Genes & Alleles - Online Tutor, Practice Problems & Exam Prep

Genes are segments of DNA that encode specific proteins, influencing traits like eye color. Different versions of genes, known as alleles, can be represented by capital and lowercase letters (e.g., B for blue eyes, b for brown eyes). Cell ploidy refers to the number of chromosome sets in a cell: haploid (n) has one copy, while diploid (2n) has two copies, one from each parent. Understanding these concepts is crucial for studying genetics and meiosis.

Genes & Alleles

In this video, we're going to briefly introduce genes and alleles. Genes are defined as small segments of DNA that encode very specific proteins. These proteins can lead to the expression of a specific trait. For example, there could be a gene for eye color. Because this is a gene, it means that it's a small segment of DNA and it's going to code for a very specific protein. This protein, in this case, is going to lead to the expression of a very specific trait or eye color. We know that people can have different eye colors; some have blue eyes, while others have brown eyes. This means that there can be different versions of genes. So, there can be a gene for eye color, but there can be different versions of the gene for eye color, which scientists refer to as alleles. Alleles are defined as alternative or different versions of specific genes. There can be a gene for eye color but different versions of the gene for eye color. There could be a version of the gene for blue eyes, and there can be another version of the gene for brown eyes. The blue eye and brown eyes represent different alleles.

It's important to note that, moving forward in our course, alleles are typically represented using capital and lowercase letters. In a different video, we'll talk more about genetics and how these different alleles can interact with each other in a diploid organism. For the sake of an example, the capital letter B could represent an allele for blue eyes and the lowercase letter b could represent another allele for brown eyes.

Looking at our image below at the alleles for eye color, you'll notice that we're showing you two replicated chromosomes. We have one replicated chromosome on the left and another replicated chromosome on the right-hand side. These are replicated chromosomes because they each have two sister chromatids. Those are the two sister chromatids for the left one, and the two sister chromatids on the right are highlighted. We know that these are replicated chromosomes. Here, there is a version of a specific gene, an allele represented with the capital letter. This would be, for example, just gene A, and the capital letter version of gene A. Notice that this chromosome also has the capital letter version of gene A. These represent identical genes, identical alleles. But notice for this other gene on the chromosome for B that this one on the left has the capital B version of this gene or the capital B allele, and this one has the lowercase b version of the gene or the lowercase b allele. Perhaps the capital B version of the gene represents the blue eye allele. Notice that the capital B here is coding for blue eyes, and you can see an individual here with blue eyes. Then, perhaps the lowercase b represents the allele for brown eyes. This is the brown eye allele, and you can see an individual here with brown eyes. These genes and alleles are responsible for many of the traits that organisms display.

Latter in our course, we'll talk a lot more about genetics and the combination of these alleles within a diploid organism. But for now, this concludes our brief introduction to genes and alleles, and we'll be able to talk more about these as we discuss meiosis moving forward in our chapter. I'll see you all in our next video.

Alternate forms of the same gene are called:

a) Chromatids.

b) Centromeres.

c) Chromosomes.

d) Alleles.

• A. Chromatids.
• B. Centromeres.
• C. Chromosomes.
• D. Alleles

Haploid vs. Diploid Cells

In this video, we're going to distinguish between haploid and diploid cells, but first, we need to define the term cell ploidy. Cell ploidy is defined as the number of copies or sets of specific genes or chromosomes found in a cell. For the purposes of this course, there are really 2 specific terms that you should be familiar with, which we have numbered down below, number 1 and number 2.

The very first term that you should know is haploid, commonly symbolized with the letter n. Haploid cells are going to be cells that only have one copy of each gene or chromosome. Now, the second term that you should know is diploid, and the term diploid is commonly abbreviated using the symbol 2n. Diploid means that the cell is going to have 2 copies of each gene or chromosome. Of these two copies, one copy is going to be inherited from each of the parents. One copy would be inherited from the mother and the other copy would be inherited from the father.

One way that helps me remember the difference between haploid and diploid is that the root 'di' in diploid means 2. Diploid means that it's going to have 2 copies of every gene or chromosome. The root 'haplo' in haploid can remind you of 'half.' Haploid cells are going to have half the number of chromosomes as diploid cells. So, they have one copy of each gene or chromosome instead of having two copies.

If we take a look at our image down below, we can further distinguish between haploid and diploid cells. Notice over here on the left-hand side we're showing you a haploid cell, commonly abbreviated with just the letter n. The haploid cell has one copy of the tall chromosome in blue and one copy of the short chromosome in a reddish color.

