So we’ve made resources for reviewing in biology: from our GIF review, to our study
tips video, to our mega biology review video called stroll through the playlist.
Our Stroll Through the Playlist was actually our longest video in Amoeba Sisters history.
This one won’t be quite that long.
But it will address an area that the Stroll Through the Playlist couldn’t go into very
much---a review of how to do general genetic problems.
By that, I mean this video will review: Mendelian one-trait and two-trait crosses.
Then non-Mendelian traits like incomplete dominance, codominance, multiple alleles,
and sex-linked traits.
We’ll end with pedigrees.
Before starting, I’ve got five things to mention first.
Number 1, a sheet of paper will be very useful for this.
You might want to pull one out now with a pen or pencil.
Just like our stroll through the playlist, when you see Gus pop up, you can pause the
video and work out a problem to check how you’re doing.
Number 2, we’ll use some genetic vocabulary here, and we are making the assumption that
you’ve already seen these words before.
Number 3, with genetic problems, the symbols used in textbooks---especially with incomplete
dominance and codominance---can vary.
Some use superscript; some use different letters.
It’s not the symbol you need to focus on but rather the concept.
But just to note: if you’re choosing the letters to write out, you might want to pick
letters that have different looking capital and lowercase letters unless you go to a lot
of effort to make them look different.
Number 4, remember that when you’re doing a genetic problem, you are determining a probability.
But just because a Punnett square tells you that 1 out of 4 guinea pig offspring will
be hairless, doesn’t mean that there will always be one hairless guinea pig every time
you have four offspring.
It can show it’s possible, but it’s still a probability.
Number 5, and this is the big one, please realize the topic of genetics is complex.
Far more complex than simple gene traits you tend to see in Punnett square problems.
Traits can be polygenic, which means MANY genes control ONE trait.
Some traits can be pleiotropic which means ONE gene controls MANY different traits.
We mention epistasis in one of our videos, which means that a gene’s expression, whether
it’s expressed or not, can be impacted by the expression of another gene.
And we released a video about epigenetics: this involves factors at work - that are not
part of the DNA sequence -but yet can influence gene expression.
Okay, with those five things said, let’s get started!
First, with understanding a Mendelian one-trait cross, which includes monohybrid crosses.
Let’s assume here we have these three guinea pigs.
We’re using the letter H to represent an allele, and we’re assuming that the presence
of a dominant allele means the guinea pig will have hair.
This guinea pig is homozygous dominant, this one is heterozygous, and this one is homozygous
recessive.
Knowing that, can you complete the genotypes?.
[PAUSE]
HH, Hh, and hh.
Remember it only takes one dominant allele and it would have hair.
Also remember there are two alleles per guinea pig here because each guinea pig gets one
allele from each of their parents.
Please cross a hairless guinea pig with a heterozygous guinea pig and give the phenotype
and genotype ratios of the offspring.
[PAUSE]
Notice that I had to figure out the hairless guinea pig’s genotype would be hh.
I remembered it only takes one dominant allele in this example and it would have hair, so
it can’t have a capital H. I arrange the Punnett square.
It doesn’t matter which side I put the parents on.
You will notice the genotype ratio of the offspring here is 2 Hh: 2 hh.
This can be reduced to 1:1: ratio.
The phenotype ratio is 2 with hair: 2 hairless which can be reduced to 1:1.
Genotype ratios and phenotype ratios don’t necessarily match although they did in this
case.
On to a Mendelian two-trait cross, which includes dihybrid crosses.
Moving on to a different animal: cats.
With our Mendelian two-trait and dihybrid cross video, we mentioned a fictional trait
of the love for sinks.
Because our cat Moo, and many cats we’ve been told, seem to have an affection for sinks.
Again, probably not a genetic trait, but for this example, let’s use this fictional trait.
We will assume here that the presence of a dominant S allele leads to the trait of a
cat loving sinks.
