Monohybrid Cross: Videos & Practice Problems

A monohybrid cross examines the inheritance of a single gene by mating organisms with different alleles. Using a Punnett square, one can visualize the potential genotypes and phenotypes of offspring. For example, crossing two heterozygous purple plants (Aa) results in three genotypes: AA, Aa, and aa, leading to two phenotypes: purple and white. In another example, crossing a wild type (homozygous) with a mutant (homozygous) yields only one genotype and phenotype, demonstrating dominance. Understanding these principles is crucial for exploring more complex genetic interactions.

Monohybrid Cross

So a monohybrid cross is just what it sounds like. So mono, you get the idea of 1, and crossing means that you're getting the idea of mating something. This is a mating that occurs between organisms to look at different alleles at one gene. That's where you get the monohybrid from. We're looking at 1 gene, we're looking at the offspring produced by this mating, from this one gene. And so, when we look at monohybrid crosses, we do something called a Punnett square, which I'm sure that you've heard of before. But remember that the genotypes of organisms can be presented in different ways. So you can have the dominant recessive with the uppercase and the lowercase letter. You can also have the wild type mutant and that's usually with a plus sign or the absence of a plus sign. Let's remember this when we go through our examples.

I have two examples here that we're going to walk through on how to do this monohybrid cross using a Punnett Square. We're going to start with this one first, the 2 heterozygous purple plants. Now, this is the mating. 2 heterozygous purple plants, that's who you're mating. What does that tell you about the genotypes and the phenotypes of the mother and the father plant? Remember, genotype refers to alleles, phenotypes are what you can see. Let's just start with genotypes. What in this question or statement gives you information about the genotypes? Right. That's heterozygous. Heterozygous means two different alleles. So, if it's heterozygous, we know that it has to have 1 dominant and one recessive. The genotypes would look like Aa and Aa because that's heterozygous. I chose A just randomly. You can choose whatever letter you want, but that's what the genotypes would be.

Now, if it asks about the phenotypes, I kind of already gave this to you, but one in here says what the phenotype is. That's going to be the purple, because that's what you can see, that's a color. The phenotypes for both the mother and the father are purple. Now, we're going to do this Punnett square, which is this thing, I'm sure you know, a Punnett square. Then we need to know, we need to have an allele and gametes. Here are the gametes, you have 1 gamete, 2 gametes, 3 gametes, 4 gametes. Remember, the gametes, do they contain 2 alleles or 1 allele? Right? They contain 1 allele. So, you want to give 1 allele from the mother. You can do this one for the mother, this one for the father. If you want to flip them, you totally can, it's completely up to you. But you need, in each one of these squares where I have numbers, you need one allele. So if we're going to say that this is the mother, then you take one uppercase letter A, which is the dominant and one lowercase letter a, which is the recessive. Remember, where did I get this from? This is the genotype and I took one allele and put them in each of the boxes. Now, for the father, you do the same thing.

Take 1 allele in each of the boxes. Now, once you have these gamete alleles, these 4 are the gametes. Let me write that, 1, 2, 4 equals gametes. Now, when these two organisms mate, those gametes will come together and fertilize. And so, when they come together, you now have to make those pairs to make the offspring. So the first thing you do is you take the letter from each corresponding row or column. So, this A comes down here, and this a comes here. So now you have an organism with 2 dominant alleles. If we repeat this again, you would get this a comes here, and you would get this A comes here. So now you have an organism with 1 recessive and 1 dominant allele. You keep repeating this. You get this A down here and this a here, and you do the same thing over here. You have A and a. Let me go over this. So this is Aa and Aa, so the colors match. There are 4 offspring. Here are the 4 offspring. They have these different genotypes. When asked about the offspring genotypes, what are you going to say? There are 4 genotypes, or how many genotypes are there first of all? You might say 4 because there's 4 boxes, but if you notice and look at them, one is actually the same genotype. So you have 2 dominants, you have a dominant and recessive, and you have 2 recessives. This is considered a genotype; it's written here because the order of the alleles does not matter. Both of them are heterozygous and both of them are the same genotype. There are only 3 genotypes. This one, this one, and this one. Now when it comes to phenotypes, how many phenotypes are there? There are 2 phenotypes. Any of the ones with a capital A in them would be dominant. We know from the question that purple is dominant because both of the parents are purple, and they're both heterozygous. That's how we know that dominant is purple. We have purple. And then, I didn't give it to you, but it's just pretty much not purple. You can choose whatever color you want. Purple is dominant, and we'll just say white is recessive. So, those are the 2 phenotypes. This one I just pulled out is pretty much just not purple, because I didn't give it to you, but it doesn't really matter.

