Worked examples: Punnett squares | Inheritance and variation | Middle school biology | Khan Academy

Summary

This educational script explains the genetic inheritance of seed shape in pea plants, focusing on the dominance of the round seed allele (R) over the wrinkled seed allele (r). It guides viewers through creating a Punnett square to predict offspring ratios for two scenarios: a cross between heterozygous plants (Rr x Rr) resulting in a 3:1 ratio of round to wrinkled seeds, and a cross between a homozygous wrinkled seed plant (rr) and a heterozygous one (Rr), leading to a 1:1 ratio. The explanation demystifies Mendelian genetics and engages learners by encouraging them to solve the problem before the solution is revealed.

Takeaways

• 🌱 The script discusses pea plant genetics, focusing on seed shape determined by two alleles: a dominant allele for round seeds (R) and a recessive allele for wrinkled seeds (r).
• 🔍 It explains the concept of heterozygous pea plants, which have one allele for round seeds and one for wrinkled seeds (Rr).
• 📊 The script uses a Punnett square to illustrate the genetic cross between two heterozygous pea plants (Rr x Rr).
• 👶 It demonstrates that the offspring from such a cross can have four equally likely genotypes: RR, Rr, rR, and rr.
• 🍀 The dominant allele (R) results in round seeds, regardless of the other allele, while the recessive allele (r) only results in wrinkled seeds when homozygous (rr).
• 🔢 The expected ratio of round to wrinkled seeds in the offspring is three to one, as three genotypes result in round seeds and only one in wrinkled seeds.
• 🤔 The script challenges the viewer to pause and attempt to solve the genetic cross problem before revealing the solution.
• 📚 It introduces the concept of phenotypes, which are the expressed traits of an organism, in this case, the seed shape.
• 🌟 The script also explores a cross between a homozygous wrinkled seed pea plant (rr) and a heterozygous one (Rr).
• 🎯 In the second cross, the Punnett square shows that all offspring will have at least one R allele, resulting in a round seed phenotype, except for the scenario where both parents contribute the r allele.
• 🧬 The final ratio for the second cross is one to one, with an equal number of round and wrinkled seed offspring.
• 📝 The script provides a clear, step-by-step explanation of Mendelian genetics principles as they apply to pea plant seed shape inheritance.

Q & A

What are alleles, and how many alleles does the gene for seed shape in pea plants have?

Alleles are different versions of a gene that determine a particular trait. In the case of the gene for seed shape in pea plants, there are two possible alleles: one for round seeds (denoted as capital R) and one for wrinkled seeds (denoted as lowercase r).

What is the difference between a dominant and a recessive allele in genetics?

A dominant allele is one that expresses its trait even when present alongside a different allele (heterozygous condition). A recessive allele, on the other hand, only expresses its trait when both alleles for a trait are the same (homozygous recessive condition). In the pea plant example, the capital R allele for round seeds is dominant.

What does heterozygous mean in the context of genetics?

Heterozygous refers to an organism having two different alleles for a particular gene. In the script, the pea plants are heterozygous for the seed shape gene, meaning they have one allele for round seeds (R) and one for wrinkled seeds (r).

How is a Punnett square used in genetics to predict offspring traits?

A Punnett square is a diagram used to predict the genotypes of offspring in a genetic cross. It is created by listing the alleles of one parent along the top and the alleles of the other parent along the side, then filling in the squares with the possible combinations of alleles from both parents.

What is the expected ratio of offspring with round seeds to those with wrinkled seeds when two heterozygous pea plants are crossed?

The expected ratio is 3:1, with three offspring having round seeds for every one offspring with wrinkled seeds. This is because the round seed allele is dominant, and most of the possible genotypes result in round seeds.

What genotypes result in round seeds in the pea plant cross?

Round seeds can result from the genotypes RR (homozygous dominant) or Rr (heterozygous), as the round seed allele R is dominant over the wrinkled seed allele r.