Only two recessive “s” alleles would result in the non-sink loving trait.
So, if we have a cat that is heterozygous for both the traits of having hair and loving
sinks, what would that cat’s genotype be?
[PAUSE]
HhSs.
Let’s cross it with another cat with that same genotype.
HhSs.
Now a dihybrid calls for a 16 square box here.
What would be the gamete combinations that we put on top and side of this square?
[PAUSE]
Recall the FOIL method---the gametes along the side would be HS, Hs, hS, and hs.
Same for the other side.
Remember it doesn’t matter which parent you put on which side.
Notice how the gametes have one of each allele type.
Meaning, you wouldn’t have a HH or a SS in a gamete; you get one of each.
Go ahead and fill that dihybrid cross in and give us the phenotype ratio.
[PAUSE]
Here we are.
Notice we put the H letters here first and then the S letters, similar to how the genotype
was written for the parent cats.
That’s a 9:3:3:1 phenotype ratio, which if you cross two organisms that are heterozygous
for both traits in a dihybrid cross, you will find that phenotype ratio to occur.
But for a two-trait example that doesn’t have two heterozygote parents, you can check
that out on our full content video.
Ok, we’re leaving Mendelian genetics now.
Mendelian genetics followed an inheritance pattern where having a dominant allele meant
the dominant trait was expressed.
But in non-Mendelian inheritance, we’ll see that’s not always how it works.
Take incomplete dominance.
Keep in mind before starting there should be clues that a problem is involving incomplete
dominance or some other non-Mendelian trait.
Incomplete dominant traits tend to have an intermediate phenotype, almost an in-between
phenotype.
The snapdragon example is a popular one.
Here is a red snapdragon with genotype RR.
A white snapdragon with genotype rr.
But the Rr genotype leads to a pink phenotype.
In incomplete dominance, one allele is not completely dominant over
the other..
What would be the genotype and phenotype ratios of the offspring from two pink snapdragons
crossed?
[PAUSE]
1 RR: 2 Rr: 1 rr would be the genotype ratio.
1 red: 2 pink: 1 white would be the phenotype ratio.
This is different from codominance, in codominance, both traits are expressed fully.
So for codominance, we like using different letters entirely for this reason.
In a certain type of chicken, genotype BB results in black chickens, WW results in white
chickens, and BW results in black and white speckled chickens!
What would be the genotype and phenotype ratios of offspring from one black chicken and one
black and white speckled chicken?
[PAUSE]
2 BB: 2 BW, reduced to 1:1 would be the genotype ratio.
And as for the phenotype ratio?
2 black chickens: 2 black and white speckled chickens reduced to 1:1.
Now it’s time for a genetic problem with multiple alleles.
Blood types are a great example of this.
If we have these four blood types: A, B, AB, and O… can you write the genotypes?
And just a hint, it’s common to write the alleles as exponents on the letter I, although
it doesn’t have to be written that way.
[PAUSE]
Here they are!
Now, if there is one parent that is heterozygous type B and another that is heterozygous type
A, what is the percent chance that the baby from these two parents will be type O?
[PAUSE] So after working this out with the correct genotypes around the Punnett square,
it is a 25% chance that the baby will be type O. Keep in mind that blood types can also
be positive or negative, which is related to Rh factor, which our video does not go
into.
Next, sex-linked traits on sex chromosomes.
In these Punnett square problems, you are usually told it is a sex-linked trait and
then also given information about whether an individual is male or female.
In these problems, you are working on Punnett squares that involve alleles on sex chromosomes.
But as we mention in our old video, please know that individuals can have more or fewer
sex chromosomes than what might be written in a Punnett square.
Let’s consider the recessive, sex-linked disorder hemophilia.
If I tell you it’s sex-linked recessive, and we use the letter “h,” how would you
write the genotype for a male that has this disorder?
[PAUSE]
You’d write XhY.
Notice that it’s only placed on the X chromosome.
