Fundamentals of Genetics - Online Tutor, Practice Problems & Exam Prep

Genetics revolves around DNA, which consists of four nucleotides: adenine (A), thymine (T), cytosine (C), and guanine (G). These nucleotides pair specifically (A with T, C with G) to form a double helix structure. Genes, segments of DNA, encode proteins essential for organism function. Alleles are gene variants that determine traits, while genotypes represent the set of alleles, and phenotypes describe observable characteristics. Chromosomes, which house genes, are inherited through gametes produced by meiosis, leading to genetic diversity in offspring.

Genetics Basics

Hi. In this video, we're going to be talking about the fundamentals of genetics. So the very first thing we need to talk about is just the genetic basis. And so most of this is going to be review from an intro biology course that you may have taken, but I want to review it here just to make sure you're refreshed on this material and everybody's on the same page. So DNA is, of course, going to be the fundamental unit of genetics. So what should we know about DNA before we move forward to more complicated things? We need to know that DNA is made up of 4 bases. We call these bases nucleotides. Those bases are going to be adenine (A), thymine (T), cytosine (C), and guanine (G). Here are the fancy names for them. Remember that there's going to be proper pairings. So, certain bases pair with other bases. The person who discovered that was Chargaff, and he made up the rules of what the pairings were. And so his rules say that A pairs with T and C pairs with G, and these pairings are preferred because of the number of hydrogen bonds they can make. So, A and T have 2 hydrogen bonds, and C and G have 3 hydrogen bonds. And so these pairings are done because these pairings are preferred. Now when these pairings occur, they're going to make the strand complementary. So what does that mean? It means that if we have a pairing here, which is A and T, C and G, C and G, T and A, that this side is going to be complementary to this side, meaning that these bonds are going to be the ones that form because the opposite pair so with an A on one side, there's always going to be a T on the other. So that means that they're complementary, and then, the DNA strand is 2 strands and the proper energetic formation of that helix. So DNA, it has these four bases. These bases form A and T, C and G, and it's a double helix. Again, like I said, things that you probably heard about before, but just want to refresh the material here. Now, DNA, we care about it in genetics because DNA contains genes. What are genes? Genes are just going to be stretches of DNA, so a DNA sequence that has some type of information on it, and that information is used to create a protein. And proteins are what allow us to function. So a gene is going to be a structural DNA. It's going to encode for a protein. It's going to tell our cells how to create a protein, and then that protein will go on to have some type of function that keeps us alive or gives us an appearance or does something in the body. Now genes also come with regulatory elements, so not just the gene that encodes for the information, but the body has to regulate when those genes are expressed. So in terms of regulatory elements, these regulatory elements are going to control whether or not the gene is expressed and when it is and where it is whether in a certain cell type or in a certain region of the body. So we have DNA. It consists of these four bases. They pair a certain way. DNA has genes. The genes are controlled through regulatory elements. And then, one of the last things you need to know about this section, about DNA specifically, is, about alleles. And pretty much what an allele is, is because genes come in different forms. Now, one of the most confusing parts of genetics for students is one of the very first confusing parts of it is understanding what an allele is. Like, you hear the word allele and you hear the word gene, you've pretty much got gene down. Like, you've heard gene enough in your life, you know what a gene is. It's a stretch of DNA that encodes for protein. But what in the world is an allele? It's like this weird term that people get really confused about. An allele is just a gene variant. Right? So we have a gene for hair color, and that's our it's a stretch of DNA, and it encodes for hair color. An allele is going to determine whether that hair color is blonde, whether it's brown, whether it's black, whether it's red, whether it's a mixture of strawberry blonde. So those alleles are the different forms that that gene can come in. So a gene is going to be hair color. A gene will not be blonde hair color or black hair color. A gene will be hair color. And then the color that it is, blonde, black, whatever, those are the alleles. And so in diploid cells, diploid is another fancy term, so in humans, we have 2 alleles per gene, and this combination of alleles will tell us what we look like, what or what the gene does. So in the case of hair color, the combination of these two alleles will determine whether our hair is blonde, black, brown, red, whatever color it's going to be. So make sure you understand that an allele is just a gene variant, because people get this confused all the time, and you're going to have a really difficult time in genetics if you do not know the difference between gene and allele, so make sure you get that now. So here we have, an example showing alleles versus gene. So we have our 2 chromosomes, right, because we're diploid, so we have 2 copies of every gene. So we have gene A, gene B, and gene C. Now, gene A is the blue gene, gene, B is the green gene, and gene C is the red gene, but you can see that the alleles represent different variants. So, this allele is like a dark blue, where this allele is a light blue. Now, it's still the blue gene, just different variants. This one is green, and so this is the same allele even on different chromosomes, and that can totally happen. And then we have our red, which we have a dark red and a light red. So make sure you completely understand the difference between alleles and genes, or else you're going to be really confused this whole semester. So, we start off with DNA, so that's what we've been talking about so far, but in genetics, we're not only concerned with DNA, we're also concerned with what that DNA encodes for, what the gene makes, and the gene makes the protein. So how do we go from DNA to protein? Well, in introductory biology, you probably talked about this, and there are 2 main steps. There's the transcription and the translation step. So the transcription step turns DNA into RNA, and then the messenger RNA is used to create protein, but there are other types of RNA, which we will talk about. If you don't necessarily know them right now, that's okay. So we get so transcription takes DNA to RNA, and then the second step translation takes that mRNA to proteins. So this is really important. We're going to spend some time talking about it. You're probably familiar with at least the basics of this, and this class will go into a little bit more detail. So we know that DNA does not encode a protein in a one to one manner. It's not that all nucleotides produce all the same type of protein. Remember, there's you have to decode it. The DNA to RNA is 1 to 1, but the RNA to protein is 3 to 1. In the case that we have this thing called a codon, and a codon is a collection of 3 nucleotides, so we have adenine (A), cytosine (C), thymine (T), for instance. A which is going to be the building block to build protein. So we start with DNA, we turn it to mRNA, and then every 3 RNA this is mRNA, so it's you, every 3 nucleotides for an RNA is going to make up one amino acid. So here we have an example of what this is. So we start out with our RNA. Remember, our RNA is going to be used to create protein in translation, and so we have our 3 nucleotides here. It encodes for 1 codon, and this codon creates the amino acid alanine. Then, we have our 2nd codon, and this codon creates the amino acid threonine, and this keeps going codon by codon until we stop, and together, all of these amino acids are going to create 1 protein. So, I hope this wasn't too fast for you. Most of this should be or pretty much all of this should be a review from an intro biology class about what DNA is made up of, how it al...

