Final answer:
The rules of dominant and recessive traits, based on Mendel's law of dominance, help predict that in the presence of a dominant allele, the dominant trait will be expressed, while recessive traits will only appear in homozygous recessive individuals.
Step-by-step explanation:
The rules of dominant and recessive traits allow us to predict which traits will appear in successive generations. According to Mendel's law of dominance, in the presence of a dominant allele, the recessive allele for the same gene will not be expressed in the phenotype. This means when predicting offspring traits, if an individual carries at least one dominant allele for a trait, the dominant trait will be expressed. Conversely, the recessive trait will only appear if the individual has two copies of the recessive allele, making them homozygous recessive. Examples include violet-flower color being dominant over white color.
Mendel's experiments with pea plants showed that heterozygous offspring (carrying one dominant and one recessive allele) will exhibit the dominant trait. However, the recessive trait can reappear in the progeny when two heterozygous organisms are crossed, as some offspring may receive two recessive alleles. This pattern was observed in Mendel's classic 3:1 ratio among the third-generation plants. Knowing the genetic makeup of the parents therefore allows us to predict the odds of certain traits in their offspring.