Final answer:
Dominant-recessive relationships determine whether a dominant or recessive trait is expressed in the phenotype of an organism. Dominant traits appear in both homozygous dominant and heterozygous genotypes, while recessive traits only appear in homozygous recessive genotypes. This is evidenced by Mendel's pea plant experiments and the inheritance patterns of certain human traits.
Step-by-step explanation:
Dominant-Recessive Relationships in Phenotypic Expression
The dominant-recessive relationship plays a critical role in determining the traits that are expressed in the phenotype of an organism. According to Mendel's law of dominance, if an individual receives two dominant alleles, one from each parent, the phenotype will exhibit the dominant trait. Conversely, if an individual receives two recessive alleles, then the phenotype will express the recessive trait. Organisms with two identical alleles for a trait are homozygous, and those with one dominant and one recessive allele are termed heterozygous. In heterozygous individuals, the dominant trait will be phenotypically expressed, making them appear identical to homozygous dominant individuals.
Evidence supporting these concepts comes from Mendel's hybridization experiments with pea plants, where dominant traits were expressed in heterozygous F1 offspring, looking identical to the homozygous dominant parent. The recessive traits remained unexpressed unless the individual was homozygous for the recessive allele. Moreover, certain human traits follow this pattern of inheritance, with dominant and recessive traits being passed down to offspring in predictable ways.
However, incomplete dominance is an exception where the heterozygote expresses a blend of both parental traits, indicating that not all traits follow the classic dominance-recessive model.