Chromosomal Fundamentals

Okay. So we talked about DNA, let's now talk about chromosomes. Chromosomes come in different types of formations. The one you're going to hear about a lot are homologous chromosomes. These are just pairs. You have 2 chromosomes that are homologous because they are paired together. Now, diploid, another term you're going to hear, is represented as 2n. These are organisms that have homologous chromosomes, meaning they have a chromosome pair. Then, you have haploid, which has one less chromosome. So instead of having a pair, it just has one chromosome copy. Diploid is sometimes written as 2n because of the two chromosomes, and haploid is written as n because of the one chromosome.

Chromosomes are so important for genetics and inheritance because the chromosomal theory of inheritance states that traits come from genes on chromosomes. Without chromosomes, you're not going to inherit anything. The genes sit on the chromosomes, and you inherit the chromosomes that are on those genes. The chromosomes that are inherited are passed through what's known as gametes, which are sex cells. So these are sperm and eggs if we're talking about humans.

Now, how do we get gametes? This is through the process called meiosis. This process produces gametes. In diploid individuals, like humans, you take a 2n cell, a diploid cell. Over time, through a series of steps which we'll discuss in great detail, you end up with four haploid cells. These haploid cells then go on to fuse or combine with other haploid cells to create a diploid organism. In our case, these haploid cells are sperm and eggs. When they come together, they create a diploid zygote. Don't worry if you don't get all of that down now. Hopefully, this is a review, but if not, we're going to talk about this in a lot more detail in the future.

Then there's mitosis, which we're not going to talk about a lot in this class. We're mainly going to focus on meiosis, but mitosis does still exist. We'll talk about it briefly. Mitosis is the process of creating somatic cells. Somatic cells are every cell that is not a gamete. So, they include your skin cells, eye cells, brain cells, toe cells, literally all the cells that aren't sperm and eggs in humans. This process takes diploid cells and turns 2n cells into more 2n cells. So with meiosis and chromosomes, what you get is you start with a diploid organism, it undergoes meiosis, and you end up with haploid cells, so one copy of each chromosome. These are the homologous chromosomes. You end up with one copy after meiosis in each cell. Then, through fertilization, the combining of the sperm and egg, that will create another diploid organism.

Like I said, we'll go over these in a lot of detail. These are just basic review fundamentals. So now, let's turn the page.

