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.
<u>Banded vs unbanded:</u>
The chance of getting bb from our testcross is <u>1/2.</u>
Bb x bb
b b
B Bb Bb
b bb bb
<u>Yellow vs brown:</u>
The chance of getting Yy or YY from our testcross is also <u>1/2.</u>
Yy x yy
y y
Y Yy Yy
y yy yy
So the combined probability is 1/2 * 1/2 = 1/4.