Final answer:
To determine the dominant allele, observe the progeny phenotype from a cross of individuals from two pure-breeding lines, with one line having the dominant phenotype and the other the recessive. Use a Punnett square to predict offspring genotypes and phenotypes to demonstrate dominance.
Step-by-step explanation:
The most effective method to determine which of two alleles of a gene is dominant is to observe the relevant phenotype in the progeny that result from a cross between individuals from two different pure-breeding lines. When a dominant allele is present, it will mask the expression of a recessive allele, thus the dominant trait will appear in the offspring. A typical experiment would involve crossing two homozygous individuals: one with the dominant phenotype (AA) and one with the recessive phenotype (aa).
For example, using Mendel's peas, if the yellow seed trait (Y) is dominant over green (y), crossing plants with YY and yy genotypes, respectively, will always result in Yy offspring expressing the yellow phenotype. This Punnett square approach will lead to understanding the allele interactions. This process not only confirms the dominance of one allele over another but also follows Mendel's principles of inheritance and segregation.
The test cross, another methodology introduced by Mendel, can further clarify if a dominant phenotype arises from homozygous or heterozygous conditions in an organism. This involves crossing an organism with a dominant phenotype against one known to be homozygous recessive, allowing the elucidation of the dominant organism's genotype based on the offspring's phenotypes.