Dihybrid and Two-Trait Crosses

Dihybrid and Two-Trait Crosses


Captioning is on. To turn off, click the CC button at bottom right. Follow the amoebas on Twitter @amoebasisters or on Facebook! Have you ever been working on a Punnett Square
and you think, all of a sudden, man I really wish there were more than just these 4 squares?
Well if you ever have, today is your lucky day. And if not, well…prepare to have more
squares. Because up until this point we have focused on one pair of alleles. Hence, monohybrid
where mono means one. In guinea pigs, for example, the trait of having or not having
hair is influenced by a pair of alleles (HH, Hh, or hh). But guinea pigs have more traits
than just hair, right? They have lots of alleles. Well if you perform a cross with two pairs
of alleles, that’s called a dihybrid. The root “di” means two. So, two traits. One of us has this cat named Moo. Moo, as
in, what the cow says. Because he kind of looks like a cow, and we’re not that creative
with names. The thing about Moo is that he is a cat that loves sinks. You get up in the
morning and try to splash water on your face and —oh no you think—there’s a cat in
my sink. Try to brush your teeth and—on no, it’s a cat in my sink. You have people
over and you’re terrified that they’re going to flip out because that cat is peeking
out of the sink… Anyway, not all cats like sinks. But Moo does.
Around the clock, chances are, he’s in the sink. We cannot find any information about
whether this is a genetic trait. We don’t know if Moo’s parents liked sinks. But then
we realized Moo is not that weird, because there is a website called catsinsinks.com.
Seriously, google it—you’re going to see TONS of cats in sinks. What if this was a
genetic trait? There is no research that we can find that demonstrates this is genetic,
and it’s probably not. Behaviors like that can be challenging to study. But for practicing
dihybrid squares, let’s imagine what it would be like if it was a genetic trait. And
that loving sinks—for cats—-is a dominant trait represented by the allele S and that
not loving sinks is a recessive trait with the allele s. Let’s also take in account
that cats—like guinea pigs in our previous video—typically have hair but they can also
be hairless. Having hair will be represented by the allele H and not having hair would
require two recessive h alleles. So let’s say we want to cross a cat that
is heterozygous for the trait of having hair and also for liking sinks. Heterozygous for
both traits would be represented by the genotype HhSs. Now we want to cross that cat with a
hairless cat that does not love sinks. To be hairless, the cat must be hh. A dominant
allele, capital H in this case, would mean it has hair. And if it does not love sinks…a
recessive trait in our example…then it is ss. A dominant allele, capital S, would mean
that it does like sinks. So the second cat is hhss. Remember how it looks like in a regular monohybrid
Punnett square? Here’s an example of what it would be like if you were crossing a Hh
cat with a hh cat. Remember how you put the parents on the top and sides like this? Well
when you are doing this, those alleles on the top and sides represent the alleles that
would be in the gametes of the parents. Gametes are sperm cells—if male—and egg cells—if
female. And they contain half of the genetic material as the cat’s body cell. So it makes
sense that if there are two alleles—letters— in the Hh parent, then a gamete would only
carry one letter—a H or a h. This is known as Mendel’s law of segregation. The gametes
only carry one allele for a gene. Well if you have a cat that is HhSs, there
are four alleles there. Two genes—one involving hair and one involving sinks—so if gametes
carry only one allele—letter– per gene (Mendel’s law of segregation), that means
each gamete is going to have two alleles. In those gametes, with each of them having
two alleles (letters), you have to account for every possible combination. Mendel’s
law of independent assortment says that those alleles are not linked. That means a cat can
have hair and love sinks or not have hair and love sinks—there is no link. So let’s work out a dihybrid with the parent
cross of HhSs x hhss. Step 1—write the parent cross with your 16 square Punnett square.
Step 2—gamete combinations from the parents are written along the top and side of the
Punnett square. But how did we get these letters along time sides? We like the FOIL method
to come up with the gamete combinations. FOIL stands for FIRST, OUTSIDE, INSIDE, LAST. This
is not the only way you can do this—we just like this way. So when you
FOIL HhSs, you get these gamete combos: HS,
Hs, hS, and hs. Place those on the top of the Punnett square like this. FOIL the other
parent hhss, you get these gamete combos: hs, hs, hs, hs. Place those on the side of
the Punnett square like this. Yes, they are all the same, because notice that was all
that parent could contribute as far as alleles. Remember again—each gamete must have one
allele (letter) of each gene. That’s why you won’t find a gamete with only H’s
or only S’s. One allele of each. Step 3—-combine the gametes to see what
the offspring prediction will be. For formatting purposes, because the parents had H’s coming
before S’s, we write it that way with the offspring as well. For formatting, you also
put capitals of each letter type before the lowercase. So in our example, what is the genotype ratio
in the predicted offspring? Remember that genotypes are the genetic make-ups—the letters
that represent the alleles. So 4/16 (25%) are HhSs, 4/16 (25%) are Hhss, 4/16 (25%)
are hhSs, and 4/16 (25%) are hhss. That’s a 1:1:1:1 ratio. What about phenotypes? Well half of the cats
have hair here and half of the cats don’t. But with dihybrids, often you are asked about
both traits. For example, what chance would be predicted for a kitten to be born that
was like our Moo? Moo has hair and loves sinks. Well it’s a 4/16—25% chance—-that a
kitten would be like Moo. We could write out the ratio 4/16 (25%) have
hair/love sinks, 4/16 (25%) have hair, dislike sinks, 4/16 (25%) hairless/love sinks, and
4/16 hairless/dislike sinks. This is a 1:1:1:1 ratio. Some big things to remember—in our example,
the genotype and phenotype ratios were the same. This does not always happen. The handout
will give you an example of when it is not. Just remember the steps to setting up the
problem correctly, and you will be fine. Also remember that Punnett squares are predictions.
This Punnett square is only predicting the chances of having offspring with certain genotypes
or phenotypes. It’s fascinating really. Well that’s it for the amoeba sisters and
we remind you to stay curious!

