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 strictly on the F₁ and F₂ results of Wilkens's initial crosses, what possible explanation concerning the inheritance of eye size seems most feasible? 

Video transcript

Welcome back. Let's look at our next question here. It says which of the following statements about the back cross experiment is true. So first let's recall what a back cross experiment is that up here where we have recurrent crosses between two lines, a recipient line and a donor line in which the offspring of those crosses keeping crossed back with an individual. Either it's the parent or an individual genetically identical to the parent. With the goal of producing offspring who are nearly genetically identical to one of the parents. So as you go further, the offspring become more and more similar to the recipient line in their genetic makeup. So that is a back cross that. So let's look at our answer choices to determine which answer is true about a back cross experiment choice A says. It can be used to determine if the person is home as I guess dominant or hetero is dominant. Well that is not the purpose of a back cross. As we said. Use back cross to generate offspring similar to one parent. If you're wanting to determine whether an individual is homos I guess dominant or hetero sex dominant. So you've got an individual showing a dominant phenotype and you want to determine whether their homes I guess or hetero guys for that dominant trait. What you're going to use is a test cross rather than a back cross. So that's when you cross your unknown genotype with a home as I guess recessive individual. So for instance if our gene is big a and little a We cross with little a little a parent and if we see offspring that show any recessive phenotype, we know that you must have a heterogeneous parent because we won't see a recessive phenotype in the offspring unless the unknown parent can also donate a recessive allele. So Joyce A. Isn't correct because that describes a test cross rather than a back cross choice B. Says the organism with an unknown genotype is bred with an organism that is home Zegas recessive. Again, that's our test cross. Not our back cross A. Choice B. Is incorrect choice C. Says it is used to determine the genetic makeup of the dominant organism that's also our test cross. So we'll eliminate that. And finally we've got choice D. It can be done by crossing a hybrid with an individual whose genetic makeup is similar to one of its parents. And that is correct. We have our hybrid offspring and cross it back to an individual with a genetic makeup similar to its parent and then do that over and over again for multiple generations. So which statement about the back cross experiment is true choice D. Can be done by crossing a hybrid with an individual whose genetic makeup is similar to one of its parents. See you in the next video