Mendel's Laws: Videos & Practice Problems

Gregor Mendel's laws of genetics include the law of segregation and the law of independent assortment. The law of segregation states that during $G^1$ formation, alleles separate so that each gamete receives one allele. The law of independent assortment indicates that the segregation of one gene does not influence another, allowing for diverse genetic combinations. These principles are foundational for understanding genetic variation and inheritance patterns in organisms.

Mendel's Laws

In this video, we're going to begin our introduction to Gregor Mendel's laws. And so through his research with pea plants, Gregor Mendel was able to propose 2 fundamental laws of genetics. And so the first fundamental law of genetics that Gregor Mendel proposed is the law of segregation. And the second fundamental law that Gregor Mendel was able to propose is the law of independent assortment. Now moving forward in our course, we're going to talk about each of these laws of genetics in their own separate video, starting with the law of segregation and then moving on to the law of independent assortment. However, it turns out that we've already covered both of these laws in our previous lesson videos when we talked about meiosis. And so really moving forward as we talk about both of these laws, what you should find is that it's more review than new information. And so that being said, in our next lesson video, we'll talk about the law of segregation. So I'll see you all there.

Law of Segregation

In this video, we're going to introduce Gregor Mendel's Law of Segregation. The law of segregation basically says that during gamete formation or during meiosis, two alleles of the same gene are going to segregate or separate from each other and end up in different gametes. In other words, the law of segregation states that gametes are haploid, meaning they will only receive one copy of a gene or allele.

Let's consider our example of the law of segregation. Notice that at the top, we have a homozygous dominant cell that has two dominant alleles. We have a heterozygous cell that has one dominant allele and one recessive allele. Lastly, there is a homozygous recessive cell that has two recessive alleles. Before meiosis can take place, the DNA needs to be replicated. After DNA replication, we have duplicated chromosomes, meaning we have an extra copy of each of these alleles. At this stage, before meiosis begins, there are a total of four alleles. After meiosis I and meiosis II, each of these four alleles segregates and separates independently, ending up in gametes so that each gamete only receives one copy of the allele for that particular gene.

The same happens in the heterozygous cell, where it undergoes DNA replication to produce an extra copy of each of the alleles. Ultimately, they will all segregate or separate from one another by the law of segregation, ensuring that each of the gametes only receives one copy of the alleles.

Similarly, for the homozygous recessive, just before meiosis begins, DNA replication occurs where the chromosomes replicate or duplicate, and there will be an extra copy of each of these alleles. These alleles once again segregate and separate from one another so that each of the gametes receives only one copy of the allele, making all these gametes haploid.

The law of segregation was previously discussed when we talked about meiosis in our earlier lesson videos. Gregor Mendel, through his study of pea plants, discovered the law of segregation. This concludes our introduction to the law of segregation. We'll practice applying these concepts as we progress in our course and then discuss the law of independent assortment. I'll see you all in our next video.

According to Mendel's Law of Segregation, which of the following is a true statement?

• A. Each gamete receives both of the parent’s alleles for each gene.
• B. Dominant alleles segregate into gametes more frequently than recessive alleles.
• C. Alleles segregate into different gametes with equal frequency.
• D. Recessive alleles segregate into gametes more frequently than dominant alleles.

Answer: (C)

Mendel's observation of the segregation of alleles in gamete formation has its basis in which of the following phases of cell division?

• A. Prophase I of meiosis.
• B. Metaphase II of meiosis.
• C. Anaphase II of meiosis.
• D. Anaphase I of meiosis.

Answer: (D)

Law of Independent Assortment

So now that we've covered the law of segregation in our last lesson video, in this video, we're going to talk about Gregor Mendel's second law, which is the law of independent assortment. Now recall that way back in our previous lesson videos when we talked about meiosis, we introduced independent assortment. And so recall from those older lesson videos that independent assortment is when homologous chromosomes independently and randomly align themselves on the metaphase 1 plate during meiosis 1. And this is going to create a large amount of genetic diversity in the gametes and in individual organisms. Gregor Mendel's law of independent assortment basically says that allele segregation of one gene does not affect the allele segregation of another gene. And this is because these genes are found on different pairs of homologous chromosomes. The law of independent assortment essentially allows for gametes with all possible combinations of alleles from different genes. And we'll be able to see that down below once we wrap up this explanation. Now Gregor Mendel was able to come up with this law of independent assortment by monitoring the inheritance of multiple genes at once to make this discovery using what are known as dihybrid crosses. And we'll talk a lot more about dihybrid crosses later in our course, but for now, you should just know that dihybrid crosses allowed Gregor Mendel to determine this law of independent assortment.

