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Course: Middle school biology > Unit 7
Lesson 5: Reproduction and genetic variation- Sexual reproduction and genetic variation
- Sexual reproduction and genetic variation
- Genetics vocabulary
- Worked examples: Punnett squares
- Genetics vocabulary and Punnett squares
- Understand: sexual reproduction and genetic variation
- Apply: genetics vocabulary
- Apply: Punnett squares
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Worked examples: Punnett squares
Solve Punnett squares that include heterozygous and homozygous genotypes. Created by Sal Khan.
Want to join the conversation?
- Just when I thought I was safe from math...(27 votes)
- we don't talk about maAaaAAAath!(2 votes)
- What is the difference between homozygous and heterozygous?(14 votes)
- A homozygous genotype has two of the same alleles. A heterozygous genotype has two different alleles. The prefix homo means the same, the prefix hetero means different.(27 votes)
- is it possible for both parents to give, for example, two capital Rs?(8 votes)
- Yes, that would happen if you crossed plants that both had RR in their genotype.(10 votes)
- how can he draw such a smooth line with ease(3 votes)
- The straight line tool and also they have a little pad that has a pen that electronically transfers it to the screen so the writing is also really good. ALSO GREAT VIDEO! I understand it now(7 votes)
- i really love punnett squares! i read a book series called Warrior cats, and the authors never made the cats genetically accurate. when i have a bit of time i like to make punnett squares and find out how the cats would REALLY look like :)(5 votes)
- Do we need to memorize the all the names?(3 votes)
- What if there are two dominant or non dominant alleles?(3 votes)
- Who came up with the punnet square?(1 vote)
- It's named after Reginald Punnett, a British geneticist. He came up with this concept back in 1905, taking inspiration from Gregor Mendel's work on pea plants.(5 votes)
- Can Punnet squares predict all allele combinations? If they cant why not?(2 votes)
- Yes, they can predict all allele combinations.(3 votes)
- What's Punnett squares and how does that work?(2 votes)
- the video is supposed to answer that =/(2 votes)
Video transcript
- 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.