Mendel and the Laws of Inheritance - Online Tutor, Practice Problems & Exam Prep

Mendel's principles of inheritance focus on alleles, which are gene variants. Individuals can be homozygous (identical alleles) or heterozygous (different alleles). Alleles can be dominant, always expressed, or recessive, expressed only in the absence of a dominant allele. Mendel's laws include the law of segregation, stating that alleles separate during gamete formation, and the law of independent assortment, which indicates that different genes are inherited independently, unless they are closely linked on the same chromosome, where crossing over can occur.

Alleles

Hi. In this video, we're going to be talking about Mendel and the principles of inheritance. So the first thing we need to talk about are alleles. And I feel like people get confused when we talk about alleles just because they're not 100% sure what they are. So, let's just define it very simply. An allele is a variant of a human gene. So one gene can have multiple alleles, and each one of them is a variant. And because every human cell or every human somatic cell has two copies of its gene, that means every human can have 2 alleles for that gene. And we define combinations of these different alleles in a couple of different ways. When we say, we classify them as homozygous or heterozygous. So it's homozygous if the 2 allele copies that a person has are the same, they're identical. And it's heterozygous if the 2 alleles are different. So if you have 2 of the same alleles, we'll say that's homozygous. If you have 2 different alleles, that's heterozygous. But alleles can also be classified in a different way as well, and that's as dominant or recessive. So dominant alleles are going to be if they're present, if you have a dominant allele, it's always going to be expressed. No matter what the second allele is, it's always going to be expressed. You're going to see it phenotypically. You're going to see a physical appearance of it. But if those alleles are recessive, they are only expressed pretty much if a dominant isn't there. So a dominant will always be expressed, and so you'll see it if you have it, but if you don't have it, then you will see a recessive allele. So what this looks like is we have this lovely animal here, I don't know what kind of animal it is, I guess you could bank it up. And here are our 2 alleles for our gene, and this gene is for, I guess, coat color. Right. And you can see there's one uppercase and one lowercase here. And you are probably familiar with this notation from either high school or your undergraduate intro classes. But essentially, this one is dominant heterozygous because dominant means black. And we see this because it has the dominant allele, the uppercase R. Now it's heterozygous because the 2 alleles are different. You have one uppercase and one lowercase. Whereas this animal, for instance, is different. It's recessive because there is no uppercase R, so instead, you just have 2 lowercase r's. But it's also homozygous because the 2 alleles are identical, they're exactly the same. So that's what those terms mean.

Now I said we were going to talk about Mendel, and so Mendel studied alleles and gene inheritance. And he did this by studying pea plants, which I'm sure you know. And he observed their offspring. So there are a few different ways to label mating and offspring that I want to go over. So the first one is called the P₁ generation, that's going to be the parental plants. So those are the mom and dad plants. Then you have the F₁ generation, so that's going to be the offspring created from the parental plants. So, this is kind of like, you know, their children, the plant's children. But then, you have the F₂ generation, and this is the offspring created by the F₁. So these are the grandchildren. You have the parents, you have the children, and you have the grandchildren. Now, in humans, this analogy entirely makes sense. But what's interesting is that typically the F₂ generation can, how we get that can be interesting and uses some kind of breeding we can think of from the F₁ generation. So we say it's their back cross if we mate the F₁ offspring, so the children, with the parents. And we think that's super weird for humans. Right? It would be, like, so gross if that happened with humans. But in pea plants, it doesn't really matter. Right? Because there are just some plants. So, we can mate the F₁ generations with their siblings or with their parents to create these F₂ generations that can tell us interesting things about genetics. And it's not gross because it's plants. So, here's an example. So these are cats for instance, just in the generation. So, you cross the 2 parents. You have the mom, mom cat, and the dad cat, and they produce these four children, these F₁ children. And then, in this case, I think the 2 of the F₁ children are mated together, so these 2 siblings mate and they produce the F₂ grandchildren for the cats. Total inbreeding, but it's okay because we're studying genetics. So, that's just some of the information that hopefully will be helpful. So now, let's move on.

Mendel's Laws

Okay. So now let's talk about Mendel's laws. So we're going to talk only about two of Mendel's laws: the law of segregation and the law of independent assortment. Mendel discovered these laws because he was studying pea plants and noticed important laws about inheritance. All this focuses on how different genes are inherited.

