Incomplete (partial) dominance __________________
Red X ivory gives a pink F1. The F2 phenotypic ratio is 1 red: 2 pink: 1 ivory. Red pigment is formed by a complex series of enzymatic reactions. Plants with the dominant allele at the I locus produce an enzyme critical for pigment formation. Individuals that are ii produce an inactive enzyme and thus no pigment. In this case, II individuals produce twice as much pigment as Ii individuals and ii individuals produce none. The amount of pigment produced determines the intensity of flower color. Note: enzymes are catalytic and heterozygotes usually produce enough enzyme to give normal phenotypes. This is the basis for complete dominance. However, upon closer examination, there are often measurable differences between homozygous dominant and heterozygous individuals. Thus, the level of dominance applies only to a specified phenotype.
Codominance __________________
Many biochemical and molecular markers show codominant inheritance. In the case of codominance, both of the alleles that are present in a heterozygote can be detected. A way of visualizing codominance is through electrophoresis. (Electrophoresis animation) An application of electrophoresis is to separate proteins or DNA extracted from tissues or whole organisms. An electric charge is run through the supporting media (gel) in which extracts, containing proteins or DNA for separation, are placed. Proteins or DNA fragments are allowed to migrate across the gel for a specified time and then stained with specific chemicals or visualized via isotope or fluorescent tags. Banding patterns are then interpreted with reference to appropriate standards. The mobility of the protein or DNA is a function of size, charge and shape.
The following illustration shows codominant alleles at a simple sequence repeat (SSR) locus (HvHVA1) in the F1 and doubled-haploid lines of barley. Lanes 1-17; 19-21 are doubled haploids and lane 18 is the F1. The forward primer was unlabeled and the reverse primer was labeled at its 5’ end with a fluorochrome. The PCR products were analyzed using an ABI PRISM 377 automated DNA sequencer. The doubled haploids are homozygous for one of the two parental alleles (BCD47 allele = 138 bp = A; Baronesse allele = 122 bp = B). The F1 shows both alleles. (Data courtesy of I Vales, OSU)
F1 ¯ A ®
B ®
More examples of codominance:
Isozymes and allozymes (Details)
Human blood groups (Details)
Lethality__________________
Chlorophyll mutants are found in a range of plant species. The chloroplasts have their own genome and in most angiosperms show maternal inheritance. However, alleles at loci in the nuclear genome can also determine albinism. The homozygous recessive individual produces no chlorophyll and will thus die. The expected F2 phenotypic ratio for normal to albino seedlings would be 3 green: 1 albino. However, the albino individuals will not survive: viewed another way, the ratio of living to dead F2 individuals is 3:1.
Overdominance __________________
Demonstration of multiple types of intra-alleleic interactions at a locus: Human hemoglobin alleles and malaria (Details)
