Question 4. In fruit flies a recessive mutation, w, causes the eyes to be white.

Question

Question 4. In fruit flies a recessive mutation, w, causes the eyes to be white. Another recessive mutation, v, causes them to be vermilion, and a third recessive mutation, bw, causes them to be brown. The wild type eye color is red. Vermilion is a bright shade of orange and, phenotypically, is distinguishable from the brown eye color. Hybrids produced by crossing any two homozygous mutants produce red eyes All the doubly homozygous mutant combinations have white eyes. Questions: (a) How many genes do these mutations define? (b) If the red color of the wild type eyes is due to the accumulation of two different pigments, one orange/vermilion and the other brown, which gene controls the expression of which pigment? (c) Can the genes be ordered into a pathway for pigment accumulation? If so, how?

Explanation / Answer

a. Several genes affect on the eye color in fruit flies. It is explained as multiple allele inheritance.

Pigments of eye color are transported by ABC transporters .In drosophila it is:white+scarlet for brown pigments and white+brown for red.Scarlet and brown double mutants are white eyed. Most other genes involved in eye pigmentation affect synthesis of either red or brown.There are some mutants affecting vesicle transport.

Eye pigmentation in fruit flies are under the influence of many genes located in different parts of genome. Mutations in such genes, resulted in altered phenotypes of its eye color. The role of white eye gene located in the X chromosome play a viral role as this gene in wild type form helps to transport the pigment to surprises appropriate area to provide normal eye pigmentation.

b. There are two classes of pigments in the eye of fruit flies. Brown pigments called ommochromes which are products of tryptophane metabolism and red pigments called pteridines which are the products of purine metabolism. The wild type eye color is due to the mixture of several different pigments.

Drosopteridine is orange under ultraviolet light while other pteridines are blue or yellow. Under ultraviolet light orange makes the wild type eye red.

c. Yes. The genes can be ordered.

The eye color of fruit fly is the sum of different pigments with different concentration.

There are two pathways:pteridines (GTP) and ommochromes (tryptophan)

=>pteridin => biopterin(blue)=>drospterin(red-orange)

=>dihydropterin(blue)

=>xanthopterin(yellow-orange green) =>isoxanthopterin(purple )

=>sepiapterin(yellow-orange)

=> ommochrome =>cynerinin => hydroxanthommatrin=>xanthommatrin(brown)

These genes are located throughout the genome.Each step of the pathways are catalyzed by a separate enzyme .If the enzyme is not produced (like mutations) the pathway is halted.

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