A Beginner's Guide to Punnett Squares

Summary

Mr. Andersen provides a beginner's guide to Punnett squares, a tool used in genetics to predict offspring traits. He explains common mistakes students make, emphasizing that Punnett squares represent the potential genetic combinations after meiosis. Starting with simple monohybrid crosses, he demonstrates how to predict outcomes for dominant and recessive traits. He then moves to more complex examples like heterozygous crosses, incomplete dominance, codominance, and sex-linked traits. Finally, he tackles dihybrid crosses, emphasizing their complexity and how they apply to traits controlled by multiple genes.

Takeaways

• 🧬 Reginald Punnett, while not directly working with Punnett squares, contributed significantly to genetics.
• 🔄 The two sides of a Punnett square represent gametes produced during meiosis.
• 🌸 A monohybrid cross focuses on one trait, and a homozygous dominant crossed with homozygous recessive produces a uniform result.
• 🧮 Heterozygous crosses result in a 1:2:1 genotypic ratio and a 3:1 phenotypic ratio (dominant vs recessive).
• 🌺 Incomplete dominance results in offspring that exhibit an intermediate trait (e.g., pink snapdragons from red and white alleles).
• 👥 Codominance involves both alleles being expressed simultaneously, unlike incomplete dominance where traits blend.
• 🧑‍🦯 In X-linked crosses, males are more likely to exhibit recessive traits (like colorblindness) due to the lack of a backup X chromosome.
• 🌱 Dihybrid crosses involve two traits and commonly result in a 9:3:3:1 phenotypic ratio.
• ⚠️ A common mistake in dihybrid crosses is forgetting to give one of each allele in the gametes.
• 📏 Most traits, such as height, are influenced by multiple genes, making Punnett squares more complex than the simple monohybrid or dihybrid examples.

Q & A

Who was Reginald Punnett, and why is his name associated with genetics?

Reginald Punnett was a geneticist known for his work with mimicry in butterflies. Although he didn't directly work with Punnett squares, his name is strongly associated with genetics due to the popular use of Punnett squares to solve genetics problems.

What do the two sides of a Punnett square represent?

The two sides of a Punnett square represent the different genetic alternatives after meiosis. Each side corresponds to the gametes, which carry half of the parent's genes.

What do the boxes inside a Punnett square stand for?

The boxes inside a Punnett square represent all the possible combinations of genes that can result from the union of the different gametes during fertilization.

What is a monohybrid cross, and what kind of traits does it involve?

A monohybrid cross focuses on a single trait. For example, it may look at the inheritance of flower color, like crossing purple flowers (homozygous dominant) with white flowers (homozygous recessive).

What is the genotypic ratio in a heterozygous monohybrid cross?

In a heterozygous monohybrid cross, the genotypic ratio is typically 1:2:1, meaning one homozygous dominant, two heterozygous, and one homozygous recessive.

What is the phenotypic ratio for a heterozygous monohybrid cross?

The phenotypic ratio in a heterozygous monohybrid cross is 3:1, meaning three individuals will show the dominant trait (e.g., purple flowers) and one will show the recessive trait (e.g., white flowers).

What is incomplete dominance, and how does it affect the phenotype?

Incomplete dominance occurs when the heterozygous genotype results in an intermediate phenotype. For example, crossing a red and a white snapdragon results in pink flowers.

How does codominance differ from incomplete dominance?

In codominance, both alleles are fully expressed in the phenotype. For example, in blood types, a person with type AB blood expresses both A and B proteins equally.

How are sex-linked traits represented in a Punnett square?

Sex-linked traits, particularly those on the X chromosome, are represented by pairing the X and Y chromosomes. For example, a colorblindness carrier mother (XcX) and a normal father (XY) can produce offspring with varying combinations of normal and colorblind traits.

What is a dihybrid cross, and how does it differ from a monohybrid cross?

A dihybrid cross involves two traits instead of one, such as seed shape (round or wrinkled) and seed color (yellow or green). It requires a larger Punnett square to account for the multiple combinations of alleles.

Outlines

00:00

🔬 Introduction to Punnett Squares and Genetics

In this section, the speaker introduces Punnett squares, commonly used in genetics to predict the outcome of a cross. He explains that students often misuse Punnett squares, not understanding the genetics behind them. The speaker emphasizes the importance of understanding meiosis, where genes are divided and passed to offspring through sperm or egg cells. Punnett squares visually represent the possible outcomes of gene combinations between parents.

