The smooth feathers on the back of the neck in pigeons can be reversed by a mutation to produce a 'crested' appearance in which feathers form a distinctive spike at the back of the head. A pigeon breeder examined offspring produced by a single pair of non-crested birds and recorded the following: 22 non-crested and 7 crested. She then made a series of crosses using offspring from the first cross. When she crossed two of the crested birds, all 20 of the offspring were crested. When she crossed a non-crested bird with a crested bird, 7 offspring were non-crested and 6 were crested. Which allele is dominant?

Video transcript

Hi everyone here's our next problem. A researcher sets three mating pairs and her observations are as follows. Male mouse having brown fur. Made it with a female mouse having brown for the observation of 10 pups with brown fur and three had white fur. Experiment two male mouse with white fur made it with female mouse having white for observation. All eight pups had white fur. And finally experiment three male mouse having Brown fur made it with female mouse having white fur observation. Five pups had brown fur and four had white fur determine which allele is dominant and which is recessive. So I'm not going to go in order of the experiments because the easiest thing to kind of start with is if we have a recessive allele if you have two parents who show the recessive trait and you made them to each other, all the offspring will only show the recessive trait. Whereas if you have two parents with a dominant trait, uh some of the offspring could show the recessive trait if the parents were heterosexual. So I'm first going to zoom in on experiment too Because experiment two shows an outcome where all of the offspring had the same color. For we have a male with white fur cross with a female with white fur and all offspring are white and it's eight offspring. So pretty significant number. So that's going to cause me to hypothesize that the recessive trait is white and that the dominant is brown. So we'll label it as capital B. For the dominant trait and little B. For the recessive trait. So let's look at the other experiments and see whether they hold up that hypothesis. So an experiment one, I have a brown male crossed with a brown female. When I look at the offspring, I show 10 with brown 2, 3 with white. Well I know that the brown parents must be hetero sickos. If I'm positing that brown is dominant and white is recessive because if the parents were homesick is dominant, we couldn't have any white offspring here. So we're going to assume this is a hetero saiget versus heterocyclic cross. So, Big B. Little B. Big B. Little B. And fill in this chart with our expected offspring. And we would see an expected ratio of 3- brown to white. Because we're going to see these three are going to be brown having at least one copy of the dominant gene. And we see a ratio of 10-3, so that's pretty close 3-1. So that sort of holds up our hypothesis here. And then let's look at experiment three where we had a brown male along here, brown male oops crossed with a white female and our offspring show a five brown 2 4 White ratio. Now then when we look at our punnett square again, a brown male could be big B. Big B. But if you could only pass on a dominant brown gene, we wouldn't expect any white offspring. So we know that our brown male must be hetero. Saga's here. Our white female. Again, if we're positing that the white is recessive, we'd expect little B. Little B. And then we look at our offspring. We're going to see a 1-1 ratio expected of brown to white. Again, I'll highlight the brown ones And that's about what we see. We have a 5-4 ratio pretty close 1-1. So given that the questions has determined which allele is dominant in which is recessive. It looks like our theory of recessive being white and dominant being brown is correct. So let's look over at our answer choices. Choice B says dominant brown. For recessive white fur. So we'll just briefly see why the other choices are not correct. Choice A. Has them reversed. But we know that the recessive cannot be brown because of the ratios that we're seeing here, particularly our headers I get crossed where we see a majority brown. So that's not correct. Choice E. Is both alleles for for color are co dominant. Well this is not correct. If we had a co dominant situation, we'd expect to see a range of three colors brown, white and like a light brown because hetero would show co dominance A blending of the two traits. Choice C. Not correct and choice D. Says insufficient data to interpret. Well are three experiments did give us enough data to and for this based on the ratios of the offspring. So again our answer is choice be dominant Brown. For recessive white. For thanks for watching. See you next.