The first law is the law of segregation. This law states that two alleles for a single gene separate during gamete formation. We know that we get one copy of a gene from our mom and one from our dad. This means we receive one allele from our mom and one allele from our dad. So, when gametes form—those being eggs and sperm—we do not get two alleles from each parent; we only get one. Therefore, the two alleles that a parent has separate to form the gametes. When the egg and sperm unite, these two alleles unite at random. There is no one egg destined for one sperm; it's all somewhat random. Any sperm can fertilize any egg, resulting in one random allele from each parent. This means we do not receive two copies of alleles from each parent because then we'd end up with double the genetic information we need. We only receive one copy of one allele because those separate during gamete formation.

The more complex law that we're going to spend a lot of time on is the law of independent assortment. This law states that the alleles of different genes are passed independently of each other. For pea plants, this translates to a pea plant of a certain color being any shape, because the genes for color and shape are different and are passed independently. In humans, this would mean that, for example, someone's height does not determine their hair color. They can be any height and still have blonde hair because these are two different genes, and therefore, they're passed independently. When discussing crosses like the monohybrid cross, it focuses on the inheritance of one trait (e.g., blonde hair), while the dihybrid cross examines the inheritance of two traits (e.g., blonde hair and a certain height).

An example of a dihybrid cross involves crossing a white cat with a short tail and a brown cat with a long tail. The offspring might exhibit a variety of combinations of these traits, illustrating independent assortment. This tells us the proportions of each combination based on genetics. Now, there's a caveat to the law of independent assortment concerning the genes on the same chromosome. Although genes on the same chromosome generally assort independently due to crossing over in meiosis, the closer two genes are to each other, the more likely they will be inherited together. If genes are very close together, they form what's called a linkage group and often do not assort independently. This distance affects the likelihood of the genes being inherited together, which is observed more commonly in model organisms like fruit flies than in humans.

If two genes are far apart on a chromosome, they are less likely to be inherited together because crossing over is less likely to affect both simultaneously. This mixing up of alleles through crossing over during meiosis allows for genes close together on the same chromosome to sometimes be co-inherited. This is the exception to the law of independent assortment, which states that genes are sorted independently unless they are very close together and undergo crossing over frequently.

So, that concludes our discussion on Mendel's laws of segregation and independent assortment—key principles in understanding genetic inheritance.

Which of the following of Mendel's laws states that two alleles for a single gene separate during gamete formation?

• A. Law of independent assortment
• B. Law of segregation

Which of the following generations is created from mating the parental generation?

• A. P
• B. F₁
• C. F₂
• D. F₃

Genes closer together on a chromosome are more likely to be inherited together?

• A. True
• B. False

Here’s what students ask on this topic:

What are alleles and how do they differ from genes?

Alleles are variants of a gene. While a gene is a segment of DNA that codes for a specific trait, alleles are different versions of that gene. For example, a gene for eye color may have an allele for blue eyes and another for brown eyes. Each individual inherits two alleles for each gene, one from each parent. These alleles can be homozygous (identical) or heterozygous (different). The combination of alleles determines the expression of the trait, with dominant alleles being expressed over recessive ones.

What is Mendel's Law of Segregation?

Mendel's Law of Segregation states that during the formation of gametes (eggs and sperm), the two alleles for a single gene separate so that each gamete receives only one allele. This means that offspring inherit one allele from each parent. This law explains why genetic variation occurs, as the combination of alleles from each parent is random, leading to different genetic outcomes in the offspring.

What is Mendel's Law of Independent Assortment?

Mendel's Law of Independent Assortment states that alleles of different genes are passed independently of each other from parents to offspring. This means that the inheritance of one trait (e.g., pea color) does not affect the inheritance of another trait (e.g., pea shape). However, this law applies only to genes located on different chromosomes or far apart on the same chromosome. Genes that are close together on the same chromosome may be inherited together due to genetic linkage.

How do dominant and recessive alleles differ in their expression?

Dominant alleles are expressed phenotypically even if only one copy is present, while recessive alleles are expressed only when two copies are present. For example, if the allele for black fur (B) is dominant and the allele for white fur (b) is recessive, an organism with the genotype Bb or BB will have black fur, while only an organism with the genotype bb will have white fur. This is because the presence of a dominant allele masks the expression of a recessive allele.

What is the significance of crossing over in genetics?

Crossing over is a process during meiosis where homologous chromosomes exchange segments of DNA. This results in new combinations of alleles on each chromosome, increasing genetic diversity. Crossing over is significant because it allows genes that are close together on the same chromosome to be inherited independently, which can affect the genetic variation seen in offspring. It is a key mechanism that supports Mendel's Law of Independent Assortment for genes located on the same chromosome.