05:04

🌸 Monohybrid Cross Example: Purple and White Flowers

This part explains a simple monohybrid cross between purple and white flowers. The purple flowers are homozygous dominant (PP) and the white flowers are homozygous recessive (pp). The speaker shows that regardless of the Punnett square size, the result is a 100% chance of heterozygous (Pp) offspring, meaning all flowers will be purple due to the dominant trait.

10:07

🔀 Heterozygous Cross Example

Here, the speaker explores a more complex heterozygous cross, where both parents carry one dominant (P) and one recessive (p) gene for purple flowers. The possible gametes are represented in a Punnett square, which shows the probability of different combinations. The cross produces a 1:2:1 genotypic ratio (PP, Pp, and pp) and a 3:1 phenotypic ratio, meaning three purple to one white flower.

🌸 Incomplete Dominance Example: Snapdragons

The speaker introduces incomplete dominance using snapdragons, where a red and white gene results in a pink flower. The example shows how a Punnett square can predict the offspring’s flower color, producing a 1:2:1 ratio for both genotypes and phenotypes. In incomplete dominance, the heterozygous condition results in a blend of the two traits.

👁️‍🗨️ Sex-Linked Inheritance Example: Colorblindness

This section discusses sex-linked inheritance, focusing on colorblindness, which is carried on the X chromosome. The speaker explains how Punnett squares can predict the likelihood of offspring inheriting colorblindness, with the mother being a carrier (XcX) and the father being normal (XY). The cross produces four outcomes, including carriers, normal offspring, and colorblind males, as males lack a second X chromosome to offset the defective gene.

🌱 Dihybrid Cross Example: Seed Shape and Color

This part covers dihybrid crosses, which involve two traits, in this case, seed shape (round or wrinkled) and seed color (yellow or green). The speaker warns against common mistakes, explaining that each parent can produce four different gametes by giving one allele of each trait. The resulting Punnett square predicts the ratio of offspring phenotypes: 9 round yellow, 3 round green, 3 wrinkled yellow, and 1 wrinkled green.

🧬 Complexity of Genes and Punnett Squares

In the final part, the speaker highlights the complexity of real genetic inheritance, where traits like height are controlled by multiple genes. He emphasizes that while dihybrid and monohybrid crosses can help illustrate simple inheritance, most human traits involve polygenic inheritance, requiring a larger Punnett square. The speaker concludes by challenging the audience with an example to calculate the necessary Punnett square size for a given genetic cross.

Keywords

💡Punnett Square

A Punnett square is a diagram used in genetics to predict the outcome of a cross or breeding experiment. It shows how different alleles from each parent combine to form offspring. In the video, Mr. Andersen explains how to organize gametes from each parent on the sides of the square and how to interpret the possible outcomes for genetic traits.

💡Monohybrid Cross

A monohybrid cross is a genetic cross that focuses on one specific trait. It examines the inheritance of a single gene, such as flower color. In the video, Mr. Andersen uses an example of purple and white flowers to demonstrate how a monohybrid cross works and the 3:1 phenotypic ratio that results from crossing heterozygous individuals.

💡Alleles

Alleles are different versions of a gene that determine specific traits. An organism inherits two alleles for each gene, one from each parent. In the video, Mr. Andersen uses 'big P' and 'little p' to represent dominant and recessive alleles for flower color, illustrating how different allele combinations affect offspring traits.

💡Heterozygous

Heterozygous refers to having two different alleles for a particular gene, one dominant and one recessive. Mr. Andersen discusses this when explaining a cross between two heterozygous plants (big P, little p) for purple flower color, showing how it produces a 1:2:1 genotypic ratio and a 3:1 phenotypic ratio.

💡Homozygous

Homozygous refers to having two identical alleles for a particular gene, either both dominant or both recessive. In the video, Mr. Andersen uses the example of homozygous purple (big P, big P) and homozygous white (little p, little p) flowers to explain how crossing them results in offspring that are all heterozygous (big P, little p).