That is how you do a Punnett Square or a monohybrid cross using dominant and recessive. Let's do this one now, and this is with wild type and mutant. These are looking at flies, and these are wild type wings and short wings. You have normal wings and short wings. This is the mating. So we have a wild type mutant short wing fly. It's asking for the genotypes and the phenotypes. The genotypes we're going to cross are going to be, and I'm going to tell you this because you can't necessarily tell this from the question. So I'm just going to give it to you. The wild type wing fly will say is homozygous or wild type. Do you remember if you're talking about wild type you use the plus? And then, the mutant short wing is going to be homozygous for the mutant. This is the short wing and this is the normal wing. The phenotypes I complete

A black and white rabbit were mated. All F₁ offspring were black, and the F₂ offspring is made up of approximately ¾ black and ¼ white rabbits. 
• Draw out two Punnet squares detailing both matings.
• Supposed two white F₂ offspring were mated. What would be the phenotype and genotype of the F₃ offspring? 

• A. White, aa
• B. White, Aa
• C. Black, Aa
• D. Black AA

Answer: (A)

Green scales (G) in a particular species of fish is dominant over blue scales (g). The following crosses are carried out, producing the progeny shown. Write out all possible genotypes of the parents in each cross. 

Okay. So this question is similar to the previous question, in which we're going to take the progeny phenotypes, and work back to figure out what the genotypes of the parents are. So in this case, we're working with fish, and in this one it says green scales, which is this uppercase G, is dominant to blue scales, which is the lowercase g, which means that if you have a blue fish, that genotype is going to be homozygous recessive. Because the only way to get blue is to have two of these lowercase g alleles, so therefore, homozygous recessive. So we're carrying out a bunch of crosses. Here they are. And it's producing this progeny. So this is the offspring. Progeny is another word for offspring. And, what we are interested in figuring out are all the possible, and it usually can be more than one, all the possible genotypes of the parents from this situation here. So let's walk through the first few, and then I want you to pause it and do the last couple by yourself and see if you can figure it out. So let's start with the first one. So the first is we have two green parents, meaning that at least each of them has at least one of these, uppercase G or dominant G alleles. What they do is they produce green and blue offspring. So how would two green parents produce some green and some blue offspring? What are the possible genotypes they can have? Well, if we draw a Punnett square, for this example, we kinda don't know these parts. Right? But we do know that some blue offspring exists. So at least one of these squares is going to be blue. It's gonna be homozygous recessive or this blue color. Now, in order for this to happen, we have to get a recessive allele from each parent. So that means one of the parents has a lowercase g, and the other parent has a lowercase g. So that's great. We figured out the alleles for each of these, each of the parents. But we also know something else about the parents, and we know that both parents are green. Right? They have green scales. And in order to be green, they at least have to have one, uppercase G allele. So that means they both have to have this uppercase G, and therefore, the possible genotypes of the parents are heterozygous for the green allele. Does that make sense? So if you had any other combination, if this parent here had two uppercase G's, or two dominant alleles, you would not produce a blue progeny. So that wouldn't be possible. And I suggest if you are a little bit skeptical, go through, think of all the different genotypes that you can produce, and do Punnett squares of them and see if I'm correct. But in order to produce blue offspring, both parents have to have a blue allele. And because they're both green, they both have to have a green allele, and, therefore, this is the only genotype that's possible. So let's do the second one. So again, it's a green and a green cross, so both parents are green. But in this case, they're producing eight green offspring. So there are, a few different options that can occur. Right? So if we were to do a Punnett square, we know that all the offspring have to be green. Right? And so they allele . So in order to do that, you have to have mostly dominant alleles, but you don't have to have all. So one of the options is that you actually do have all. Right? So it would produce these offspring here. And so this is an option for one of the parents. But there's another allele, right, one of them could still be heterozygous. Right? So we would get, in this case, we would get this. And all of them would still be green, even if one of the parents is heterozygous. So this is one option, and this is one option. Now, you might say, okay, how did you do that? You kind of knew going in what the answer was going to be, and so you were able to figure this out. Well, if you're struggling with this and you don't know, the best way to do it is to think of all the possible genotypes and just do the Punnett squares. And over time, as you start practicing these Punnett squares, you're going to recognize patterns, and those patterns will become familiar to you. So even if you're not just saying, oh, these are the only two options immediately, like I just did, try some other ones. Try this and see what you get. Try this and see what you get. And see if it actually matches the problem. And if it doesn't, you know that it can't be it. And if it does, you know it's a possibility. So until you start, like, being able to do this, you know, very frequently on your own, quickly recognize these patterns, feel free to draw out as many Punnett squares as you need to answer the question. So the third one here is you have a green and a blue parent, and they produce all green children. So let me back up here. So all green offspring. So because we know that all the offspring are green, that means we have to have all the offspring have at least one dominant allele. So we figured this out here. Now, we know something else about the parents. One of them is blue. And a blue parent is going to have what genotype? Right. It's going to be homozygous recessive because that's the only way to get, that blue phenotype. So if the offspring have all dominant, at least one dominant, uppercase green allele, and we know that they're getting a lowercase allele from this other parent, this other blue parent, then the only other option is for this parent here to be homozygous dominant for the two greens. Right? So the answer here is homozygous dominant and homozygous recessive. And that's the only combination that will produce 12 green from a green and a blue parent. So, oh, so now we've got 3 of them. Take a second, pause the video, see if you can do d and e by yourself, and then come back in a second and see if you did it correctly. And remember, practice all the Punnett squares you need. Okay. So welcome back. Hopefully, you went through the practice. So now we have two blue parents, which makes this super easy because in order to be blue, you have to be homozygous recessive. So the options for the parental genotypes are two homozygous recessives. I know that was a lot. I'm going through each of these, but I think it's good to be able to practice going from the progeny to the parental genotypes, and understanding, you know, how we make these sort of, we critically think about these genotypes or these phenotypes, and turn them into parental genotypes. So feel free to rewatch this video, practice these all yourself, see if you can do that because it is important. Because you likely will have to do this at least once on some kind of examination or quiz or something, in your class. So with that, let's now move on.