What is the only genotype that results in wrinkled seeds in the pea plant cross?

The only genotype that results in wrinkled seeds is rr (homozygous recessive), where both alleles are for wrinkled seeds.

What is a phenotype in genetics, and how is it related to the pea plant seed shape example?

A phenotype is the observable physical or biochemical characteristics of an organism, as determined by the interaction of its genotype with the environment. In the pea plant example, the phenotype for seed shape is either round or wrinkled, depending on the genotype.

What is the expected ratio of round to wrinkled seeds when a homozygous wrinkled seed pea plant is crossed with a heterozygous one?

The expected ratio is 1:1, as each offspring has an equal chance of inheriting the dominant round seed allele (R) or the recessive wrinkled seed allele (r) from the heterozygous parent.

How can the dominance of an allele affect the observed traits in a population?

The dominance of an allele can lead to certain traits being more common in a population. If a dominant allele masks the expression of a recessive allele, as seen in the pea plant example, most individuals will display the trait associated with the dominant allele.

What happens when a homozygous recessive individual is crossed with a heterozygous one, and what is the resulting phenotype ratio?

When a homozygous recessive individual (rr) is crossed with a heterozygous one (Rr), all offspring will have at least one recessive allele (r). The resulting phenotype ratio will be 1 round seed (Rr) to 1 wrinkled seed (rr), as the dominant allele from the heterozygous parent can only produce round seeds when paired with another dominant allele.

Outlines

00:00

🌱 Pea Plant Seed Shape Genetics

This paragraph discusses the genetic basis for seed shape in pea plants, focusing on the round and wrinkled seed traits. It explains that there are two alleles for seed shape, denoted as 'R' for round (dominant) and 'r' for wrinkled (recessive). The paragraph outlines a genetic cross between two heterozygous pea plants (Rr) and uses a Punnett square to demonstrate the expected offspring ratio of round to wrinkled seeds, which is three to one. It also invites the viewer to pause and attempt the problem before revealing the solution.

05:02

🧬 Cross Between Homozygous and Heterozygous Pea Plants

The second paragraph extends the genetic discussion to a cross between a homozygous wrinkled seed pea plant (rr) and a heterozygous one (Rr). It describes the genotypes of the parents and uses another Punnett square to illustrate the possible genetic outcomes. The summary explains that since the round seed allele is dominant, any offspring with at least one 'R' allele will exhibit round seeds. The expected phenotypic ratio for this cross is one to one, with an equal number of round and wrinkled seed offspring, reflecting the equal contribution of alleles from both parents.

Keywords

💡Pea Plants

Pea plants are the subject of the video, used to illustrate genetic inheritance patterns. They are known for Mendel's experiments which laid the foundation for modern genetics. In the script, pea plants with round and wrinkled seeds are used to demonstrate how dominant and recessive traits are passed from one generation to the next.

💡Round Seeds

Round seeds represent the dominant trait in the pea plants discussed in the video. Dominant traits are those that are expressed when at least one dominant allele is present. The script uses round seeds to explain how a dominant allele (denoted as 'R') can result in round seeds even when paired with a recessive allele.

💡Wrinkled Seeds

Wrinkled seeds are an example of a recessive trait in the pea plants. This trait is only expressed when an individual has two copies of the recessive allele (denoted as 'r'). The video script discusses how the expected ratio of offspring with wrinkled seeds can be determined through genetic cross analysis.

💡Alleles

Alleles are different versions of a gene that determine particular traits. In the context of the video, the alleles for seed shape are 'R' for round seeds and 'r' for wrinkled seeds. The script explains how these alleles interact during genetic crosses to produce offspring with different phenotypes.

💡Dominant

The term 'dominant' describes a gene or trait that is expressed when an individual has at least one copy of the dominant allele. In the script, the 'R' allele for round seeds is dominant, meaning it will be expressed over the 'r' allele for wrinkled seeds in a heterozygous individual (Rr).