💡Genotype

Genotype refers to the genetic makeup of an organism, specifically the combination of alleles it possesses. Mr. Andersen explains how the genotype is represented by the letters in a Punnett square, such as big P, little p, and how the 1:2:1 genotypic ratio is derived from a heterozygous cross.

💡Phenotype

Phenotype refers to the physical appearance or expression of a genetic trait. It is determined by the genotype. In the video, Mr. Andersen describes how a 3:1 phenotypic ratio (purple to white flowers) results from a heterozygous cross because the dominant allele (big P) masks the effect of the recessive allele (little p).

💡Incomplete Dominance

Incomplete dominance occurs when neither allele is completely dominant, and the heterozygous offspring have a phenotype that is a blend of both parents. Mr. Andersen uses snapdragons as an example, where a cross between red and white flowers produces pink flowers, demonstrating how the heterozygous phenotype is intermediate.

💡Codominance

Codominance occurs when both alleles in a heterozygous individual are fully expressed, leading to a phenotype that displays both traits simultaneously. Mr. Andersen mentions codominance as a concept similar to incomplete dominance but explains that in codominance, both traits are visible rather than blended, as seen in certain cases of animal coat color patterns.

💡Dihybrid Cross

A dihybrid cross involves two different traits, each controlled by two genes, and examines how they are inherited together. Mr. Andersen introduces a dihybrid cross using pea plants, discussing seed shape (round vs. wrinkled) and seed color (yellow vs. green), and demonstrates the typical 9:3:3:1 phenotypic ratio from a dihybrid cross.

Highlights

Introduction to Punnett squares and common mistakes made by students when using them.

Reginald Punnett's contribution to genetics, though not directly related to Punnett squares, but associated with mimicry in butterflies.

Punnett squares are often overused as a quick solution without fully understanding the genetic mechanisms.

The two sides of a Punnett square represent the alternatives after meiosis, where gametes receive half of the genes.

In a monohybrid cross, with homozygous dominant and recessive traits, only one possible outcome occurs.

For heterozygous crosses, the Punnett square shows a 1:2:1 genotypic ratio and a 3:1 phenotypic ratio.

Incomplete dominance results in a 1:2:1 genotypic and phenotypic ratio, as seen in snapdragons with red, white, and pink flowers.

Codominance differs from incomplete dominance because both alleles are fully expressed in hybrids.

X-linked traits, like colorblindness, can be tracked using Punnett squares, with outcomes depending on the inheritance of X and Y chromosomes.

Monohybrid crosses are generally well-understood, but students often struggle with dihybrid crosses.

Dihybrid crosses involve two traits, and gametes must include one allele for each trait, leading to a 9:3:3:1 phenotypic ratio.

Mistakes often occur when students fail to correctly identify all possible gametes in dihybrid crosses.

The dominant and recessive traits in dihybrid crosses determine the phenotypic ratios, such as round/yellow seeds vs. wrinkled/green seeds.

Most human traits are polygenic, meaning multiple genes influence them, making Punnett squares more complex for such traits.

A final example question on determining Punnett square size based on possible gametes produced by parents with different alleles.

Transcripts

Hi. It's Mr. Andersen and today I'm going to give you a beginner's guide to

punnett squares. There are a few mistakes that students always make when they're doing

punnett squares, so I hope to kind of clear those up. First of all we should talk about

the name sake. This is Reginald Punnett. He didn't really work with punnett squares however

he did work with genetics quite a bit. He did work with mimicry in the butterflies.

And so his name is probably associated with genetics just as much as Mendel. And it's

through the use of the punnett square. Now the punnett square is often over used as just

a quick way to solve genetics problems without really understanding what's going on with

the genetics. And so I want to kind of get to that route. And if you could remember one

thing from this whole video podcast, it's this right here. The two sides of a punnett

square represent the alternatives after meiosis. In other words, you have a bunch of genes

and you give half of those genes to a sperm or an egg. And that happens through meiosis.

And so the organization of those gametes, in this case it's just a monohybrid cross,

are going to be on either side of this. Just like a flip of a coin. This would be for one

parent. And then this would be the other parent on the other side. And so what are the boxes

on a punnett square stand for? They simply stand for all the alternatives that could

occur if we had mating between each of these different gametes. And so let's get to some

examples and hopefully that will help. So we're going to start with a monohybrid cross.

