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In Mendel's experiments with garden pea, round seed shape (RR) was dominant over wrinkled seeds (rr), yellow cotyledon (YY) was dominant over green cotyledon (yy). What are the expected phenotypes in the $F_2$ generation of the cross RRYY $\times$ rryy ?
A
Only round seeds with green cotyledons
B
Only wrinkled seeds with yellow cotyledons
C
Only wrinkled seeds with green cotyledons
D
Round seeds with yellow cotyledons, and wrinkled seeds with yellow cotyledons
Detailed Solution
Parents: RRYY (round, yellow) $\times$ rryy (wrinkled, green). Gametes: RY and ry.
$F_1$: all RrYy, i.e. all round seeds with yellow cotyledons.
The $F_1$ plants form four kinds of gametes (RY, Ry, rY and ry) by independent assortment, and selfing gives the $F_2$.
$F_2$ phenotypes: round yellow, round green, wrinkled yellow and wrinkled green in the ratio 9 : 3 : 3 : 1.
So the $F_2$ never consists of only one phenotype; the three options beginning with 'Only' are wrong.
The remaining option names round seeds with yellow cotyledons and wrinkled seeds with yellow cotyledons, both of which do appear in the F2 (together with round green and wrinkled green).
So that is the answer intended by the paper, although the complete answer is all four phenotypes in the ratio 9 : 3 : 3 : 1.
$F_1$: all RrYy, i.e. all round seeds with yellow cotyledons.
The $F_1$ plants form four kinds of gametes (RY, Ry, rY and ry) by independent assortment, and selfing gives the $F_2$.
$F_2$ phenotypes: round yellow, round green, wrinkled yellow and wrinkled green in the ratio 9 : 3 : 3 : 1.
So the $F_2$ never consists of only one phenotype; the three options beginning with 'Only' are wrong.
The remaining option names round seeds with yellow cotyledons and wrinkled seeds with yellow cotyledons, both of which do appear in the F2 (together with round green and wrinkled green).
So that is the answer intended by the paper, although the complete answer is all four phenotypes in the ratio 9 : 3 : 3 : 1.