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  1. Card Note: At 4:50, we made a card edit that this example is a "test cross" two-trait cross as to be a true dihybrid cross, both parents should be heterozygotes. However, the steps to solve two-trait crosses are the same, regardless of whether the two-trait cross is is a true dihybrid cross (meaning 2 heterozygote parents). See our handout for a true dihybrid cross with two heterozygous parents! We appreciate an email we received about this topic- thank you! 😀

  2. This video was really good helpful thank you so much!!! I was having trouble on my genetics packet and this just explained it perfectly and also in a fun way so I was able to pay attention🤗💙

  3. Made a dry topic relatable and engaging so that I actually wanted to follow. Refreshing, relatable, uplifting, and informative–thanks!!

  4. BOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO!!!!…. I sent that cz the teacher sent us a linkto this vid… shoutout to my homies at CTI XD

  5. You saved my life! Thank you so much! You make it so much easier to understand. Please make more videos like these!!

  6. Sorry but there is something that I find confusing. So take your Cat in Sink analogy. If you considered sink loving an eccentricity, then wouldn't SS or Ss mean they are normal and ss means they are crazy about sinks? My point is wouldn't it all be on a basis of whether what we're tracking is 'sink-loving' or being a 'normal' cat (no offence to Moo).😾Wouldn't this affect the percentage probability?
    Thanks for the great video btw. 👍🏼😻

  7. That 'cat loving sink' thing and parents of moo (Mr.&Mrs.mooington) was ridiculous !!! 😂❤😻LOL

  8. In my opinion its easier to lable it as HShs instead of HhSs. This is because it looks more similar the foil we do in math.

  9. this video was kind of silly but informative and i liked the fact that its kind of like a cartoon and that the lady is speaking with energy to engage us kids into the lesson

  10. My alevel exams are saved! Here’s to last minute refreshers. Thumbs up if you’ve got your second biology paper tomorrow

  11. How to solve In mice , dur color can be the dominant colored fur or the recessive albino colored fur. Whiskers can be the dominant bent or the recessive straight. If two mice, heterozygous for both trait s mated , what are the chances that their offspring.
    A. Albino?
    B. Colored?
    C with bent whiskers
    D with strait whiskers
    E give the genotypic ratio of the progenty

  12. I love the Amoeba Sisters! Your illustrations and teaching style reminds me of my favorite childhood computer games by Humongous Entertainment. I know you guys will go far – maybe an interactive computer game for kids/young adults could be an idea.

  13. Thank you so much for this..Im currently getting ready for the exam and this video is a great help… thank you Amoeba sisters.. I'll always be curious 😉⚘❤

  14. The video is excessively long, I just needed to remember that I have to foil each gene that's all…

  15. I love your page, and have learned so much for Bio. The humor, jazzy music and simple detail are what I love most.

  16. How do you draw a punnet square when one parent is heterozygous (AbBb) for two different alleles, and the other parent is heterozygous for one, but homozygous for the other (aaBb)?
    For the parent that has (aaBb) I still used four columns, but just repeated "aB" and "ab" twice, so as not to affect the final percentage of outcomes. Is this correct, or should I have just used two columns for that parent?
    This is my punnet square:

    http://oi66.tinypic.com/345jxpx.jpg

  17. I'm out of school but I forgot how to do this and I want to know the tea on how my favorite webtoons baby dragons gonna come out… Good luck on all your quizzes genetics came pretty easy for me back then😂

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