A monohybrid cross is simply a cross that is looking at one trait. And so let's do one

that's really, really simple. And so let's say we're crossing purple flowers that are

homozygous purple with those that are homozygous white flowers. In other words this is the

dominant trait. This is the recessive trait. And so if you look at the parents what you

want to do first of all is figure out what are the possible gametes that could be produced

in meiosis. In other words, this one could either give a big P or it could give a big

P. What does that mean? It can only give one thing. It can only give a big P or that dominant

allele. And so if you're doing a problem like this you don't even need a big punnett square.

One parent can only contribute big P. So let's look at the other parent. The other parent

can either give a little p, and I try to make them really small or a little p. Because p's

look the same. And so the other parent can only give a little p. And so we could put

that on the other side. And so what are the opportunities that we could have as far as

fertilization could goes? Well this one's automatically going to give big P. The other

one's going to give little p. And so this is the only possible outcome we could get

between a cross of a homozygous dominant and homozygous recessive. And so you don't really

need a big punnett square. Now you could do that. You could fill it in, big P over here,

little p over here. But if you do that, you're going to get the same thing in all of the

boxes. And so it's still a 1 to 1 ratio. In other words 100% of the time you're going

to have that. Okay. So let's get to one that's a little more complicated. Let's say we have

a heterozygous cross. And so this is for purple flowers as well. Well if you look at this

one, now the problem has changed a little bit. This one could give a big P. But it also

could give a little p. And so we have to show both of those possible gametes of meiosis.

And so this would be the big P. And then this would be the little p over here. So half of

the time it's going to give the big P. Half of the time it's going to give the little

p. But if you look at the other parent, same thing, it's going to give the big P half of

the time. And it's going to give the little p half of the time as well. So we're going

to put that here. Now we simply fill in the boxes. And so this would be a big P with a

big P. Because I'm taking this here and that there. This is going to go over to here to

give us a big P and a little p. By convention we usually write the dominant allele first.

So that would be one alternative here. Here would be a big P little p as well. We get

the big P here and the little p here. And then finally we're going to get little p over

and a little p over here. Since they're each contributing a little p. And so what do we

get from this cross? Well we get a 1 to 2, since these are exactly the same, to 1 genotypic

ratio. Because the genotype is the letter. So there is one that's like this. There's

two that look like this. And there's one that looks like that. So that's going to be our

1 to 2 to 1 genotypic ratio. What about phenotypic? Well this one's going to be a purple flower.

And so are these other two. And so if we're looking at the phenotypic ratio, the phenotypic

ratio now is going to be a 3 to 1 ratio. We're going to have three purple to every one white

that we have. Okay. Let's try another one. Let's say we're looking at incomplete dominance.

So incomplete dominance, a snapdragon would be an example of that. A snapdragon has two

genes. If it has a red gene and a white gene, then it's going to be pink. And so this one

actually has two alleles that it can contribute and the same on the other side. And so we

just write those out. So this would be the parent. This would be 50% chance of giving

the red, 50% chance of the white. And then the same thing on this side over here. So

now if we fill in our punnett square like that, what do we get for all the different

choices? Well now we have a 1 to 2 to 1 genotypic ratio. But we also have a 1 to 2 to 1 phenotypic

ratio. So if you're doing a question where it's incomplete dominance, you use a punnett

square the same way. Codominance would be the same way. The difference is in incomplete

dominance the heterozygous or the hybrids are going to be somewhere between the two.

If it's codominance it will actually, they're going to express both of those genes. Both

of those proteins. And now let's try one that's sex-linked chromosome or a X linked chromosome.

And so in this one we've got a parent, so this is a mom, because she's X X chromosome.

And she's a carrier of let's say colorblindness, the gene. And this is a dad that is normal.

And so I'm going to put that up here. X Y because half of the time he's going to give

the X. And half of the time he's going to give the Y. If we put mom over here, I'm going

to put that carrier up there, she is not colorblind because she has one deficient gene, colorblind

gene. But she has another gene that works well on her other X chromosome. So if I fill

in this one here it's going to be XcX. So this would be a female, because two X chromosomes.

