Final answer:
A Punnett square cross between true-breeding red (RR) and white (rr) four-o'clock flowers results in pink offspring (Rr), showing incomplete dominance. In a trihybrid cross of heterozygous parents for three traits, 27 genotypes and 8 phenotypes are expected. A cross between two heterozygous tall pea plants results in offspring with a genotypic ratio of 1:2:1.
Step-by-step explanation:
The question involves using a Punnett square to determine the possible genotypes and phenotypes that can result from a genetic cross. To address the specific case presented, we consider the cross between true-breeding four-o'clock flowers, where red is dominant (RR) and white is recessive (rr). If we cross these, all the offspring will be heterozygous (Rr), resulting in pink flowers. This is an example of incomplete dominance where the heterozygous phenotype is a blend of the two homozygous phenotypes.
For the question on the number of genotypes and phenotypes in a trihybrid cross, if each parent is heterozygous for all three traits (AaBbCc), the number of genotypes can be found by calculating 3^n, where n is the number of traits, resulting in 3^3 = 27 genotypes. The number of phenotypes will be 2^n, which for three traits is 2^3, equating to 8 phenotypes.
Similarly, for the cross between two heterozygous tall pea plants (Tt x Tt), we expect the following genotypic ratio among the offspring: 1:2:1, that is one TT, two Tt, and one tt. This covers the predicted genotypic ratios.
We can also discuss a more complex dihybrid cross (two trait cross), where each gene has two alleles and can occur in 16 different combinations, as each diploid individual has two sets of chromosomes.