To determine which cross yields four phenotypes in the 1:1:1:1 ratio, we need to understand what's happening during the cross-breeding of organisms with different genotypes.
1. Phenotype Ratios: The phenotypes refer to the observable traits of the organisms, such as shapes, colors, etc. A 1:1:1:1 ratio indicates that there are four different phenotypes present in equal proportions.
2. Dihybrid Cross Explanation: A dihybrid cross involves organisms that are heterozygous for two traits. For example, RrYy means the organism has one dominant and one recessive allele for two separate traits (R and Y).
Let's analyze each given cross:
1. myy x rry:
- This seems to be an invalid genetic representation because typically, the letters used for alleles should be the same case format (R/r for one trait and Y/y for another). Disregard this as it's an invalid format.
2. RrYy x RrYy:
- This is a dihybrid cross between two heterozygous organisms.
- The expected phenotype ratio for a dihybrid cross of this nature is typically 9:3:3:1, not 1:1:1:1. This isn't the correct cross.
3. RRYY x rryy:
- Crossing an organism that is homozygous dominant for both traits (RRYY) with one that is homozygous recessive (rryy).
- All offspring would be RrYy, resulting in a single phenotype ratio (all dominant traits expressed). This too is incorrect.
4. RrYy x rryy:
- This cross involves one organism that is heterozygous for both traits (RrYy) and one that is homozygous recessive (rryy).
- The heterozygous organism can pass on either R or r and Y or y, while the homozygous recessive organism will only pass on r and y.
- This results in the following genotypes: RrYy, Rryy, rryy, and rrYy.
- These genotypes translate to four different phenotypes in a 1:1:1:1 ratio.
Thus, the correct cross that will yield four phenotypes in the 1:1:1:1 ratio is:
RrYy x rryy
