Lesson Video: Alleles | Nagwa Lesson Video: Alleles | Nagwa

Lesson Video: Alleles Biology

In this video, we will learn how to define and explain the key terms in genetics and explain how alleles provide genetic variation in humans.

17:55

Video Transcript

In this video, we’ll learn how to define and explain key terms in genetics. And this knowledge will help you to understand and communicate important concepts of genetic inheritance.

What is genetic inheritance? Well, traits like the texture of our hair or the shape of a bird’s beak usually have at least some basis in our DNA. And DNA stores information about our traits in sections called genes. So, genetic inheritance is all about how we inherit our particular DNA and where it came from. Here’s a simplified cell from our character Dan that we’ll use to investigate the origins of his curly hair. And here’s an extra big nucleus so we can see his DNA a little better.

DNA coils around special proteins to form structures called chromosomes. Each chromosome contains only one DNA molecule or two identical DNA molecules. And Dan, like most humans, has 46 chromosomes. But so far, I only have 23 chromosomes in this nucleus because these are the 23 that Dan inherited from his mother. Dan also inherited 23 similar but not completely identical chromosomes from his father, shown here in green. And let’s say that the gene that controls hair texture is located on Dan’s chromosome pair number one. Even though hair texture is a lot more complicated than just being a single gene trait, for simplicity, let’s imagine it is for now. The funny thing is that both Dan’s parents have straight hair, while Dan has wildly curly hair. What if Dan has children? Will any of them get Dan’s curly hair? Let’s find out.

When Dan produces sperm, each sperm will contain only one chromosome from each of his 23 pairs. And you can see that Dan will pass on some of the yellow chromosomes he inherited from his mom and some of the green chromosomes from his dad. Here’s Dan’s wife, Chloe. She has straight hair, and so do her parents. This is one of Chloe’s normal body cells. Again, it shows the chromosomes from her parents. In this case, the blue are from her father, and the pink are from her mother. The location of genes on chromosomes is the same for members of the same species. So, Chloe’s hair texture gene is at the same location as Dan’s. And just like Dan, when Chloe produces eggs, each receives only one randomly selected chromosome from each pair again, some from her mother and some from her father.

Now, if this sperm fertilizes this egg, we can see where the resulting baby’s chromosomes come from. Some come from Dan’s mother, some from Dan’s father, some from Chloe’s mother, and the rest from Chloe’s father. In other words, the new baby here, Destiny, who will be growing straight hair is like the rest of us, the expression of merged genetic inheritance from four grandparents. And they were the product of countless generations that continuously shuffled and merged DNA for billions of years.

So, how does that help us understand how Dan ended up with curly hair? Well, Destiny’s chromosome pair number one both came from her grandmothers, and both her grandmothers’ hair were straight. So, should we draw out all the chromosomes of Destiny’s grandparents next? No, definitely not because it’s easier and less cumbersome to use some vocabulary terms to represent all those merging and shuffling and divvying up of chromosomes. So, next, we’ll continue with our investigation, but we’re gonna use more vocabulary and fewer diagrams.

We already know that a gene is a section of DNA that generally codes for a protein and can affect our traits. We also know that that DNA coils up into structures called chromosomes and that according to our scenario, Dan’s hair is curly because of the hair texture gene on chromosome number one. We also know that the genetic information in the chromosomes of Dan’s parents or anybody’s parents is similar but not identical, since parents are not identical to each other. So, the hair texture gene can vary allowing for different textures of hair such as straight or curly.

Different versions of the same gene are called alleles, and letters are used to represent them. Let’s let the letter capital S represent the straight hair allele, and we’ll let the letter lowercase s represent curly hair. These two terms, gene and allele, parallel the next two terms, characteristic and trait. A characteristic is a common feature among the members of a species, and our example here is hair texture. So, in our example, hair texture is a characteristic, and hair texture is a gene. And traits since they’re versions of a characteristic, like the straight or curly hair in our example, are based on the combination of alleles an individual inherits from their parents. So, genes are to characteristics as alleles are to traits.

Next, let’s take a look at dominant and recessive alleles. Here, we have chromosome pair number one with the hair texture gene we’ve been talking about in our scenario and the two alleles for straight or curly hair texture and the letters capital S and lowercase s that we’re using to represent them. If a person inherits two capital S alleles, they’ll have straight hair. And their hair will still be straight even if they only inherit just one capital S allele. But inheriting a lowercase s allele from each parent results in having the trait of curly hair. It’s easy to see that having two capital S alleles is gonna lead to straight hair and that two lowercase s alleles is gonna result in curly hair.

But why would a person with a capital S and a lowercase s have straight hair? Although the details can vary, the expression of some alleles can mask or cover up the expression of other alleles. In this case here, a straight hair allele will cover up a curly hair allele. So, we say the straight hair allele is dominant over the curly hair recessive allele. And we use capital letters to represent dominant alleles and lowercase letters to represent recessive ones.

Next, we’ll take a look at another set of terms, genotype and phenotype. The term phenotype refers to the physical characteristics of an organism. And in our example, these characteristics can be either straight hair or it can be curly hair, while the term genotype refers to the genetic makeup of an organism and often consists of a pair of letters that represent the alleles of a gene. In our example, the genotypes that result in straight hair are capital S capital S or capital S lowercase s. And the only genotype that results in curly hair are two lowercase s’s.

