In 1988, Horst Wilkens investigated blind cavefish, comparing them with members of a sibling species with normal vision that are found in a lake [Wilkens, H. (1988). Evol. Biol. 25:271–367]. We will call them cavefish and lakefish. Wilkens found that cavefish eyes are about seven times smaller than lakefish eyes. F₁ hybrids have eyes of intermediate size. These data, as well as the F₁×F₁ cross and those from backcrosses (F₁×cavefish and F₁×lakefish), are depicted below. Examine Wilkens's results and respond to the following questions:

Based on the results of the F₁ backcross with lakefish, is your explanation supported? Explain. 

Video transcript

Hi everyone. Let's look at our next problem. It says by the BC four generation and back crossing, the offspring are blank genetically similar to the recurrent parent. So let's remind ourselves of how back crossing works. And I'm just going to put up a little diagram to kind of go through that. So in back crossing we have a hybrid pairing here of a recipient line. Uh This will also be the recurrent parent. Draw that over here and you have the recurrent parent paired with the donor line. And this will be the parent with a desired trait. And that's the trait you're trying to introduce into the recipient line. So here's the first pairing creating a hybrid offspring down here and that's the F one generation. Then you take that hybrid offspring and cross back to either the parent or an individual genetically identical to the parent. So our result will be in our first generation, our first back cross generation here labeled B. C. One. You see now the uh the hybrid offspring has only 1/4 The genes of the donor parent, 75% genetically someone or to the recurrent parent. We select in these crosses the offspring that have the desired trait. So among this 25% genetic similarity of the donor parent that includes the gene that we're trying to introduce into the recipient line. Now this Bc one generation crossed back to the recurrent parent and now we have a B. C. Two generation. You can see now the amount of genetic similarity to the donor parent is you know, becoming half each time. Um This little red dot here represents we've we've now gotten down to, you know, that portion but we have the gene of interest because we keep selecting for offspring that show the gene of interest. And so we can see, I've just put dot dot dot here. We can see as we carry forward each generation, the overall genetic similarity to the recurrent parent will become larger and larger and larger until you get to the point where you have an individual pretty much genetically almost identical to the recurrent parent. But including the gene we wanted to introduce from the donor line. So if we're looking for the B. C. Four generation, you know, looking carrying this forward, the offspring of that generation are going to be choice of greater than 96% genetically identical to the recurrent parent. As you can see when you kind of trace forward here, we're only with the B. C. Two generation, 75% is going to be. Uh we see that's only in the BC one generation here. So that's why Choice C. Is incorrect. And then choice a. And b. are 50 and 60% even less genetically similar genetic similarity to the recurrent parent. So it's easy to see that they're also incorrect. So again by the Bc four generation and back crossing the offspring are choice d greater than 96% genetically similar to the recurrent parent. See you in the next video