If we take a look at our example image below, what we're showing you is independent assortment that occurs during metaphase 1 of meiosis 1. Again, independent assortment occurs during metaphase 1 of meiosis 1. Notice that over here on the left-hand side and over here on the right-hand side, we're showing you two possibilities of metaphase 1 during meiosis 1. Here this is representing meiosis 1, more specifically metaphase 1 of meiosis 1. During metaphase 1 of meiosis 1, homologous chromosomes are going to align themselves on the metaphase 1 plate. There are different possible alignments for these homologous chromosomes, and that's why we have possibility 1 and possibility 2 shown here. Possibility number 1 shows the red chromosomes lined up on the left-hand side and the blue chromosomes lined up on the right-hand side. Whereas, possibility number 2 shows one of the blue chromosomes on the left, the other blue chromosome on the right, one of the red chromosomes on the left, and one of the red chromosomes on the right. These are different possibilities for the alignment of these homologous chromosomes. And because each of these possibilities is equally likely during meiosis 1, that is what the law of independent assortment is all about.

What you'll notice here is that we've got two pairs of homologous chromosomes. We've got the larger pair of homologous chromosomes at the top and a smaller pair of homologous chromosomes at the bottom. Notice that the larger pair of homologous chromosomes contains the color gene. The color gene has different alleles. It has different versions of the gene. It has the capital Y version of the color gene which says yellow color and then it has the lowercase y version of the color gene which says green color, green peas. Then we have the smaller pair of homologous chromosomes containing a different gene, the shape gene. The shape gene also has different alleles. It has the capital R allele, which is for a rounded shape of the pea, and then it has a lowercase r version of the gene, a lowercase allele version of the gene, which basically says make a wrinkled pea, a wrinkled-shaped pea. And so what you'll notice is that the yellow bands over here represent the yellow allele, and the green bands represent the green allele. The blue bands here represent the capital R round allele, and the orange bands here represent the lowercase r wrinkled allele. Again, you can see how the alignment of these alleles, and the alignment of these chromosomes, is going to lead to different possibilities in the gametes. And because of the law of independent assortment and the segregation of one gene does not affect the segregation of the other gene, it allows for gametes with all combinations of alleles from different genes, which means that we can have a yellow round peas or a yellow wrinkled pea, or we could have a green round pea or a green wrinkled peas because of independent assortment. You can see that we have our yellow round peas over here, represented here, our green wrinkled peas represented here, our green round peas represented here, and our yellow wrinkled peas represented here. And so, again, it's because of the law of independent assortment creating these different possibilities that allows for all of these different combinations that we see down below. This once again refers to the law of independent assortment, how these homologous chromosomes, how they align on this metaphase 1 plate is independent and random from each other. And so this here concludes our brief introduction to the law of independent assortment and we'll be able to get some applying these concepts as we move forward in our course. So I'll see you all in our next video.

Mendel's law of independent assortment has its basis in which of the following events of meiosis I?

• A. Synapsis of homologous chromosomes.
• B. Crossing over of homologous pairs of chromosomes.
• C. Alignment of pairs of homologous chromosomes along the middle of the cell.
• D. The division of cells during cytokinesis.

Answer: (C)

Here’s what students ask on this topic:

What is Mendel's Law of Segregation?

Mendel's Law of Segregation states that during gamete formation, the two alleles for a gene separate so that each gamete receives only one allele. This occurs during meiosis, where homologous chromosomes are divided into different gametes. As a result, gametes are haploid, containing only one copy of each gene. For example, if an organism has a genotype Aa, the alleles A and a will segregate into different gametes. This law is fundamental for understanding how traits are inherited and explains why offspring receive one allele from each parent.

What is Mendel's Law of Independent Assortment?

Mendel's Law of Independent Assortment states that the segregation of alleles for one gene does not affect the segregation of alleles for another gene. This occurs because genes are located on different pairs of homologous chromosomes, which align independently during metaphase I of meiosis. This independent alignment results in gametes with various combinations of alleles, contributing to genetic diversity. For instance, in a dihybrid cross involving genes for seed color (Yy) and seed shape (Rr), the alleles for color segregate independently from the alleles for shape, leading to combinations like YR, Yr, yR, and yr.

How did Mendel discover the Law of Segregation?

Gregor Mendel discovered the Law of Segregation through his experiments with pea plants. He observed that traits such as flower color and seed shape were inherited in specific patterns. By cross-pollinating plants with different traits and analyzing the offspring, Mendel noticed that each trait was controlled by two alleles, which separated during gamete formation. His meticulous record-keeping and statistical analysis led him to propose that alleles segregate independently into gametes, forming the basis of the Law of Segregation.

How does the Law of Independent Assortment contribute to genetic diversity?

The Law of Independent Assortment contributes to genetic diversity by allowing different genes to segregate independently during meiosis. This means that the combination of alleles for one gene does not influence the combination of alleles for another gene. As homologous chromosomes align randomly at the metaphase plate during meiosis I, various combinations of maternal and paternal chromosomes are distributed to the gametes. This random assortment results in a wide variety of genetic combinations in the offspring, enhancing genetic diversity within a population.

What is the significance of Mendel's experiments with pea plants?

Mendel's experiments with pea plants were significant because they laid the foundation for modern genetics. By studying traits such as flower color, seed shape, and pod color, Mendel formulated the Laws of Segregation and Independent Assortment. His work demonstrated that traits are inherited in predictable patterns and that alleles segregate and assort independently during gamete formation. Mendel's findings provided a scientific basis for understanding inheritance, genetic variation, and the role of meiosis in producing diverse offspring.