In a test of his chromosome theory of heredity, Morgan crossed a female Drosophila with red eyes to a male with white eyes. The females were produced from Cross A shown in Figure 3.19. Predict the offspring Morgan would have expected under his hypothesis that the gene for eye color is on the X chromosome in fruit flies.

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Hi, everyone. Let's take a look at this practice problem together, Morgan chose to investigate the color of the eyes in the mono hybrid cross he conducted with Joola. He crossed a red eyed female with a white eyed male during the parent generation. It was unknown at the time whether the gene for eye color was on sex chromosomes or autosome. Morgan concluded that the gene for eye color inro soila was on the X chromosome based on which of the following observations option. A presence of only males with red eyes in the F two generation B, no females with white eyes were born in the F two generation C. Only males with white eyes were born in the F two generation in option D, both B and C. So on the screen, I am putting up a diagram to uh help us understand Morgan's experiments. The diagram depicts three generations of droid test crosses and together we work through them and fill in eye color and discuss Morgan's findings. So Morgan began his experiments when he came across a mutant wide eyed male. He bred a red eye female with a white eye male in the parental generation. So let's add red color to the female's eyes in the parental generation. And that is the fly on the left. Now, recall the results were that all F one offspring had red eyes. So let's add red color to the eyes of the F one offspring. He hypothesized correctly that the white eye color trait was recessive since white eye color was not present in the F one offspring to support his hypothesis. Morgan did a test cross between a red eyed female and a red eye male from the F one generation. Now, if the white eye color was recessive and inheritance followed Mendelian laws, we would expect a 3 to 1 phenotypic ratio with three red eyes to one white eye. And indeed, that is what happened. However, after several of the test crosses, he notices something different and there's something different is something important. So there is the 3 to 1 phenotypic ratio of three red eyes to one white eye. However, only males in the F two generation have white eyes and it is only half of the males that have white eyes. There is not a single female in the F two generation with white eyes. From this. Morgan concluded that the eye color gene was inherited differently between females and males and thus, it must be sex linked and in particular X linked. So let's take a look at those answer options. Again, option A that only males with red eyes are in the F two generation is false. In the F two generation, half of males are red eyed and half are white eyed. So option A is incorrect. Option B that there are no white eyed females in the F two generation is true. So option B is a correct statement. Option C states that only males with white eyes were born in the F two generation. And again, this is also true. Remember only males with white eyes exist in the F two generation. So because both B and C are correct, the correct answer is option D, both B and C. All right, everyone. I hope you found this helpful and I'll see you soon for the next practice problem.

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Key Concepts

Chromosome Theory of Heredity

Sex-linked Inheritance

Drosophila Melanogaster Genetics