Final answer:
The most commonly observed phenotype in natural populations is simply called 'phenotype'. Options such as genotype, homozygous, and allele refer to different aspects of genetic variation and not the expression of traits.
Step-by-step explanation:
The phenotype for a character that is most commonly observed in natural populations, such as red eyes in Drosophila, is called a wild-type phenotype. However, between the given options, the correct answer to represent the observable traits like red eyes in Drosophila is simply 'phenotype'. A phenotype is the composite of an organism's observable characteristics or traits, such as its morphology, development, biochemical or physiological properties, and behavior.
An allele is a variant form of a gene. Some alleles are dominant, meaning they are expressed if present, while others are recessive and only affect the phenotype when an individual is homozygous for that recessive allele. For example, a tall and green plant that is homozygous for each trait could have a genotype represented by 'TTGG' if 'T' and 'G' are the dominant alleles for tallness and green color, respectively. Therefore, the phenotype of this plant would be tall and green.
It is important to note that two individuals with the same phenotype may not necessarily have the same genotype. For instance, an organism with a dominant phenotype could be either homozygous dominant or heterozygous; both genotypes would result in the same phenotype due to the presence of at least one dominant allele.