Lesson Video: Monohybrid Inheritance Biology

In this video, we will learn how to recall Mendel’s laws of inheritance and interpret genetic diagrams of monohybrid crosses.


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.

Nagwa uses cookies to ensure you get the best experience on our website. Learn more about our Privacy Policy.