Mendel's Experiments and Laws - Online Tutor, Practice Problems & Exam Prep

Gregor Mendel's experiments with pea plants established foundational principles of genetics. He identified pure lines, leading to the first filial generation (F₁), which he self-fertilized to produce the second filial generation (F₂). Mendel's observations revealed a 3:1 ratio of dominant to recessive traits, leading to his laws: the law of segregation, where alleles separate during meiosis, and the law of dominance, indicating that some alleles are dominant over others. These principles are crucial for understanding inheritance patterns in genetics.

Mendel's Experiments

Hi. In this video, we're going to be talking about Mendel's experiments and laws. Okay. So, Mendel is Gregor Mendel. He was an Austrian monk, and he studied genetics and is really mainly the founder of genetics. And his main focus was pea plants. And the reason he chose pea plants was because you can breed them, you can breed a lot of them, you can breed them easily, but also there were a lot of pure lines available. So what is a pure line? A pure line is going to be when all of the offspring from that line produces another pure line. So that means that the trait that you're looking for will be identical in the parent and in the offspring. And so, for instance, a yellow seeded pure line will produce yellow seeded offspring if, you know, if they're mated together. Pure line with another yellow seeded pure line, it's going to produce yellow-seeded offspring. And so that was a major reason why Mendel chose pea plants because you can you know, you know what your offspring is going to be if you use these combinations. And so that allowed him to make combinations of pure lines to see what happened. And, he was very meticulous, when doing these studies, and he labeled each generation in a very specific way. And, it's important that you understand what these terms and these labels mean because this is how you're going to understand everything in genetics from now on. So, the first is the parental generation, and this is the first mating that occurs, the very first one. Male and female are plant and another plant, this is what happens. The offspring of that mating is the first filial generation or F₁, which is what you'll pretty much always see it as F₁, and this is the offspring produced from the parental mating. Now when you have an F₁ generation, there's lots of things you can do. You can self-mate them. I suppose with plants, with certain sexually reproducing or certain organisms, you can't do this. Like, with humans, you can't do it. But with plants, you can self-mate. And that means that the plant pollen can be used to fertilize itself. Self-mating is a technique that Mendel used and still a lot of geneticists use today. You can also cross-fertilize them, which means that one plant's pollen is used to fertilize another plant. Obviously, we're talking about plants because we're talking about Mendels, but you can do this with other organisms as well. So when you have your F₁ generation, you can self-mate it, you can mate it with another F₁, you can mate it back to its parent. You can mate it with anything you want. And, generally, what will happen is the second filial generation will be produced, also, shorthand F₂ generation. This is the offspring from the F₁ mating. So, those are the 3 generations. Now, let's go through one of Mendel's crosses as our example. So, here we have the parental generation. You can see that he took yellow seeds and green-seeded plants and crossed them. So, here's the parental. The offspring of that is called what? Offspring of that parental mating is called what? Called the F₁ generation. And what Mendel saw is that when you mated the yellow seeds with the green seeds, you got all yellow seeds in the F₁ generation. Then what he did is he took the F₁ and he self-fertilized, meaning that he self-fertilized them. So he took the pollen from that F₁ plant and used it to fertilize the same F₁ plant. When he did this, he saw not only yellow, but he also saw green in this F₂ generation. So, we went from parental F₁ and F₂. So, now we're in the F₂ generation. And, so, he started to count the number of these plants and to figure out, you know, what ratio. Is it mostly yellow? Is it mostly green? You know, what's going on here? And, what he saw is that there were 6,022 yellow plants and 2,001 plants, which equals a total of 8,023 total F₂ plants. Now if you do ratios, so if you do 60228023 and you do 20018023, what you'll get approximately is 3 fourths and approximately 1 fourth. Now, it's not perfect, it's never perfect in genetics. It doesn't have to be exactly perfect but that's approximately what you'll get. And so he noticed that this was a 3 to 1 ratio, and that this ratio kept popping up a lot, and this will become very important and we'll talk about this ratio more. But this is just an example of one of Mendel's crosses. So then he took the F₂ generation. So here we're dealing with this one. So we have yellow and we have green. So he took each one of those. So he took a yellow plant and then made the F₃. So you're getting the pattern here. So we went from parental, we went from F₁, F₂, and then now we're at F₃. Now, most of the time in this class, we're going to stop at F₂, but just for this example, we're going to keep going through F₃. So in F₃, he took a yellow F₂ and selfed it. So he took a yellow F₂ and took that pollen and fertilized that same yellow F₂ plant. And what he found is he saw this 3 to 1 ratio again of yellow and green plants, but when he did the same thing with the green F₂ plant he got 100% or 4 out of 4 green plants. So that was this is a really important cross to know what Mendel did. So make sure you go back and review and you understand each step of this process. So this is one major cross that he did. Parental, F₁, all yellow, then he got the F₂, which was 3 fourths yellow and 1 fourth green. Then he took each of those F₂ and selfed them and got different ratios. Then he did a second cross and what the second cross was was a yellow F₁, This is from the F₁ generation and a green. And when he mated the yellow F₁ with a green, what he saw is that half the offspring were yellow and half the offspring were green.