Generally sex-linked traits will be found on the X chromosome, although there are some
exceptions.
If a male was XHY, this individual would not have the disease since the disease is a recessive
sex-linked disorder.
Which of these female genotypes would have the disease hemophilia?
[PAUSE]
Only the female genotype with XhXh.
Remember, the heterozygous genotype XHXh still has a dominant allele, represented by the
capital H, which means this individual does not have this disorder.
If a male with hemophilia and a female who is homozygous dominant decide to have a child
together, what percent chance is it that their child would have hemophilia?
[PAUSE] It is a 0% chance.
Also, do you notice how the male children receive their X chromosome from the female?
They receive their Y chromosome from the male.
All right, that’s a lot of genetic problems.
Now, our last topic, pedigrees.
Pedigrees can be used to track a trait of interest, and they use many of the concepts
we’ve been reviewing.
A reminder that the shaded shape in a pedigree is generally the trait of interest.
Some people will also do a half-shading to represent the heterozygous genotype, as we
mention in our pedigree video, however, this isn’t always done and we’re going to assume
half shadings have not been done in our examples.
Take a look at this pedigree.
What shape is supposed to represent females?
[PAUSE]
That’s right, the circles.
The males would be represented as squares.
So in this pedigree, assume you are told the shaded shapes represent individuals that have
an autosomal recessive trait.
Because it is autosomal, the trait is not on a sex chromosome.
In our pedigree video, our trait of interest was tracking attached earlobes, but as we
mentioned in that video, this trait may be more complex than a single gene trait.
We’re going to use the letter “e” here so any of the shaded shapes must have the
genotype ee.
So, if given this pedigree, can you determine the genotypes of the rest of the individuals?
Just a reminder, with pedigrees, it’s often ideal to fill out the genotypes of the shaded
shapes first before determining the others.
And in this case, you know the shaded shapes will all be ee.
So now try and fill out the rest of the genotypes!
[PAUSE]
Here are all the genotypes!
A few things to point out.
Notice in generation I, individual I, the individual must be Ee.
That’s because there is a ee offspring and so if the individual was EE, that would not
be possible.
This is the same situation with individual 1 in generation 2.
Notice in generation 3, individual 2, this male must be Ee.
This male cannot be EE, because it would not be possible to receive a dominant allele from
both parents.
All offspring receive one allele from each parent.
Notice in generation II, individual 4, this male could be EE or Ee.
We don’t know.
Even if this individual had 10 children that did not have the trait, you still would not
know the genotype for sure.
The only way you would know for sure is if they had a child with the genotype ee.
Because a child with the ee genotype would reveal that both of these parents would have
to be the heterozygous genotype.
But there is not a child represented by a shaded shape here.
What if I didn’t tell you this trait was autosomal recessive?
Could you show why this pedigree is likely NOT tracking a sex-linked recessive trait?
So if this was tracking a sex-linked recessive trait, the shaded shapes would then represent genotypes
that are sex-linked recessive.
Filling out the genotypes of shaded shapes first.
Here they are.
So can you determine why this is NOT likely?
[PAUSE]
So take a look at parents 1 and 2 in generation II.
We know that Generation II, individual 1 is a male.
The individual must have genotype XEY, because if the genotype was XeY, the shape would be
shaded.
Notice a Punnett square with parents 1 and 2 of generation II shows it is not possible
to have those offspring genotypes in generation III from the pedigree.
This specific pedigree is not likely to be tracking a sex-linked recessive trait.
So that was a lot to review!
What if you’re still stuck?
Check out the full content videos, which are each under 10 minutes, in our genetic series.
Practice.
Practice a lot.
And, finally, connect this to why it matters.
We have some links to check out with more fascinating real-life examples in our video
details so you can discover why gaining an understanding of genetics is such a worthwhile
endeavor.
Well, that’s it for the Amoeba Sisters, and we remind you to stay curious!