The progeny from today's mating (F1 generation) will be used as the parents in a subsequent cross, and you will analyze the offspring from that cross (the F2 generation) in four weeks (Lab #5). As will be explained in more detail in Lab #5, these flies differ at three genes. These genes code for eye color (wild type=red [gene w+], mutant = white [gene w], wing shape (wild type = normal [gene m+], mutant = miniature [gene m], and bristle shape (wild type = normal [gene f+], mutant = forked [gene f]. We know that these genes are linked, meaning they are on one chromosome not on different chromosomes and will therefore be inherited together, and that they exist in this order (wm/). The two fly stocks you will be handling today are either all wild type or all mutant at these three loci. You will be setting up a mating between mutant females and wild type males. 1. Identify a male and female fly. The most reliable way of distinguishing the sex of flies is by examining the external genital organs under low magnification, i. e., using a dissecting microscope or magnifying lens. Only male flies exhibit darkly colored external genitalia, which are visible on the ventral side of the tip of the abdomen. The following characteristics may also be helpful in distinguishing males from females. The female is larger and has alternating dark and light dorsal bands on the abdomen. The dorsal abdomen of the male is uniformly pigmented. Conspicuous sex combs are visible on each foreleg of the male. In ventral view, the male's darkly pigmented genital arch and penis are visible.
Size: Females are usually larger than males.
Shape: In a dorsal view, the abdomen of the male is round and blunt, whereas that of the female is sharp and protruding.
Color: Black pigment is more extensive on the abdomen of the male than on that of the female. On the male, the markings extend completely around the abdomen and meet on the ventral side. On the female, the pigment is present only in the dorsal region.
Sex Combs: Only males have a small tuft of black bristles, called a sex comb, on the anterior margin at the basal joint of each front leg.
To perform a cross:
1. Lay a culture bottle on its side and brush 3-4 anesthetized virgin females from one strain and 5-7 anesthetized males from the other strain into the bottle. Cap the bottle. Do not stand the bottle upright until the flies are fully recovered.
2. Make sure the bottle is labeled with: the date, your name/group, the cross designation (genotypes being crossed).
3. Give the bottle to your instructor. They will incubate the bottle at 25°C; at this temperature the parent flies will be cleared out after one week and a subsequent cross with the offspring can be made a week later (i. e. crosses can be done on a semiweekly basis). Discard extra flies in the morgue when you are finished with this activity.
1) Write out the cross you did today, using the correct nomenclature and symbol designations, give the genotypes and phenotypes for the parents and the expected F1 progeny.
2) Write out the cross, assuming the F1 flies are selfed, give the genotypes and phenotypes for the parents (i. e. the Fl) and the expected F2 progeny.
3) How would it change the expected cross results if the three genes were not located on the same chromosome?