In a species of fish, a black spot on the dorsal fin is observed in males and females. A fish breeder carries out a pair of reciprocal crosses and observes the following results.

    Cross I      Parents: black-spot male x nonspotted female
                      Progeny: 22 black-spot male
                                     24 black-spot females
                                     25 nonspotted males
                                     21 nonspotted females
     Cross II     Parents: nonspotted male x black-spot female
                       Progeny: 45 black-spot males
                                      53 nonspotted females

Identify which sex is heterogametic. Give genotypes for the parents in each cross, and explain the progeny proportions in each cross.

Video transcript

Welcome back. Here's our next question. It says which of the following is an example of an organism with a female heterogametic sex determination system. And our answer choices are a fruit flies. B birds, C human or D grasshopper. Well, our answer we're going to look for here is choice B birds, because in birds, the females are the heterogametic sex, their sex chromosomes are labeled Z W while the males and birds, birds here have the sex chromosomes, Z Z. So the males are the homogametic sex. We'll just go through the other answer choices to see why they're not correct. Choice. A fruit flies, fruit flies have an X Y male, X X female system like humans. So choice A is not correct. And of course, choice. C humans also like fruit flies, the X Y male X X female. So not our correct answer. And then choice the grasshopper. Um and grasshoppers, it's slightly different, but they do have males having a single X chromosome while females have two X chromosomes, that's the distinction between sex, regardless of uh the presence or how many chrome Y chromosomes there are. So again, you don't have a female heterogametic sex because the females have two X chromosomes. So choice D is not correct. So which is an organism with a female heterogametic sex determination system? Choice B birds. See you in the next video.