Mendel's work on pea plants involved traits that had a completely dominant/recessive pattern of inheritance and only two alleles for a gene. For example, flower color in pea plants is either purple or white and the purple allele is completely dominant to the recessive white allele.
We now know that inheritance is not always this straightforward. Alleles can show different dominance relationships and most genes have more than one allele. We also know that most aspects of an organisms' phenotype are the result of multiple genes as well as a complex interplay with an organisms' environment.
In Mendel's experiments, offspring always looked like one of two phenotypes due to the complete dominance of one allele over the other for characters that had two traits. This is not always the case because some genes display incomplete dominance . In this type of dominance relationship between two alleles, individuals who are heterozygous exhibit a phenotype intermediate between individuals who are homozygous. For example, this figure depicts the outcome of a cross between a snapdragon with red flowers and one with white flowers the offspring in the F1 have pink flowers. In this case neither of the alleles for flower color is completely dominant over the other. Therefore, individuals who are heterozygous have a phenotype unlike those with either set of homozygous alleles.
The Punnett square for this cross is like that for any other monohybrid cross. However, the ratio of phenotypes in the F2 generation is not 3:1 (dominant:recessive), as seen with completely dominant alleles, but rather a 1:2:1 ratio of red:pink:white flowers. Note that in this example the alleles are represented differently than in the previous examples. Since neither allele is dominant over the other, the use of an uppercase and lowercase version of the same letter is not appropriate. In this example the character (flower color) is indicated by a letter (C ), and the alleles encoding the trait (white or red) are listed as uppercase subscripts (they are both uppercase because neither is dominant to the other). You may see other symbolic representations for incomplete dominance, but don't let this confuse you. The important thing to know is that some genes are expressed in an incomplete dominant manner.
At the following Web site, find the correct answer to the multiple-choice monohybrid or dihybrid cross questions. Work out each problem for yourself. To view an explanation of the problem, select the "TUTORIAL" button. After viewing the correct answer, close the Monohybrid Cross Problem Set or Dihybrid Cross window to return to this page. (note: These sites are a part of the Monohybrid and Dihybrid Problem Sets provided by The Biology Project at the University of Arizona.)
Problem 9 : Incomplete dominance - This problem is a part of the Monohybrid Cross Problem Set.
Problem 10 : Disappearance of parental phenotypes in the F1 generation - This problem is also a part of the Monohybrid Cross Problem Set.
Problem 11 : Incomplete dominance in a dihybrid cross - This problem is a part of the Dihybrid Cross Problem Set.
In a third type of dominance, codominance , both alleles are expressed in the phenotype of individuals that have are heterozygous. The human MN Blood group, with alleles designated M and N is an example of codominance. This blood group is determined by the presence of a specific protein on the surface of red blood cells, such as those shown in this figure. Group M individuals have one variant (allele) of this proteins, group N have the other variant protein, and group MN individuals express both variants (alleles) of this protein. In the latter case, the heterozygote does not have an intermediate phenotype; instead, the M and N proteins are both expressed in the phenotype.
Distinguishing between these three types of dominance is sometimes difficult. Think of complete dominance, incomplete dominance, and codominance as a continuum of dominance relationships among alleles at a gene. At one end is complete dominance, in which the phenotype for only one of the two alleles is expressed in individuals that are heterozygous. At the other end is codominance, in which both alleles are equally expressed in individuals that are heterozygous. In between there are various levels of incomplete dominance, in which individuals that are heterozygous for the alleles display an intermediate phenotype. The key to understanding the difference between the three types is to look at the phenotype of the individuals with heterozygous alleles, then classify the relationship accordingly.
Expression of Multiple Alleles Complex Expression VoiceThread Transcript