Wild type flies have brick-red eyes. Flies can also have bright-red eyes. This t
ID: 55665 • Letter: W
Question
Wild type flies have brick-red eyes. Flies can also have bright-red eyes. This trait is determined
by a recessive allele, called "scarlet" & labeled as "st". The dominant allele of this gene is labeled
as "st+". Thus, flies with the genotype "stst" will have bright-red eyes, & flies with the genotype
"st+st+" or "st+st" will have Wild type eyes. Flies can also have brown eyes. It is also a recessive
trait determined by the gene "brown". Flies with the genotype "brbr" will have brown eyes, &
flies with the genotype "br+br+" or "br+br" will have Wild type eyes. If you cross flies with brightred
eyes & flies with brown eyes, all the offspring will have Wild type eyes. A) What are the
genotypes of P & F1 flies? If you cross 2 F1 flies, you observe 9/16 Wild type, 3/16 brown, 3/16
scarlet, & 1/16 white-eyed flies. B) Explain these results?
Explanation / Answer
(a) Parental genotypes
st st X br br
st br
F1 genotype (wild type)
(b) Crossing two F1 flies (st br)
st br X st br
gametes : st, br, st, br
st st (bright eyed, scarlet)
st br (wild)
st br (wild )
br br (brown)
Though it looks like a case of independent assortment of gametes as we got the 9:3:3:1 ratio. In reality, it is a case of epistasis, where effect of one gene is dependent on the presence of one or more ‘modifier’ gene. As here, eye color of a fruit fly is determined by the synthesis of two pigments – red and brown. Recessive allele ‘br’ in the homozygous state blocks the synthesis of red pigment, hence we get brown pigment. Similarly, recessive allele ‘st’ in the homozygous state blocks the synthesis of brown pigment, hence we get red pigment. Thus the interaction of the genes is epistatic in nature.
st st (bright eyed, scarlet)
st br (wild)
st br (wild )
br br (brown)
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