Question

You have isolated two loss-of-function, recessive mutants in elves: blunt (blt) mutants have smooth, rounded ears and spikey (spy) mutants have extra pointy ears. You create bly; spy double mutants and find that the double mutants have extra pointy ears. Which of the following indicates the correct genetic relationship between bly and spy? (In other words, which gene acts first in the pathway and does it activate or repress the downstream gene.)

spy ---> blt

blt ---> spy

spy ---| blt

blt ---| spy

Answer 1

Answer: Option D is correct.

Explanation:
In a simple genetic cross, the phenotype seen in the double mutant corresponds to the downstream factor.
This rule is applicable only to recessive mutations.

In the given case, blt spy double mutant is like spy. So, spy is downstream to blt.
blt is inhibiting spy in the WT scenario. When blt and spy are absent, the double mutant shows spy phenotype.