In Drosophila melanogaster, the brick-red eye color is due to the presence of tw
ID: 302682 • Letter: I
Question
In Drosophila melanogaster, the brick-red eye color is due to the presence of two distinct protein pigments: pteridines and ommochrome. These two pigments are synthesized by two separate biochemical pathways and are transported to the compound eyes by transporter proteins (ABC proteins) coded by white, brown and scarlet genes. The enzymes that are specific to each pathway are coded by the nuclear genes.
If a mutation occurs in one or both of the pigment pathway, the mutant fly will have different eye color than the wild-type brick red eye color. If a mutation in a gene coding for an enzyme in the ommochrome pathway, the brown pigment will not be synthesized, and the resulting mutant fly will have bright red color. If a mutation occurs in one of the genes coding for any one of the intermediate compounds in the pteridine pathway, the resulting fly might have brown, dark brown or even black eyes. If both pigment pathways are blocked, no pigments are produced, and the mutant fly will have white eye.
In Drosophila, the gene for white (w) eyes is on the X chromosome and it is epistatic over brown and scarlet. White eyes can also result by crossing brown-eyed females (bw) with scarlet eyed males (st) (that is, when both mutant alleles are present). A cross is made between white-eyed females with the genotype w//w (otherwise, homozygous for wild type alleles) and white-eyed males hemizygous for the wild type allele (w+ ).
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List the expected:
a) Genotypes of the P1 and P2 flies
. b) F1 phenotypes and genotypes. 26
c) Expected F2 phenotypic ratios. Show your work.
CONCEPT PROBLEM 4 How can the same phenotype stem from many different genotypes? Explain.
Explanation / Answer
a) genotype of P1- white eyed females - ww
P2- white eyed males - (w- w+)
b) F1 phenotype - all white eyed flies
Genotype - w+w-, w+w+
C) F2 Genotypic ratio 1:2:1 Al phetypes will be white.
Many genotype might have same phenotype because of the dominant or recessive alleles. Dominant allele will be expressed.