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.

9/16 Dark Blue 6/16 light blue 1/16 white

A) At least one copy of each dominant allele results in dark blue, at least one copy of either dominant allele produces light blue, and the absence of both dominant allele produces white. B) If both dominant alleles are present, the result is dark blue. At least one copy of one specific dominant allele is required for light blue. If that dominant allele is not present, the result is white, regardless of whether the other dominant allele is present. C) At least one copy of both dominant alleles results in dark blue; at least one copy of one of the dominant alleles also results in dark blue, but at least one copy of the other dominant allele produces light blue; and the absence of either dominant allele produces white.

Explanation / Answer

ANSWER

The statement A is correct.

At least one copy of each dominant allele results in dark blue, at least one copy of either dominant allele produces light blue, and the absence of both dominant alleles produces white

Explanation:

The present cross is a type of epistasis namely Duplicate genes with cumulative effect.

Duplicate genes with cumulative effect: Certain phenotypic traits depend on the dominant alleles of two gene loci. When dominant is present it will show its phenotype. The ratio will be 9: 6: 1. Complete dominance at both gene pairs, interaction between both dominance to give new phenotypes.

From the data given

Gene pair R Blue dominant over white or albino.

Gene pair T Blue dominant over white or albino.

Interaction at RT when present together, forms a Dark blue coloured progeny.

RRrr               X                     TTtt

Blue coloured                                   Blue coloured

Gametes produced are Rr and Tt

The progeny produced in F1 generation are RrTt – Dark Blue

If we crossed F1 progeny with RrTt then as follows

RrTt                X         RrTt

Then F2 progeny as follows

RT

Rt

rT

rt

RT

RRTT, Dark blue

RRTt, Dark blue

RrTT, Dark blue

RrTt, Dark blue

Rt

RRTt, Dark blue

RRtt, Light blue

RrTt, Dark blue

Rrtt, Light blue

rT

RrTT, Dark blue

RrTt, Dark blue

rrTT Light blue

rrTt Light blue

rt

RrTt, Dark blue

Rrtt, Light Blue

rrTt , Light blue

rrtt, white

Dark Blue-9/16: 6/16 Light blue: 1/16white    

At least one copy of each dominant allele in present in Dark blue.

At least one copy of either of the dominant allele is present in Light blue.

Both the dominant alleles are completly absent in White.

RT

Rt

rT

rt

RT

RRTT, Dark blue

RRTt, Dark blue

RrTT, Dark blue

RrTt, Dark blue

Rt

RRTt, Dark blue

RRtt, Light blue

RrTt, Dark blue

Rrtt, Light blue

rT

RrTT, Dark blue

RrTt, Dark blue

rrTT Light blue

rrTt Light blue

rt

RrTt, Dark blue

Rrtt, Light Blue

rrTt , Light blue

rrtt, white

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