Concept 8: One allele can be dominant over another or show a lack of dominance

The idea of dominance was first proposed by Mendel. He observed that when crossing two lines of peas that expressed alternative versions of a trait, the first generation of offspring (first filial or F1) all had the same phenotype. Mendel proposed the F1s were heterozygous. They had an allele from one parent (‘A’) that was dominant and masked the presence of the allele from the second parent (‘a’) (Fig. 11). The F1 was ‘Aa’, had a different genotype than the AA parent but the same phenotype. Mendel was convinced the F1 still held the ‘a’ or recessive allele because if the F1 was selfed, some of the F2 offspring expressed the recessive trait again.

In other experiments performed by other geneticists, two alleles may have a lack of dominance. In this case, an F1 would be heterozygous but have a phenotype that was unique or intermediate to either of the parent line phenotypes (Fig. 12).