In a study of black guinea pigs and white guinea pigs, 100 black animals were crossed with 100 white animals, and each cross was carried to an F₂ generation. In 94 of the crosses, all the F₁ offspring were black and an F₂ ratio of 3 black:1 white was obtained. In the other 6 cases, half of the F₁ animals were black and the other half were white. Why? Predict the results of crossing the black and white F₁ guinea pigs from the 6 exceptional cases.

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Hi everyone, welcome back, let's look at our next problem. It says in crossing a black and short guinea pig, Big B. Little B. Little s little s with a white and long, little B. Little B. Big s little S. Guinea pig, what is the fanatic ratio of black to white offspring phenotype? So we have here a cross with two different traits involved. But let's note when we look at our question that it's only asking us about the offspring phenotype of one trait the color of the guinea pigs. It's not asking us whether they're short or long. Um So by the principle of independent assortment, when we're looking to figure out what the ratio of the offspring phenotype sar, we only need to look at the one trait of color. So we can see from the parent genotype were given. We have a black short guinea pig and we see that the color genes for black hetero, zika's big B. Little B. The white parent colored jeans are little B, little B. So from that we can see that the black alil is the dominant allele Bigby and the white is the recessive little B. So let's look at our punnett square again, remembering that we only need to look at the smaller punnett square because we only need to look at the ratio of offspring based on the color trait. So we'll look at the first parent, the black parent we know is big B. Little B. The white parent is little B, little B. So if we fill out our punnett square here and look at our offspring Jenna types and then convert that to Fiona types. We've got half of our offspring here. Um Are we know it inherited the dominant black alley and so they will be black And then half the offspring will be white. So we've got our offspring ratio of 2-2 or 1-1 black to white ratio. So our answer here is choice b. 1 - one. Now if you like. And you want to just confirm that even if we showed the whole cross, both traits that we end up with the same exact ratio of color in the offspring phenotype. Um Keep watching this video and we'll do the larger punnett square showing both traits in the cross. So I'll just paste a nice paste a nice big 16 square upon it square for our parent cross. And we recall that with two different two different traits. We see four different types. So here's our black short parent and the different gametes will get from that, the wrong letter here. So that's the black short parent and then the white long parent. Those are the gametes we get from that. So we know that any offspring that inherits a big be a legal is going to have the trade black. That's the dominant allele from the white parent. They'll always inherit the recessive allele. But from the black parents, they'll inherit that. So all of these squares here will be offspring that inherit that black hole and be black. Whereas these other squares will not inherit black and the white. So we're gonna end up with and 8-8 ratio Equaling 1-1 of black to white. And rather than watch me fill this all out, if you'd like to see the filled out square and what that looks like, hang on just a moment and I'll paste in those offspring. So we put in all of the genotype for the offspring. And indeed we see that eight of these squares, the offspring of an inherited the black their hetero sickness and therefore will be black. And eight of these squares they've inherited two recessive white alleles and will be white. So again that confirms our answer choice B 1 to 1. And you can see the ratio is exactly the same. Whether you look at just the one trait in the smaller pendant square or both traits out in a larger opponent square. See you in the next video

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Key Concepts

Mendelian Inheritance and Dominance

Genotype and Phenotype Ratios in F₁ and F₂ Generations

Genetic Explanation for Deviations in Expected Ratios