Punnett Squares: Study with Video Lessons, Practice Problems & Examples

A Punnett square is a diagram used to predict the genotypes and phenotypes of offspring from specific parental traits. The process involves three steps: drawing the square, aligning parental alleles, and filling in the squares to represent fertilization. Analyzing the results reveals the possible genotypes, such as heterozygous combinations, and their corresponding phenotypes. For example, a cross between a homozygous dominant and a homozygous recessive pea plant results in all offspring displaying the dominant phenotype. Understanding this tool is essential for studying inheritance patterns and genetic probabilities.

Punnett Squares

In this video, we're going to begin our introduction to Punnett squares. And so a Punnett square is really just a diagram. It's a very specific diagram that is going to show the possible genotypes and phenotypes of offspring for a specific trait. Now as we'll see moving forward in our course, Punnett squares are going to represent both meiosis or gametogenesis, as well as fertilization or gametofusion. Now again, Punnett squares show the possibilities that offspring will inherit a specific trait. And we'll get to talk more about how to use Punnett squares in our next video. But down below, what we have is this really interesting image which is showing you a pea plant over here that's saying, "Hey, wanna make baby peas with me?" to this other pea plant over here. And notice that this other pea plant is saying, "Only if one of them will be green." Let's check the Punnett square. And so notice here is the Punnett square in this pea plant's hand, and again, you can use Punnett squares to determine the possibilities that the offspring will inherit specific traits. And so, again, we'll be able to talk about how to use a Punnett square in our next video. So I'll see you all there.

How to Use Punnett Squares

In this video, we're going to talk about how to use Punnett squares, which it turns out that it is a 3-step process to using Punnett squares. Now we're going to be making a Punnett square for these 2 pea plant individuals from our last lesson video. This pea plant over here, which is homozygous dominant because it has 2 dominant alleles or 2 uppercase Ys, and this pea plant over here which is homozygous recessive because it has 2 recessive alleles or 2 lowercase Ys. And so we'll be able to create a Punnett Square for these two organisms. And so in the very first step of creating a Punnett square, step number 1, of course, you have to draw the square itself, which is going to be a square, with 4 squares within it. But then after you draw the square, you are going to simply align the alleles of the parent gametes on the top and the left side of the square. And this is going to represent the process of meiosis, and recall meiosis is gamete formation. And so notice in step number 1, again, all we're going to do is align the gametes of the parent on the top and the side. And so notice that parent number 1 up above here, is, homozygous dominant, it has 2 capital Ys. So when it undergoes meiosis, each of the gametes can only have a capital Y, one capital Y. And for this parent number 2 over here, which is homozygous recessive to lowercase Ys, each of its gametes can only have one lowercase Y, which I'll draw with a lowercase cursive here to distinguish it easier from the capital Y. And so that's it for step number 1, align the alleles of the parent gametes on the top and left side of the square representing meiosis, gamete formation. And then in step number 2 of using the Punnett square, all we need to do is actually just fill in the Punnett square itself. And this process is going to represent fertilization or the fusion of the gametes. And so, when we take a look at the square this first square over here at the top left, this one's going to represent the fusion of this gamete with this gamete up here at the top. And so what we need to do is, bring down those, fill in these gametes. So this capital Y here is gonna go in this position and this lowercase Y here will go in this position. And that represents again the fusion of these 2 gametes right here in this box. So that represents fertilization. But then what if this gamete fuses with this gamete over here? Well, then we need to fill in, this box right here. So that would be bringing down this capital Y to this position and bringing across this lowercase Y over to this position over here, And so we would get this combination. And then of course this box down here represents the fusion of this gamete with this gamete here at the top. And once again you just bring down the capital Y here to this position, and you bring across the lowercase Y here to this position. And then, this last box over here represents the fusion of this gamete with this gamete here at the top. And so again, all you need to do is take the capital Y here and bring it all the way down, and bring the lowercase Y here and bring it all the way across. And so now what you can see is that we've completely filled in each of these 4 squares within our Punnett square. So we've completely filled in our Punnett square and we've completely finished step number 2, which again represents fertilization or the fusion of gametes. And so step number 3 here is really just to analyze the results, or in other words, analyze the possible genotypes and phenotypes of the offspring. And so when you take a look at step number 3 over here, analyze the results, of each of these squares, what you'll notice is that each of these 4 squares is showing a heterozygous genotype, 1 dominant allele and one recessive allele. And so, recall that the dominant allele, the capital Y, is going to dominate over the recessive allele. So that means that the capital Y, the yellow allele, is going to dominate, and each of these squares here represents a heterozygous yellow offspring. And so what we can say by analyzing the results is that there are 4 possibilities for yellow phenotypes in the offspring, and there are 0 possibilities for green phenotypes in the offspring. Again, to get a green phenotype, one of these squares would have to have 2 lowercase Ys, but that is not the case here. And so what's important to note is that each of these squares that you see here represents an equally probable genotype and phenotype that one single offspring can inherit. So these represent the possibilities. And so, later in our course, we'll be able to talk more about calculating probabilities when it comes to Punnett squares. But what's really important to note is that each fertilization event producing an offspring is going to be independent of each other. And so one fertilization event will not impact another fertilization event. And so that's exactly what we mean by independent. One fertilization event does not impact another fertilization event, which technically means that for each fertilization, for each offspring, this Punnett square would need to be reconsidered and redone essentially. And, again, we'll get to talk more about probabilities as they apply to Punnett squares later in our course. But for now, this here concludes our lesson on how to use Punnett squares, and we'll be able to apply these concepts as we move forward in our course. So I'll see you all in our next video.