Which of the following offspring ratios is expected from a Mendelian heterozygous cross examining one gene?

• A. 2:2
• B. 3:1
• C. 9:3:3:1
• D. 4:2:1

Answer: (B)

Human albinism is a simple recessive trait. Determine the genotypes of the parents for each offspring combination 
i. A wild-type male and albino female have 6 wild-type children 

• A. AA x aa
• B. Aa x Aa
• C. aa x aa
• D. AA X AA
• E. AA x Aa

Answer: (A)

Human albinism is a simple recessive trait. Determine the genotypes of the parents for each offspring combination
ii. A wild-type male and albino female have 8 children, 4 wild-type, and four albino 

• A. AA x aa
• B. Aa x Aa
• C. Aa x aa
• D. AA X AA
• E. AA x Aa

Answer: (C)

Here’s what students ask on this topic:

What is a monohybrid cross and how is it used in genetics?

A monohybrid cross is a genetic cross between two organisms that are heterozygous for a single trait. It is used to study the inheritance of one specific gene. By using a Punnett square, we can predict the possible genotypes and phenotypes of the offspring. For example, crossing two heterozygous purple-flowered plants (Aa) can result in three genotypes: AA, Aa, and aa, and two phenotypes: purple and white. This helps in understanding how traits are passed from parents to offspring and the concept of dominance and recessiveness in alleles.

How do you set up and interpret a Punnett square for a monohybrid cross?

To set up a Punnett square for a monohybrid cross, draw a grid with four boxes. Place one parent's alleles on the top and the other parent's alleles on the side. For example, if both parents are heterozygous (Aa), place 'A' and 'a' on the top and side. Fill in the boxes by combining the alleles from the top and side. The resulting genotypes will be AA, Aa, Aa, and aa. To interpret, count the frequency of each genotype and phenotype. In this case, the genotypes are 1 AA, 2 Aa, and 1 aa, and the phenotypes are 3 purple (dominant) and 1 white (recessive).

What are the possible genotypes and phenotypes in a monohybrid cross between two heterozygous organisms?

In a monohybrid cross between two heterozygous organisms (Aa x Aa), the possible genotypes of the offspring are AA, Aa, and aa. Using a Punnett square, we find that the genotypic ratio is 1:2:1 (1 AA, 2 Aa, 1 aa). The phenotypes depend on the dominance of the alleles. If 'A' is dominant and 'a' is recessive, the phenotypic ratio will be 3:1, with three showing the dominant trait and one showing the recessive trait. For example, if 'A' represents purple flowers and 'a' represents white flowers, the phenotypes will be 3 purple and 1 white.

How does dominance affect the phenotypes in a monohybrid cross?

Dominance affects the phenotypes in a monohybrid cross by determining which trait is expressed in the offspring. In a dominant-recessive relationship, the dominant allele (represented by a capital letter, e.g., 'A') masks the expression of the recessive allele (represented by a lowercase letter, e.g., 'a'). For example, in a cross between two heterozygous organisms (Aa x Aa), the phenotypes will be determined by the presence of the dominant allele. If 'A' is dominant for purple flowers and 'a' is recessive for white flowers, the phenotypic ratio will be 3 purple (AA and Aa) to 1 white (aa).

What is the difference between genotype and phenotype in a monohybrid cross?

In a monohybrid cross, the genotype refers to the genetic makeup of an organism, specifically the combination of alleles it possesses (e.g., AA, Aa, or aa). The phenotype, on the other hand, refers to the observable traits or characteristics of the organism that result from the interaction of its genotype with the environment. For example, in a cross between two heterozygous purple-flowered plants (Aa), the genotypes of the offspring could be AA, Aa, or aa. The phenotypes would be the physical appearance of the flowers, which could be purple (dominant) or white (recessive).