Question

In a snail, an autosomal allele causing a banded shell (b) is recessive to the allele for unbanded shell (B). Genes at a different locus on the same chromosome determine the background color of the shell; here, yellow (y) is recessive to brown (Y). The genes for shell banding and background color have a recombination frequency of 0.25. A banded, yellow snail is crossed with a homozygous brown, unbanded snail. The F1 are then crossed with banded, yellow snails (a testcross). What proportion of progeny will be banded and brown

Answer 1

Answer:

1/4

Explanation:

We are crossing a banded yellow snail with a brown unbanded snail.

bbyy x BBYY

Next we cross the F1, BbYy, with a banded yellow snail.

BbYy x bbyy

The question asks us what proportion will be banded brown, which is bbYY or bbYy. Separate this into two Punnett squares, one per trait.

Banded vs unbanded:

The chance of getting bb from our testcross is 1/2.

        Bb x bb

          b     b

    B  Bb    Bb

    b  bb    bb

Yellow vs brown:

The chance of getting Yy or YY from our testcross is also 1/2.

        Yy x yy

           y    y

    Y  Yy    Yy

    y   yy    yy

So the combined probability is 1/2 * 1/2 = 1/4.