and female reproductive parts of the
same plant.
Importantly for Mendel’s
experiments, pea plants show distinct and contrasting traits for many of their
physical characteristics. For example, one plant may produce
smooth seeds and another plant may produce wrinkled seeds. One plant may have red flowers, but
another may have white flowers. These contrasting traits for
certain characteristics were incredibly useful for Mendel to show the inheritance of
traits with his experiments.
Let’s take a look back at the
answer choices to determine our correct answer. Remember, we are looking for the
answer which is not a correct reason that Mendel chose pea plants. We know that Mendel chose pea
plants because they could be self-, cross-, and artificially pollinated. So (A) is not the correct answer to
this question. Pea plants grow quickly and
importantly for Mendel show contrasting traits. So let’s go ahead and eliminate (B)
and (C) from our options. Pea plants are fairly common garden
plants, so they are not rare. Therefore, option (D) is not a
reason that Mendel chose pea plants. So the correct answer to our
question is pea plants are very rare, so Mendel knew no one else was doing these
experiments.
Let’s take a look at some key
points from this video. Gregor Mendel is sometimes known as
the father of genetics because his experiments with pea plants demonstrated
inheritance of traits. Mendel used pea plants in his
experiments as they were quick and easy to grow. They could self-pollinate or
cross-pollinate, in addition to being artificially pollinated by humans, and they
had obviously contrasting traits, such as wrinkled or smooth seeds. From Mendel’s experiments, three
laws of inheritance were produced: the law of dominance, the law of independent
assortment, and the law of segregation.
