In fruit flies as recessive mutation in either of two independently assorting ge
ID: 55937 • Letter: I
Question
In fruit flies as recessive mutation in either of two independently assorting genes, brn and prp which prevent the synthesis of wild type red eye pigment. Homozygotes for either of these mutations have brownish purple eyes. Heterozygotes for both these mutations have wild type eyes. (A) If double heterozygotes are crossed with each other what are the phenotypes and expected ratios for the offspring? [Let the wild type (normal) alleles of each gene be brn+ and prp+ respectively.] (B) What type of genic interaction is occurring here?
Explanation / Answer
Let the wild type alleles be brn+ and prp+ and mutant alleles be brn- and prp-
The different phenotypes possible are as follows:
brn+brn+prp+prp+ (homozygous wild type: red eyes)
brn+brn-prp+prp- (Heterozygous: red eyes)
brn-brn-prp-prp- (Homozygous recessive: brownish purple eyes)
The below Punnett Square shows the cross between 2 double heterozygous fruit flies.
The different allelic combinations possible for a heterozygous fruit fly are:
brn+prp+ ; brn+prp- ; brn-prp+ and brn-prp-
Parental genotypes
brn+prp+
brn+prp-
brn-prp+
brn-prp-
brn+prp+
brn+prp+ brn+prp+
(Red eyes)
brn+prp- brn+prp+
(Red eyes)
brn-prp+ brn+prp+
(Red eyes)
brn-prp- brn+prp+
(Red eyes)
brn+prp-
brn+prp+ brn+prp-
(Red eyes)
brn+prp- brn+prp-
(Purple eyes)
brn-prp+ brn+prp-
(Red eyes)
brn-prp- brn+prp-
(Purple eyes)
brn-prp+
brn+prp+ brn-prp+
(Red eyes)
brn+prp- brn-prp+
(Red eyes)
brn-prp+ brn-prp+
(Purple eyes)
brn-prp- brn-prp+
(Purple eyes)
brn-prp-
brn+prp+brn-prp-
(Red eyes)
brn+prp-brn-prp-
(Purple eyes)
brn-prp+brn-prp-
(Purple eyes)
brn-prp-brn-prp-
(Purple eyes)
This gives us the phenotypic ratio of 9 red eyed fruit flies to 7 purple eyed fruit flies, which is the modified 9:3:3:1 ratio related to dihybrid crosses.
This is an example of Epistasis wherein 2 genes control a single phenotype. In this case brn and prp both control eye color.
Parental genotypes
brn+prp+
brn+prp-
brn-prp+
brn-prp-
brn+prp+
brn+prp+ brn+prp+
(Red eyes)
brn+prp- brn+prp+
(Red eyes)
brn-prp+ brn+prp+
(Red eyes)
brn-prp- brn+prp+
(Red eyes)
brn+prp-
brn+prp+ brn+prp-
(Red eyes)
brn+prp- brn+prp-
(Purple eyes)
brn-prp+ brn+prp-
(Red eyes)
brn-prp- brn+prp-
(Purple eyes)
brn-prp+
brn+prp+ brn-prp+
(Red eyes)
brn+prp- brn-prp+
(Red eyes)
brn-prp+ brn-prp+
(Purple eyes)
brn-prp- brn-prp+
(Purple eyes)
brn-prp-
brn+prp+brn-prp-
(Red eyes)
brn+prp-brn-prp-
(Purple eyes)
brn-prp+brn-prp-
(Purple eyes)
brn-prp-brn-prp-
(Purple eyes)