Final answer:
Dominant alleles in diploid organisms, such as the one for brown eyes, can alter the normal phenotype even when paired with a recessive allele, like the one for blue eyes. While Mendel suggested only two alleles exist per gene, there are often multiple alleles within a population, with some mutant alleles becoming dominant due to interference with normal genetic function.
Step-by-step explanation:
Mutations in a single gene can lead to different alleles, which are gene variants that exist on homologous chromosomes. In situations where one allele is dominant, it is because this single copy of the mutation in a diploid organism is enough to alter the normal phenotype, even when paired with a wild-type allele. The concept of dominant alleles is essential to understand, especially when we consider Mendel's law of inheritance, which describes how dominant traits can mask the presence of recessive traits in heterozygotes.
A straightforward example of allele dominance can be found when considering the inheritance of eye color. The allele for brown eyes is dominant over the allele for blue eyes. This means that even if an individual carries one allele for brown eyes and one for blue eyes (heterozygous), the phenotype of brown eyes will be expressed. Conversely, a recessive allele, such as the one for blue eyes, only manifests when an individual has two copies of the recessive allele (homozygous).
It's essential to note that while Mendelian genetics provided the foundation for understanding how traits are inherited, the reality is more complex. Multiple alleles can exist at the population level, and the interaction between them can produce a variety of phenotypes. Sometimes, a mutant allele becomes dominant because it interferes with the genetic message, even if there is a wild-type allele present. An example of this is the Antennapedia mutation in Drosophila, which leads to the development of legs in place of antennae.