Now, if we compare that to the diploid cell over here on the right-hand side, again, diploid is commonly abbreviated with the symbol 2n. Notice that there are 2 copies of the tall chromosome: one copy in blue inherited from the father, and another copy of the tall chromosome inherited from the mother in reddish-pink. There are also 2 copies of the short chromosome: one copy in blue inherited from the father and one copy in reddish-pink inherited from the mother. So, there are 2 copies of every chromosome, and of course, there are genes on the chromosome, so there are also 2 copies of every gene in the diploid cell. Here you can see a cell, and its nucleus, with the diploid number of chromosomes here.

Distinguishing between haploid and diploid cells is going to be very important as we move forward in our course and talk about meiosis. This here concludes our brief introduction to haploid versus diploid cells, and we'll be able to get some practice applying these concepts as we move forward in our course. So, I'll see you all in our next video.

Genes & Alleles Example 1

Oh, everyone. Which of the following statements is true of a species that has a chromosome number of n=16. Okay. So first off, what do we know? What does 2n stand for? Remember that 2n means diploid, meaning 2 of each chromosome. And the diploid number here is 16. So let's write that down. So 2n=16. Now, what do we know about the haploid cells? Well, haploid is symbolized by simply an n. So to get n, simply divide 16 by 2, and you're going to get 8 chromosomes. A diploid cell of this species is going to have 16 chromosomes and a haploid cell of the species has 8 chromosomes. So knowing that, let's look at our answers.

Answer choice A says, the species is diploid, okay, perfect, with 32 chromosomes per cell. Remember, the diploid number is 16. It's not 32. So that's not the answer. Answer choice B says each haploid cell of the species has 16 chromosomes. Remember, that's not true either because we just mentioned that the haploid number is 8. So that's not what we're looking for. Answer choice C says each diploid cell of this species has 16 chromosomes from the father and 16 chromosomes from the mother. Well, if there's 16 from mom and 16 from dad in a single cell, that means there's 32 in total. And we know that can't be true because the diploid number is 16, not 32. So that's not the answer we're looking for.

Answer choice D says each diploid cell of the species has 8 chromosomes from the father and 8 chromosomes from the mother. Eight chromosomes from mom plus 8 chromosomes from dad equals 16 total chromosomes, which is the diploid number, making it a perfect answer. Thus, for this mystery species that happens to have diploid cells with 16 chromosomes, 8 of those chromosomes are coming from mom, and 8 of those chromosomes are coming from dad. Okay, everyone. Let's move on to our next video.

A cell that has 2 copies of each chromosome is called a __________ cell:

a) Sperm.

b) Diploid.

c) Haploid.

d) Gamete.

• A. Sperm.
• B. Diploid.
• C. Haploid.
• D. Gamete.

All human cells, except sex cells, are diploid and have 23 pairs of chromosomes. Human sex cells, such as egg and sperm, are haploid and have _______ chromosomes.

a) 23

b) 12

c) 11.5

d) 46

• A. 23
• B. 12
• C. 11.5
• D. 46

Here’s what students ask on this topic:

What is the difference between a gene and an allele?

A gene is a segment of DNA that encodes a specific protein, which in turn influences a particular trait, such as eye color. An allele, on the other hand, is a variant form of a gene. For example, the gene for eye color can have different alleles, such as one for blue eyes and another for brown eyes. These alleles can be dominant or recessive, and their combination determines the trait expressed in an organism.

How are alleles represented in genetics?

In genetics, alleles are typically represented using capital and lowercase letters. A capital letter denotes a dominant allele, while a lowercase letter represents a recessive allele. For instance, if B represents the allele for blue eyes (dominant) and b represents the allele for brown eyes (recessive), an individual with the genotype Bb would have blue eyes, as the dominant allele masks the effect of the recessive allele.

What is cell ploidy and how does it relate to haploid and diploid cells?

Cell ploidy refers to the number of sets of chromosomes in a cell. Haploid cells (symbolized as n) contain one set of chromosomes, meaning they have one copy of each gene. Diploid cells (symbolized as 2n) have two sets of chromosomes, with one set inherited from each parent. This means diploid cells have two copies of each gene, which is crucial for sexual reproduction and genetic diversity.

Why is understanding haploid and diploid cells important in genetics?

Understanding haploid and diploid cells is essential in genetics because it helps explain how genetic information is passed from parents to offspring. Haploid cells, such as gametes (sperm and egg), contain one set of chromosomes. When two gametes fuse during fertilization, they form a diploid zygote with two sets of chromosomes. This process ensures genetic diversity and the correct number of chromosomes in offspring, which is fundamental for studying inheritance patterns and genetic disorders.

How do dominant and recessive alleles interact in a diploid organism?

In a diploid organism, each individual has two alleles for each gene, one inherited from each parent. Dominant alleles are expressed even if only one copy is present, while recessive alleles are expressed only when two copies are present. For example, if B is a dominant allele for blue eyes and b is a recessive allele for brown eyes, an individual with the genotype Bb or BB will have blue eyes, while only an individual with the genotype bb will have brown eyes.