💡Recessive

A 'recessive' gene or trait requires two copies of the recessive allele to be expressed. In the video script, the 'r' allele for wrinkled seeds is recessive and will only result in wrinkled seeds if the individual is homozygous recessive (rr).

💡Heterozygous

Heterozygous refers to an individual having two different alleles for a particular gene. In the script, a heterozygous pea plant for seed shape has one 'R' allele and one 'r' allele, resulting in round seeds due to the dominance of the 'R' allele.

💡Punnett Square

A Punnett square is a tool used to predict the genotypes of offspring resulting from a genetic cross. The script describes how to use a Punnett square to determine the expected ratio of offspring with round versus wrinkled seeds when crossing two heterozygous pea plants.

💡Genotype

Genotype refers to the genetic makeup of an individual, specifically the alleles they possess for a particular trait. The video script uses the term to discuss how the combination of 'R' and 'r' alleles in the genotype of pea plants affects the phenotype of their seeds.

💡Phenotype

Phenotype is the observable expression of a trait, such as the shape of a pea plant's seeds. The script explains how the phenotype (round or wrinkled seeds) is determined by the interaction of the genotype (the combination of 'R' and 'r' alleles).

💡Homozygous

Homozygous describes an individual having two identical alleles for a trait. In the video script, a homozygous individual for wrinkled seeds has the genotype 'rr', ensuring the expression of the recessive trait.

Highlights

In pea plants, seed shape is determined by two alleles: one for round seeds (denoted as R) and one for wrinkled seeds (denoted as r).

The round seed allele (R) is dominant over the wrinkled seed allele (r).

Pea plants heterozygous for the seed shape gene have the genotype Rr.

A Punnett square is used to determine the genotypes of offspring from a cross between two heterozygous pea plants.

The four equally likely genotypes from a cross between Rr x Rr are RR, Rr, Rr, and rr.

Three out of four genotypes result in round seeds, while only one results in wrinkled seeds.

The expected ratio of offspring with round seeds to wrinkled seeds is 3:1.

A homozygous wrinkled seed pea plant has the genotype rr.

Crossing a homozygous wrinkled seed pea plant (rr) with a heterozygous one (Rr) results in a different Punnett square.

The genotypes from a cross between rr x Rr are Rr and rr.

All offspring from this cross will have round seeds (Rr) or wrinkled seeds (rr), with an equal likelihood.

The expected ratio of round to wrinkled seeds in this cross is 1:1.

Dominance plays a crucial role in determining the phenotype of offspring in pea plants.

Understanding allele dominance is essential for predicting genetic outcomes in pea plant crosses.

The Punnett square is a valuable tool for visualizing genetic inheritance patterns.

Genotype and phenotype are not always the same due to the influence of dominant alleles.

A homozygous dominant genotype (RR) results in round seeds, while a homozygous recessive genotype (rr) results in wrinkled seeds.

Heterozygous genotypes (Rr) can also result in round seeds due to the dominance of the round allele.

The genetic ratio of 3:1 for round to wrinkled seeds is a fundamental concept in Mendelian genetics.

Transcripts

- We're told that in a population of pea plants,

some plants have round seeds and others have wrinkled seeds.

The gene for seed shape in this population

has two possible alleles.

Remember, alleles are just versions of the gene.

One allele or one version is for round seeds,

which we're denoting capital R,

and the other allele is for wrinkled seeds

which we're denoting lowercase R.

And they're telling us that the round seed allele,

the capital R, is dominant.

For a cross between two pea plants

that are heterozygous for the seed shape gene,

determine the expected ratio of offspring

with round seeds to offspring with wrinkled seeds.

All right, like always, pause this video

and see if you can have a go at this

by yourself before we do this together.

All right.

Now let's think about the genotype

for the two pea plants that are heterozygous.

Heterozygous means they have two different versions

of the gene, or two different alleles.

So these are going to be two pea plants

that are capital R, lowercase R.

They have one of each.

And to see what happens in a cross between them,

I will draw a Punnett square.

