Final answer:
The allele with the most influence in a trait's expression is the dominant allele, which masks the effect of a recessive allele in a heterozygous genotype. Dominant traits can be seen in homozygous dominant and heterozygous individuals, whereas recessive traits only show up when an individual has two copies of the recessive allele.
Step-by-step explanation:
The allele that most influences a trait and will show up in the offspring's phenotype when paired with another allele is known as the dominant allele. A dominant allele has the ability to mask the presence of a recessive allele in heterozygous genotypes, which means that if an individual has one dominant allele and one recessive allele, the dominant trait is the one that will be expressed. When considering genetic crosses, such as a monohybrid cross, we can use principles like Mendel's laws to predict the outcome.
For instance, if we cross a homozygous dominant individual (AA) with a homozygous recessive individual (aa), all offspring will be heterozygous (Aa) and will display the dominant trait. This is because the dominant allele (A) overshadows the effects of the recessive allele (a). Population genetics studies the frequency of alleles within a gene pool to determine the distribution of genotypes and phenotypes within a population. However, it's crucial to remember that there may be more than two alleles for a trait in a population, which is the case with multiple alleles systems like the ABO blood type.