To determine which genetic cross will yield four phenotypes in the 1:1:1:1 ratio, let's analyze each cross:
1. rrY x rryy:
- This cross involves a homozygous recessive for both traits (rryy) and one that is homozygous recessive for one trait and heterozygous for the other trait (rrY).
- The genotype possibilities here do not produce a variety of phenotypes because there are multiple homozygous recessive traits involved.
2. RrYy x RrYy:
- This is a dihybrid cross where both parents are heterozygous for both traits.
- The resulting phenotypic ratio from this cross is typically 9:3:3:1, not 1:1:1:1.
3. RRYY x rryy:
- This cross involves a homozygous dominant for both traits (RRYY) and a homozygous recessive for both traits (rryy).
- All offspring from this cross would be heterozygous (RrYy) for both traits, resulting in only one phenotype.
4. RrYy x rryy:
- This cross involves a parent that is heterozygous for both traits (RrYy) and one that is homozygous recessive for both traits (rryy).
- The gametes produced by RrYy can be: RY, Ry, rY, and ry.
- The gametes produced by rryy can only be ry.
- The resulting genotypes when these gametes combine are as follows:
- RY x ry -> RrYy
- Ry x ry -> Rryy
- rY x ry -> rrYy
- ry x ry -> rryy
- Each genotype occurs with equal probability, resulting in four phenotypes with a 1:1:1:1 ratio.
Therefore, the cross that yields a 1:1:1:1 phenotypic ratio is RrYy x rryy.