So just like this, and I will explain

what I am doing in a second here.

So I'll draw it like this.

So what you need to think about

is you have your two parents,

and so each parent is capital R, lowercase R.

Now that means that let's say parent one

can contribute the capital R to the offspring

or it could contribute the lowercase R allele

to the offspring, and the same thing could happen

for the other parent.

It could contribute the capital R

or it could contribute the lowercase R.

So what are all of the equally likely

possibilities for the offspring?

Well, the offspring might get a capital,

the dominant allele, the round seed allele,

from both parents, capital R, capital R.

All I'm doing is I'm taking whatever letter's in the column

and whatever's in the row.

It could get a capital R from this parent

and a lowercase R from that parent.

It could get a lower case R from this parent

and a capital R from that parent,

or in this situation, it's getting

a lowercase R from either one.

So these are the four equally likely genotypes.

Now they're not asking us about genotypes.

They're saying determine the expected ratio of offspring

with round seeds to offspring with wrinkled seeds.

So what does the genotype have to look like

in order to have round seeds?

Remember the round seed allele is dominant.

Well, this would be a situation where you could be

capital R capital R, but it's dominant.

So you could also be capital R lowercase R

and still have round seeds.

The only way that you're going to have

wrinkled seeds right over here

is if you are homozygous for the wrinkled seed allele.

So this is the only wrinkled seed scenario.

So if you look here, you look and see

out of the four equally likely scenarios,

three of them end up with a phenotype of round seeds

and only one of them ends up with a phenotype.

Remember, phenotype is what's being expressed.

Only one of these four equally likely scenarios

has a phenotype of wrinkled seeds.

So the expected ratio is going to be,

you're gonna expect to see three offspring

with round seeds for every one with wrinkled seeds

or you would expect a three to one ratio.

Now I have another part of this question

that is asking us, what about a cross between

a pea plant that is homozygous for wrinkled seeds

and one that is heterozygous?

Pause the video again and see if you can

have a go at this one.

All right, so the first thing that I like to do

is just think about, what are going to be the genotypes

for each of these parents?

So if we're talking about homozygous for wrinkled seeds,

remember the wrinkled seed allele, we're denoting

with a lowercase R.

So that's going have a genotype

of lowercase R, lowercase R,

and then they say one that is heterozygous.

Well, we already saw that before.

That's going to be capital R and lowercase R. so

let's draw a Punnett square again, to be able to do this.

So I'll do it like this, like that, like that

and then make sure I have two columns and two rows,

and let's put the homozygous parent up here.

They could either contribute a lowercase R

or they could contribute a lowercase R.

They're gonna contribute one of these two,

and then let's think about the heterozygous parent.

Well, they can contribute a capital R,

the dominant allele, or they can contribute a lowercase R.

So let's think about the equally likely scenarios here,

at least for genotype.

Well, these are all scenarios

in which we get a capital R from this parent over here,

the heterozygous parent, I guess I could say.

So let me write capital R, capital R.

And in both of these scenarios,

we're getting the lowercase R from the homozygous parent.

And both of these situations are we are getting

the wrinkled seed allele from both parents.

You're getting the lowercase R from the heterozygous parent,

and in either scenario, you're getting a lowercase R

from the homozygous parent.

So let me write that right over here.

So, which of these genotypes would have

a phenotype of round seeds?

Well, round seeds are going to be scenarios

where you have at least one of the round seed alleles.

So these are all going to be round seeds, right over here.

They're going to have a phenotype,

I should say, of round seeds

because remember, the round allele is dominant.

Even though you have one of each,

the dominant allele is what you're actually

going to see in the phenotype here.

And then we see these two

are going to be wrinkled, wrinkled.

So what's the ratio of round to wrinkled going to be?

Well, it's going be one to one.

For every two round, you're going to have two wrinkled,

or for every one round, you're going to have one wrinkled.

Two to two is the same thing as one to one

when we're talking about ratios, and we're done.