But they're going to be a carrier of that gene. If we look down here, this would just

be a normal female. If we look down here, I'm grabbing the X from here and the Y from

up there, so that would be X Y. So that would be a normal male. And then if we look at this

one right here, this would be a male whose colorblind. The reason he's colorblind is

that he doesn't have an X chromosome or another gene as a back-up copy to that. So those are

monohybrid crosses. And usually students do fairly well on those. Next are dihybrid crosses.

And this is where mistakes really start. And so if we look at this parent. This is a typical

dihybrid cross. Let me tell you what the letters stand for. The R stands for round pea seeds.

And the Y stands for yellow seeds. If it's the recessive that stands for wrinkled and

if it's a little y that stands for green. And so we're looking at a dihybrid, so that

mean two traits. We're looking at seed shape, round or wrinkled. And seed color, yellow

or green. So now we have to do a dihybrid cross. And so the tendency is we look at this

parent, the tendency is to see that there are four letters over here. There's four boxes

over here and then you just simply write them out. Big R little r big Y little y. And then

you get a weird answer and you don't know what to do with it. Okay? That's wrong. That's

a mistake. Okay? Whenever you're figuring out the gametes, remember that you have to

give one of each letter. In other words, each of the gametes is going to have one of each

of the alleles. And so let me clear this mess out of the way. So what do we do? Let's say

this parent right here, and I'm going to write it up here so it makes a little more sense.

Big R little r big Y little y. What possibilities could they produce? They're giving one of

each letter remember. Well they could give the big R and the big Y. So big R big Y. That

would be one possibility. They could also give the big R and the little y. So they could

give the big R little y. They could also give the little r big Y or the little r little

y. Since they're giving one of each color, there's only four, one of each letter excuse

me, there's only four possibilities that they can give. So it's going to be kind of a mess

but those are the four right here. And so those four and going to go across the top.

So we've got big R big Y, big R little y, little r big Y, little r little y. So those

are the four gametes that you could produce. In other words with this parent you can only

get four combinations of each of the two letters. Same thing down this side. And so it's going

to be the same thing written down this side because this other parent is going to be exactly

the same. Okay. So it would take me a long time to write all of those out, so let me

throw those in here. So these would be the parents. All the possible gametes you could

get. And if I fill those in, these first nine, if you look at them, let me get a color that's

different. Let me grab a yellow color. And so this one right here is going to be round

and it's going to be yellow. So it's going to be round and yellow. And this one you can

see is going to be round and yellow. And round and yellow. And round and yellow. Round and

yellow. And round and yellow. And round and yellow. And round and yellow. And round and

yellow. In other words nine of them are going to be round and yellow in shape. And the only

reason why is that the R is dominant. And so you could have on like this where they're

hybrid for both and they're still going to be round and yellow. And so let me try to

add the next ones. So what about these next ones? Well the next ones are going to be round

and green. So let me get a different color. So these ones are going to be round and green.

Because the round is dominant. But they don't have the dominant for the yellow. So they're

not going to be. Let me get rid of that. So the next ones, these ones are going to be

wrinkled and yellow. So again I've got to get a different pen. So these ones are going

to be wrinkled and yellow. And then if we look at the last one, it's going to get all

of the recessive alleles. And so that one is going to be, getting green, it's going

to be green and wrinkled. Okay. And so when we say there's a 9 to 3 to 3 to 1 ration,

that's just a typical dihybrid cross. We're going to have 9 of the phenotypes that are

round and yellow. Three of the phenotypes that are round and green. Three of the phenotypes

that are yellow and wrinkled. And then only one of one's that's not. And that's only going

to work if you are able to set up your gametes correctly on the side. Now it's super hard

for you to answer a question like this on a test. You're rarely going to have to draw

a dihybrid cross. But it's important that you understand the concept of it. Because

most of the genes inside your body are not caused by one gene, they're cause by multiple

genes. So how tall you are is caused by probably a dozen different genes inside your body.

So you can imagine how big the punnett square is going to be for that. An example that I'll

leave you with would be this one. So let's say we have a parent here. And the parent

is big R little r big Y little y, little r little r little y little y. The question I'm

asking you is how big would your punnett square have to be? And so you're going to have to

figure out what are all the possible gametes that you could get from both of those and

then, then draw it out. Figure out all the possibilities you can get. If you're thinking

it's going to be a 4 by 4 you're doing way too much work. And so those are punnett square

squares and I hope that's helpful.