Descriptive Genetics

Okay. So now let's talk about a few terms that we use to describe things in genetics. The first is sort of an end on an individual level. We can say that an individual has a certain genotype. What we're referring to with the genotype is the set of alleles. These are going to be the actual alleles for a given trait. For instance, if we want to know about hair color or eye color, then the genotype is going to be which alleles you have. The phenotype is different; it doesn't necessarily care what alleles you have; it just looks at what you look like. So, if we're looking at hair color, it could be blonde, brown, black, red, any of these hair colors; that's the phenotype. You can't actually see the genotype; you have to know what alleles you have. Genes present themselves in many different types of traits. We have morphological traits which are things that affect appearance, like skin color, feather color, height, or size. Then we have physiological traits that affect the ability of an organism to function properly. For example, the shape and function of the lungs can help with the physiological aspect of taking in air and breathing. We also have behavioral traits that affect how organisms respond to their environment. It's harder to think of for humans, even though it exists, but it's easier to consider different mating dances that birds or other organisms perform. When reading about this, you'll notice that the male does some kind of fancy mating dance to attract the female. These are actually behavioral traits that can be inherited through genetics.

Looking at this further, you can see the phenotype is yellow or green, and the genotype is presented here. These refer to different types of alleles. The uppercase letter represents a dominant allele, which you're going to see if it's present, and the lowercase letter refers to something that's recessive, which you'll only see if the dominant is absent. Don't worry if you don't necessarily remember dominant and recessive now; we'll go over this in much more detail. But I just wanted to get you to understand that the phenotype is what you see. The genotype is what you can't see, but we're going to spend a whole semester learning how to actually figure it out.

There are three divisions of genetics. First, there's transmission genetics, which studies inheritance and the ability to pass traits onto the next generation. Then you have molecular genetics, which studies gene activity at the molecular level. Examples of this are DNA replication, transcription, or translation. These are molecular genetic events. Finally, you have population genetics, which studies genes in terms of an entire population. So if we take the entire population of the United States, we can compare traits and genetics with people in Australia or Kenya. These types of population genetics studies will be discussed towards the very end of the semester.

With that overview, let's now turn the page.

In a diploid cell, the number of alleles is the same as the number of genes.

• A. True
• B. False

What is an allele?

• A. A nucleotide variant
• B. Different variants of a gene
• C. Proteins
• D. Codon

The chromosomal theory of inheritance states that….

• A. Traits come from the mixing of fluids from two parents
• B. Traits come from only chromosomes found in the egg cell
• C. Traits come from genes on chromosomes
• D. Traits comes from complementary DNA strands

Which one of the following is NOT a division of Genetics?

• A. Transmission genetics
• B. Molecular genetics
• C. Breeding genetics
• D. Population genetics

Here’s what students ask on this topic:

What are the four nucleotides that make up DNA and how do they pair?

DNA is composed of four nucleotides: adenine (A), thymine (T), cytosine (C), and guanine (G). These nucleotides pair specifically due to hydrogen bonding: adenine pairs with thymine (A-T) through two hydrogen bonds, and cytosine pairs with guanine (C-G) through three hydrogen bonds. This specific pairing is crucial for the double helix structure of DNA, ensuring that the two strands are complementary to each other.

What is the difference between a gene and an allele?

A gene is a segment of DNA that encodes for a protein, which performs a specific function in the body. An allele, on the other hand, is a variant form of a gene. For example, a gene might determine hair color, while different alleles of that gene might result in blonde, brown, black, or red hair. In diploid organisms, such as humans, each gene has two alleles, one inherited from each parent.

How does meiosis contribute to genetic diversity?

Meiosis is a process that produces gametes (sperm and egg cells) with half the number of chromosomes (haploid) compared to the original diploid cell. During meiosis, homologous chromosomes undergo recombination, where they exchange genetic material. This recombination, along with the random assortment of chromosomes, results in gametes with unique genetic combinations. When these gametes fuse during fertilization, they create a genetically diverse diploid organism.

What is the difference between genotype and phenotype?

Genotype refers to the set of alleles an individual possesses for a particular trait. It is the genetic makeup that determines potential characteristics. Phenotype, on the other hand, is the observable expression of the genotype, such as physical appearance or behavior. For example, the genotype might include alleles for both brown and blue eyes, but the phenotype would be the actual eye color observed, which could be brown if the brown allele is dominant.

What are the three main divisions of genetics?

The three main divisions of genetics are: 1) Transmission genetics, which studies the inheritance of traits from one generation to the next; 2) Molecular genetics, which focuses on the molecular structure and function of genes, including DNA replication, transcription, and translation; and 3) Population genetics, which examines the distribution and behavior of genes in populations, often comparing genetic variations across different groups or species.