Another set of terms can help replace all the repetitive talk about capital letter this and lowercase letter that. And these terms are homozygous and heterozygous. Homozygous means having two of the same alleles for a gene. So, you can have two capital letters for a genotype and be homozygous dominant. Or you can have two lowercase letters at a gene and be homozygous recessive. The term heterozygous, on the other hand, means having two different alleles for a gene such as the capital S lowercase s in our example.

Well, okay, we’ve got our key genetics terms that we were gonna cover all covered. So, let’s put them to use and finally figure out how Dan inherited his curly hair.

Here’s Dan’s family. And they all have straight hair except for Dan. So, let’s use what we’ve learned to make sense of this. Here’s chromosome pair number one and the location of the hair texture gene, which determines a person’s hair texture. This gene comes in two different versions or alleles. One allele results in straight hair and the other one in curly hair. Dominant alleles are represented by a capital letter, and recessive alleles are represented by lowercase letters. Since each of the family members will inherit two of these alleles, one from their mom and one from their dad, there are three possible combinations: capital S capital S, capital S lowercase s, and lowercase s lowercase s.

And these allele combinations are called genotypes. Genotypes with two of the same alleles are called homozygous. If both the alleles are dominant, we can say that the genotype is homozygous dominant. And if there are two recessive alleles, the genotype can be said to be homozygous recessive. And a genotype with both a dominant and recessive gene is called heterozygous. Finally, phenotype is the physical expression of the genotype. So, as long as there’s at least one dominant allele, the phenotype will be straight hair. But if both alleles are recessive, the hair will be curly.

Now, we can use that information to add a capital S to the genotype of every family member that has straight hair since you only need one capital S to have the straight hair trait. We can also fill in Dan’s genotype with two lowercase s’s since that’s the only way he can have naturally curly hair. Where else can we put lowercase s’s? If you guessed one for each of Dan’s mother and Dan’s father, you’re right. They had to each have a lowercase or recessive curly hair allele to give to Dan in order for Dan to have curly hair.

We still have one lowercase s that we can place on one of the four remaining blanks. Where should it go? Yes, to Destiny. Her father is only able to pass on lowercase s alleles to his daughter, where her mother, Chloe, gave Destiny a dominant straight hair allele. And we know that for sure because Destiny’s phenotype is straight hair. Well, we did it! We used our new vocabulary and understanding of genetic inheritance to determine how Dan’s able to have this curly hair in a family full of straight-haired people.

Let’s work a genetic inheritance practice problem next.

A pea plant has the following two alleles: capital P capital P, for purple flowers. Which of the following terms can be used to describe this pea plant? (A) Homozygous recessive, (B) homozygous dominant, (C) heterozygous recessive, (D) heterozygous dominant.

Key knowledge required to select the correct option here is an understanding of genetic inheritance and the terms used to discuss such things, especially alleles. So, let’s start reviewing by connecting the term alleles with a pea plant. To get to the alleles, we need to take a look inside one of the pea plant cells, more specifically, inside the nucleus, where the DNA-containing chromosomes are located. Pea plants have 14 chromosomes, but our cell here only contains seven blue chromosomes right now to show that half its chromosomes come from one parent and this green half from the other parent. But to see the alleles, we should take a closer look at chromosome pair number one here, where the flower color gene is located.

Recall that genes are sections of DNA that usually code for proteins and affect an organism’s traits, like color. And this is also where we find the two alleles. Since pea plants, humans, and other sexually reproducing species receive one chromosome of each pair from each parent, the genetic information on the chromosomes is highly similar but often not completely identical. So, alleles then are different versions of the same gene and lead to varying traits such as pea plants having more than one possible flower color.

The two alleles from the question, though, are identical and cause purple flower color. And each allele is represented by the capital letter P. But why a capital rather than a lowercase p? Well, capital letters represent dominant alleles. And dominant is an important term in our question, along with the term recessive. Recessive alleles are represented by lowercase letters. And that brings us to the term genotype. Since there’s only two possible alleles for this gene, capital P and lowercase p, we’re gonna have three possible genotypes. We can have capital P capital P as in the question, we can have capital P lowercase p, or we could have lowercase p lowercase p.

The term homozygous can be used for genotypes that have identical alleles like capital P capital P or lowercase p lowercase p. We can extend these terms by adding dominant and recessive. Capital P capital P genotype can also be called homozygous dominant, and the lowercase p lowercase p genotype can also be called homozygous recessive. A genotype that contains both a capital and a lowercase allele is called heterozygous. And we’re ready to answer our question. Which of the following terms can be used to describe a pea plant with two capital P alleles? And the answer is homozygous dominant.

Here’s some of the key points from the video. Many of our characteristics are determined by sections of DNA that are called genes. And DNA coils into structures called chromosomes. We inherit half our chromosomes from each parent, forming chromosome pairs. A gene can have more than one form called alleles. And we get one allele from each parent for most of our genes. Dominant alleles are expressed when one or two such alleles are present, and they’re represented by capital letters. For a recessive allele to be expressed, two copies must be present, and they’re represented by lowercase letters.

Genotype refers to the genetic makeup of an organism, and it’s usually written as pairs of letters for a particular gene. So, from our diagram, we could have combinations such as capital B capital B, capital B lowercase b, or lowercase b lowercase b. We can also use the terms homozygous dominant, heterozygous, or homozygous recessive to describe these genotypes. And finally, the term phenotype is the physical expression of a trait.

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