Question

A cross was made between a male fruit fly with purple eyes and two different female flies with red eyes. The first female produced 41 progeny that all had red eyes, whereas the second female produced 18 progeny with red eyes and 21 progeny with purple eyes. a. Diagram the two crosses in words (you don't need to show the genotypes yet). b. Which eye color is dominant and which is recessive? How do you know? c. Choose a symbol for eye color in flies, and write in the genotypes of the flies in your crosses above. d. Are there any genotypes that you are uncertain about? If so, why? e. If one of the red-eyed males from the first female's progeny were mated with a purple- eyed female, what types of progeny would you predict, in what ratio? Show the steps in your solution here: f. If a purple-eyed male and a purple-eyed female, both from the second female's progeny, were mated with each other, what types of progeny would you predict, in what ratio? Show your work g. Read the information about loss-of-function alleles on p. 32 of the rextbook. At the molecular level, what could explain the dominance or recessiveness of the red and purple eye colors in the cross above?

Answer 1

a) cross 1

the cross is between male flies with purple eyes and female with red eyes

purple-eyed males* red-eyed females

the second cross is also phenotypically same

purple-eyed males* red-eyed females

b) red eye colour is dominant to purple eye colour since all the progenies of cross 1 has red eyes, ( female is homozygous for red eyes so all progenies receive alleles for red eyes so all progenies show red eyes).

c) let R be the allele for red eyes and r for purple

male parent is rr and the first female is RR and the other female is Rr ( so she can produce progenies with red and purple eyes when mated with a male with purple eyes)

d) No, because male shows recessive phenotype, so he is rr and progenies of the first female, is all red so she has to be homozygous for R and another female produces both purple and red-eyed females so she has to be Rr to produce Rr and rr progenies.

e) red-eyed males of the first female's progeny are Rr is mated with rr female

	R	r
r	Rr ( red eyes)	rr (purple eyes)

red eyes and purple eyes are produced in the equal ratio

f) both male and female are purple eyed so both are rr

rr*rr

	r
r	rr

so all progenies are purple eyes.

g) loss of function mutation means the mutated allele does not have any function so the eye colour may be the result of a pathway where the product of gene R produces an active enzyme which converts purple pigment to red so in the absence of an active copy of R gene Purple eyes are produced.