Differences Between Dominance and Overdominance Hypotheses

What is Dominance Hypotheses?

The Dominance Hypothesis refers to a concept in plant breeding and genetics that deals with the expression of traits in offspring based on the dominance of alleles. It essentially posits that in a heterozygous state, one allele (the dominant allele) can mask the expression of another allele (the recessive allele). Here’s a breakdown of the idea:

1. Dominant and Recessive Alleles: Alleles are different forms of a gene. In a gene pair, one allele may be dominant and the other recessive. The dominant allele’s traits are expressed in the phenotype (observable characteristics), while the recessive allele’s traits are only expressed when two copies are present (homozygous recessive).
2. Phenotypic Expression: According to the Dominance Hypothesis, in a heterozygous organism (one dominant allele and one recessive allele), the phenotype will reflect the dominant allele’s trait. For example, if the gene for flower color has a dominant red allele and a recessive white allele, a plant with one red allele and one white allele will have red flowers.
3. Genotypic vs. Phenotypic Ratios: When breeding plants, understanding dominance helps predict the ratios of different traits in offspring. For instance, in a cross between two heterozygous plants (both with one dominant and one recessive allele), the offspring’s phenotypic ratio is typically 3:1, where 3 plants show the dominant trait and 1 plant shows the recessive trait.
4. Implications for Breeding: The Dominance Hypothesis aids breeders in predicting trait inheritance and in designing breeding strategies. By understanding how traits are inherited and expressed, breeders can more effectively select plants with desirable characteristics and improve crop varieties.

What is Overdominance Hypotheses?

The Overdominance Hypothesis, also known as the heterozygote advantage hypothesis, refers to a genetic concept where heterozygous individuals (having two different alleles for a particular gene) exhibit a greater fitness or superior traits compared to homozygous individuals (having two identical alleles). Here’s a closer look at the hypothesis:

1. Heterozygote Advantage: According to this hypothesis, the heterozygous genotype has a selective advantage over both homozygous genotypes. This means that individuals with two different alleles for a gene (heterozygotes) perform better in terms of survival, reproduction, or overall fitness compared to individuals with two identical alleles (homozygotes).
2. Phenotypic Expression: In the context of overdominance, the phenotype of the heterozygote is often superior or more beneficial than that of either homozygote. For instance, in certain plant or animal species, heterozygous individuals might have better resistance to diseases or environmental stress compared to homozygous individuals.
3. Genetic Diversity: Overdominance can help maintain genetic diversity within a population. Because heterozygotes have an advantage, there is a selective pressure to maintain a balance of different alleles within the population, preventing the loss of genetic variation.
4. Examples in Nature: One well-known example of overdominance is the sickle cell trait in humans. Individuals who are heterozygous for the sickle cell gene (carrying one normal allele and one sickle cell allele) have a higher resistance to malaria compared to both homozygous normal individuals and homozygous sickle cell individuals. This heterozygote advantage helps maintain the sickle cell allele in malaria-endemic regions.
5. Breeding Implications: In plant breeding, understanding overdominance can influence the selection of breeding strategies. Breeders might select for heterozygous individuals to harness the benefits of overdominance, leading to improved traits and greater resilience in crop varieties.

Differences Between Dominance and Overdominance Hypotheses

The Dominance and Overdominance Hypotheses offer different explanations for the phenomenon of heterosis, or hybrid vigor. Although both hypotheses explain aspects of hybrid performance, they diverge in their specific predictions and underlying mechanisms. Below is a detailed comparison between these two hypotheses.

Similarities:

1. Inbreeding Depression:
   • Both hypotheses acknowledge that inbreeding results in inbreeding depression. This occurs because inbreeding increases homozygosity, exposing deleterious recessive alleles, which reduces the overall vigor and fertility of the progeny.
2. Outcrossing Effects:
   • Both theories predict that outcrossing or hybridization restores vigor and fertility. By increasing heterozygosity, outcrossing can counteract the negative effects of inbreeding depression.
3. Genotypic Influence on Heterosis:
   • Both hypotheses agree that the degree of heterosis depends on the genetic diversity between the parental lines. Generally, greater genetic divergence between the parents leads to higher levels of heterosis.

Differences:

1. Heterozygote Superiority:
   • Dominance Hypothesis: This hypothesis posits that heterozygotes are as good as or only slightly better than the dominant homozygotes. The key feature is the masking of harmful recessive alleles by dominant alleles.
   • Overdominance Hypothesis: In contrast, the overdominance hypothesis suggests that heterozygotes are superior to both homozygotes. This superiority arises because the heterozygote can benefit from the combined functional effects of divergent alleles, which cannot be fully realized in either homozygous state.
2. Mechanistic Explanations:
   • Dominance Hypothesis: Focuses on the masking of deleterious recessive alleles by dominant alleles, implying that heterosis results from preventing the expression of harmful recessive traits.
   • Overdominance Hypothesis: Proposes that heterosis results from the superior functionality of the heterozygote, which can perform better due to the cumulative or synergistic effects of divergent alleles.

Current Understanding and Limitations:

• Experimental Evidence: Despite extensive research, it remains challenging to conclusively determine whether overdominance or dominance is the primary mechanism driving heterosis. