Two genes interact to produce various phenotypic ratios among F2 progeny of a di

Question

Two genes interact to produce various phenotypic ratios among F2 progeny of a dihybrid cross. Design a different pathway explaining each of the F2 ratios below, using hypothetical genes R and T and assuming that the dominant allele at each locus catalyzes a different reaction or performs an action leading to pigment production. The recessive allele at each locus is null (loss-of-function). Begin each pathway with a colorless precursor that produces a white or albino phenotype if it is unmodified. The ratios are for F2 progeny produced by crossing wild-type F1 organisms with the genotype RrTt.

A) 9/16 green: 3/16 yellow: 3/16 blue: 1/16 white

B) 9/16 red: 7/16 white

C) 15/16 black : 1/16 white

D) 13/16 white : 3/16 green

Explanation / Answer

colorless precursor 1--activated by exresseion of T allele --- enzyme A- colorless precursor 2 --- activated by exresseion of R allele --- Enzyme B - compound C

Suppose the genotype of the parents is Tt and Rr then the F1 generation genotype would be TtRr which seem to be a case for dihybrid cross. Where T and R being dominant causes Red and Black color respectively.

Now F1 (TtRr * TtRr) genotype crossed to form the following offspring:-

TTRR- 9

T_R_- 9/16 - Red

T_rr – 3/16 - Red

ttR_ - 3/16 - Black

ttyy – 1/16 – no color/ white since there is absence of dominant allele at both loci.

Hence, the genotypic ratio would be 9:3:3:1

Now here comes the cases of epistasis that means the presence of one allele is masked the expression of other allele.

So, if Phenotypic ratio is Red – 9/16 and white - 7/16 that means it is the case of Recesive epistatsis

T_R_- 9/16 – Red

T_rr – 3/16 – No color/ white

ttR_ - 3/16 - No color/ white

ttyy – 1/16T - No color/ white

Hence, the ratio becomes 9:7

So, if phenotypic ratio is Balck –15/16 and white - 1/16 that means it is the case of Duplicate genes that means where a dominant gene is present, the trait is expressed, one allele is sufficient to produce the pigment.

T_R_- 9/16 – Red

T_rr – 3/16 – Red

ttR_ - 3/16 – Red

ttyy – 1/16T - No color/ white (since no dominant allele is present)

Hence, the ratio becomes 15:1

So, if Phenotypic ratio is White – 13/16 and Green - 3/16 that means it is the case of Dominant recessive epistasis which suggest that dominant allele at one locus and recessive allele at another locus give rise to same effect.

T_R_- 9/16 – Red

T_rr – 3/16 – No color/ white

ttR_ - 3/16 - No color/ white

ttyy – 1/16T - No color/ white

Hence, the ratio becomes 13:3

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