Hey guys, it's Melichosis Perfectionatus where medicine makes perfect sense. Let's continue our genetics series inside the biology playlist. Today we shall talk about the Punnett squares, the monohybrids, as well as the dihybrid crosses. With that said, now let's get started.
Before we get started, always remember, parents are here, offsprings are in between. This is my biology playlist. Please watch these videos in order. In the last videos, we talked about many definitions in genetics. Remember, genotype is your genetic characteristics.
Phenotype is how you appear as an individual, your observable trait, what we see. The way I remembered is that the phoenix has an observable characteristic shape to the eye. Parents are on the outside, offsprings are on the inside. Uppercase letters are dominant, lowercase letters are recessive. If both of your copies, i.e. alleles, are the same, you are homozygous.
If they are different, they are heterozygous. You could be homozygous dominant, like this doofus, or you could be homozygous recessive, like this door cry here. First, we start with parents, then the first generation of the offspring, then the second generation of the offspring. By the way, we say because it's for parents, but we do not say f for offspring, it's actually for filial.
So let's put parents on the outside and offspring on the inside. Here is uppercase and lowercase. They will meet each other. Boom.
Uppercase, lowercase. That's a tall individual because the big T dominates over the small t. Therefore, he appears tall.
He appears tall. Yeah. So his phenotype is tall.
What's his genotype? Uppercase T, lowercase t. And you keep repeating the same thing. This meets this.
Boom. This one meets this one. here, and this one meets this one here, you will see that 100% of the offspring are tall to the naked eye.
That's their phenotype. And all of them are capital T, small t. That's their genotype.
So what's the frequency of the genotypes here? 100%. Capital T, small t.
What's the frequency of the phenotype? 100% tall individuals. Mendel and his freaking peas.
Same concept. uppercase P is purple, lowercase p is the white flower. The uppercase dominates over the lowercase, i.e. the purple is the dominant characteristic, but the lowercase or the small p or the white is the recessive characteristic.
Suppose that one parent was purple and the other was white. Let's do it. This one meets this one here. This one meets this one here. This one meets with this one here.
This one makes love to this one here. All of them are genotypically uppercase P, lowercase p. All of them are phenotypically purple. So let's take one of these doofuses from the first generation and make him mate with another doofus and see what's going to happen to the next generation.
Let's go. You do this. One parent is here.
The other is here. Bingo. Here's what we found. One fourth was capital P, capital P, which appears purple. One half was capital P, small p, which appears purple.
How about small p, small p, only 25%, or one quarter, which appeared white. So what's the frequency of the genotypes? Well, here is one, two, to one.
Oh, 25%, 2, 50%, 2, 25%. How about the frequency of the phenotypes? 3 purple to 1 white. 75% purple to 1 white. 75% dominant color, the purple, to 1 recessive color.
So in the first generation, we had 100% purple. With the second generation of monohybrid crosses, you get 75% to 25% purple to white. Perfect. dominant to recessive.
Next, test cross, aka, who's your daddy? Suppose that your mom is small p, small p, white. This is a cougar pea plant, and all of her offsprings are purple.
All of them are cookie monsters. The question is, who's your daddy? Can we tell?
Well, since all of her offspring are purple, we can guess that their daddy had to be purple. But the daddy could be capital P, capital P. which is purple, or capital P, small p, which is also purple. How can we tell?
Well, we will try and assume it first, as if it was a capital P, and we'll run the same test, assuming that the unknown was a small p. And of course, you know that all her offsprings are purple. And then bingo, you match the mom with the girl. which is known with the dad.
Once this way, and once this way. And you see what happens. See here? 100% of the offsprings are purple. Does this match the criteria?
Yeah, it matched what I told you. How about here? Only half of the offsprings are purple. The other half are Karens. I mean whites.
I'm joking. Therefore, what's the result? The result is that this is your daddy.
This is the idea behind a test cross. You start with known offsprings and you go up a step trying to guess the unknown daddy. How about dihybrid cross? Same thing but with two characteristics at the same time.
For example, here is one individual and another individual and here is some matrimony action. The first individual is tall and purple at the same moment. The second one is also tall and purple.
And let's do this. These are the individuals. but these are their gametes. When you do all of this, you will find the following results.
Some of them were tall and purple, some of them were short and purple, others were tall and white, and few were short and white. And this is color-coded. So, what's the frequency of the phenotype? 9 tall and purple, 2, 3 short and purple, 2, 3 tall and white, 2, 1 short and white.
This is the result of a dye hybrid cross. Now contrast that with the mono-hybrid cross if you remember. This is the second generation ratio, 3 to 1. But in dihybrid cross, this is what you get, 9 to 3 to 3 to 1. If you liked this video, you will enjoy my general pharmacology course, pharmacokinetics and pharmacodynamics, lots of math equations.
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