Mendel's Laws

Okay. So now let's talk about Mendel's Law. Mendel studied pea plants, and he was able to make a bunch of observations, which led him to conclude certain properties and certain laws about inheritance. Some of these properties that he deduced from these pea plant experiments were that there was some type of factor that was important for inheritance, and we now call this factor a gene. And this gene is absolutely necessary for producing a certain trait, such as seed color—yellow or green. This gene comes in two forms, which we now call alleles, and we know that these two forms are yellow and green, as mentioned above. So the alleles come in two forms. One form or allele is dominant to the other. As previously discussed, yellow is the dominant allele, as yellow seeds consistently produce yellow offspring. Green seeds produce green offspring, but since yellow is often present, the green trait is mostly masked by the dominant yellow allele.

Now, Mendel's laws were able to take that information and actually formulate laws that govern genetics. These are super important, and we're going to discuss all three. The last one, the law of independent assortment, is going to get its own chapter, which is great. The first two we're really going to focus on. The first one is the law of segregation, which means that alleles separate during meiosis, the cell division that creates sex cells. Therefore, alleles separate to form gametes, which are sex cells. Each gamete contains a single allele for each trait. It either contains the allele for yellow or green or the dominant or the recessive allele. Gametes only contain one allele, and this principle is the essence of the law of segregation. The law of dominance states that some alleles are dominant and others are recessive. In the examples discussed with Mendel, it's clear that yellow is dominant because it appears frequently, and green is recessive because the only guarantee of obtaining a green-seeded plant is if there is no yellow allele present.

The third law, the law of independent assortment, though more complex and detailed than what we've discussed so far, essentially states that genes for different traits segregate into gametes independently. This is about considering more than one trait, such as color and shape of seeds, which separate independently into sex cells. It's not predetermined that all yellow seeds must be round; they can segregate into gametes completely independently. If you're confused about this, that's okay, as this law will be elaborated on in its own chapter. However, it's crucial to understand now the law of segregation, how there's one allele per sex cell, and the law of dominance, that one allele is more dominant.

Here's another example involving a cross of white and red flowers. The notation in this image is slightly different than what you would normally see. But looking at this, we first revisit the law of segregation, which reminds us there will be one allele per gamete. As observed in the organisms, each has two alleles, but during the formation of gametes, these alleles are separated out into single alleles combined to create the next generation. The law of dominance then informs us that one of these alleles will be more dominant than the other. Looking at the presence of red and white flowers, it is clear that the red allele is more dominant because there are considerably more red than white flowers. These are the first two laws that Mendel proposed. With that, let's now move on.

Which of the following Mendel's postulates states that alleles separate in the formation of gametes?

• A. Law of segregation
• B. Law of dominance
• C. Law of independent assortment
• D. Law of dividing cells

Breeding two pure-lines of yellow-seeded flowers will always produce yellow-seeded offspring

• A. True
• B. False

What is the official genetics term for the second generation of offspring?

• A. Parental generation
• B. F₁ generation
• C. F₂ generation
• D. Grandchildren

Here’s what students ask on this topic:

What were the key findings of Gregor Mendel's experiments with pea plants?

Gregor Mendel's experiments with pea plants led to several key findings that form the foundation of genetics. He discovered that traits are inherited in specific patterns, leading to the formulation of the law of segregation and the law of dominance. The law of segregation states that alleles separate during meiosis, ensuring each gamete contains only one allele for each trait. The law of dominance indicates that some alleles are dominant over others, as seen in his 3:1 ratio of dominant to recessive traits in the F2 generation. These principles help explain how traits are passed from parents to offspring.

What is the law of segregation in Mendelian genetics?

The law of segregation is one of Gregor Mendel's foundational principles of genetics. It states that during the formation of gametes (sex cells) in meiosis, the two alleles for a given trait separate, so each gamete receives only one allele. This ensures that offspring inherit one allele from each parent. For example, in Mendel's pea plant experiments, the alleles for seed color (yellow or green) segregate so that each gamete carries only one allele, either for yellow or green. This principle is crucial for understanding how genetic variation is maintained through sexual reproduction.

How did Mendel's experiments lead to the discovery of dominant and recessive traits?

Mendel's experiments with pea plants led to the discovery of dominant and recessive traits through his observation of inheritance patterns. By crossing pure lines of pea plants with different traits (e.g., yellow and green seeds), he noticed that the F1 generation always exhibited the dominant trait (yellow seeds). When he self-fertilized the F1 generation, the F2 generation showed a 3:1 ratio of dominant to recessive traits (yellow to green seeds). This consistent ratio led him to conclude that some traits are dominant and mask the presence of recessive traits, which only appear when both alleles are recessive.

What is the significance of the 3:1 ratio observed by Mendel in the F2 generation?

The 3:1 ratio observed by Mendel in the F2 generation is significant because it provided evidence for the law of dominance and the law of segregation. This ratio indicates that when two heterozygous individuals (F1 generation) are crossed, the dominant trait appears in approximately three-fourths of the offspring, while the recessive trait appears in one-fourth. This pattern supports the idea that alleles segregate during gamete formation and that dominant alleles mask the expression of recessive alleles. The 3:1 ratio is a fundamental concept in understanding Mendelian inheritance and genetic variation.

What are pure lines, and why were they important in Mendel's experiments?

Pure lines, also known as true-breeding lines, are groups of organisms that consistently produce offspring with the same traits when self-fertilized or crossed with another pure line of the same trait. In Mendel's experiments, pure lines were crucial because they ensured that the traits he was studying were consistent and predictable. For example, a pure line of yellow-seeded pea plants would always produce yellow-seeded offspring. This consistency allowed Mendel to accurately track the inheritance of traits across generations and formulate his laws of inheritance, such as the law of segregation and the law of dominance.