Final answer:
To find the number of phenotypes after crossing, identify the number of heterozygous gene pairs in the cross and use rules such as the product and sum rule to calculate phenotype probabilities. For a dihybrid cross, the resulting phenotypic ratio is typically 9:3:3:1, and more complex crosses follow specific ratios based on the number of traits and alleles involved.
Step-by-step explanation:
Finding the Number of Phenotypes After Crossing
In genetics, finding the number of phenotypes after crossing is crucial for understanding Mendelian inheritance. For instance, when performing a dihybrid cross, you first need to identify how many gene pairs are heterozygous (represented by n). A dihybrid cross with heterozygous pairs AaBb × AaBb, would have n = 2, leading to a 9:3:3:1 phenotypic ratio. This ratio can be further expanded into specific phenotype probabilities for individual traits using the product rule for independent events, which in the case of two traits, both with a dominant and recessive pattern, would collapse into two 3:1 ratios.
Additionally, when analyzing complex crosses such as a trihybrid cross, a forked-line method is often used to visualize the probability of each phenotype based on independent assortment. For such a cross, an F2 generation would follow a phenotypic ratio of 27:9:9:9:3:3:3:1. In cases of tetrahybrid crosses, to find the proportion of offspring showing the dominant phenotype for all traits, you apply the product rule across all loci, after determining probabilities for homozygous dominant and heterozygous combinations using the sum rule.
For example, if we consider one gene where the probability of being homozygous dominant is 1/4 and heterozygous is 1/2, the overall probability of being dominant (either homozygous or heterozygous) is 3/4. When assessing more genes, you multiply these probabilities together for all loci to find the ratio of showing all dominant phenotypes.