Mendel found that green pea pod color (y) was recessive to yellow pea pod color (Y). For the cross Yy × yy, what percentage of offspring are expected to be yellow?

• A. 100%.
• B. 75%.
• C. 50%.
• D. 25%.

A female dog with black fur (Ff) mates with a male dog that also has black fur (Ff). Determine the possible genotypes and phenotypes of their puppies using a Punnett Square. Black fur (F) is dominant to grey fur (f).

a)  # of possible Genotypes:

FF: ________ 

Ff: ________ 

ff: _________

b)  % of possible Phenotypes: 

Black fur: __________

Grey fur: ___________

Okay, everyone. A female dog with black fur mates with a male dog that also has black fur. Determine the possible genotypes and phenotypes of their puppies using a Punnett square. And then they tell us that black fur is dominant to gray fur. Okay. So there's a lot of stuff going on on the screen in this question, but let's just do the Punnett square and then see if we can fill in all of these blanks. This shouldn't be too difficult. Okay. So remember, for our Punnett square, we're going to draw our square. And then, we're going to have one parent on the top and one parent on the side. Okay. So now, let's just figure out the offspring. So we're going to have one homozygous dominant offspring. We're going to have two heterozygous offspring and one homozygous recessive offspring. So now, let's have a look at our fill-in-the-blanks. Okay. So the first fill-in-the-blanks says the number of possible genotypes for homozygous dominant, that's 1. For heterozygous, that's 2. And for homozygous recessive, that's 1. So now let's talk about our possible phenotypes. So remember that they tell us that black fur is dominant to gray fur. So that means that this individual has black fur because they have two of the dominant alleles. And this individual and this individual here are also black-furred puppies because they have at least one dominant allele. So all the ones in yellow have black fur. And this one here, the homozygous recessive, has to have gray fur because gray is the recessive allele. So be careful. This says percent of possible phenotypes. So for Black fur, 3 out of the 4 possible puppies have Black fur. So that is 75%. And one out of the 4 puppies has gray fur, so that is 25%. Okay, everyone. Let's move on to our next video.

Here’s what students ask on this topic:

What is a Punnett square and how is it used in genetics?

A Punnett square is a diagram used in genetics to predict the possible genotypes and phenotypes of offspring from specific parental traits. It involves three steps: drawing the square, aligning the parental alleles on the top and left side, and filling in the squares to represent fertilization. This process helps visualize how alleles from each parent combine and the resulting genetic makeup of the offspring. For example, crossing a homozygous dominant (YY) with a homozygous recessive (yy) results in all heterozygous (Yy) offspring, displaying the dominant phenotype. Understanding Punnett squares is crucial for grasping genetic inheritance and probability in offspring traits.

How do you create a Punnett square for a monohybrid cross?

To create a Punnett square for a monohybrid cross, follow these steps: 1) Draw a square divided into four smaller squares. 2) Align the alleles of the parent gametes on the top and left side of the square. For example, if one parent is homozygous dominant (YY) and the other is homozygous recessive (yy), place 'Y' and 'Y' on the top and 'y' and 'y' on the side. 3) Fill in the squares by combining the alleles from the top and side. This represents fertilization. The resulting squares will show the possible genotypes of the offspring. In this case, all squares will be 'Yy', indicating heterozygous offspring with the dominant phenotype.

What are the steps involved in using a Punnett square?

Using a Punnett square involves three main steps: 1) Draw the square, which is a larger square divided into four smaller squares. 2) Align the alleles of the parent gametes on the top and left side of the square. This step represents meiosis, where gametes are formed. 3) Fill in the squares by combining the alleles from the top and side, representing fertilization. This step shows the possible genotypes of the offspring. Finally, analyze the results to determine the possible phenotypes. For example, a cross between homozygous dominant (YY) and homozygous recessive (yy) results in all heterozygous (Yy) offspring, displaying the dominant phenotype.

How do you interpret the results of a Punnett square?

Interpreting the results of a Punnett square involves analyzing the genotypes and phenotypes of the offspring. Each square within the Punnett square represents an equally probable genotype. For example, if you cross a homozygous dominant (YY) with a homozygous recessive (yy), all four squares will show 'Yy', indicating heterozygous offspring. The dominant allele (Y) will mask the recessive allele (y), resulting in the dominant phenotype. Therefore, all offspring will display the dominant trait. This analysis helps predict the likelihood of specific traits appearing in the offspring, which is crucial for understanding genetic inheritance and probability.

What is the significance of independent fertilization events in Punnett squares?

Independent fertilization events in Punnett squares mean that each fertilization event is separate and does not affect the outcome of another. This concept is crucial because it ensures that the probabilities calculated using Punnett squares are accurate for each offspring. For example, if a Punnett square predicts a 75% chance of a dominant phenotype and a 25% chance of a recessive phenotype, these probabilities apply to each offspring independently. This independence is essential for understanding genetic inheritance and accurately predicting the distribution of traits in a population.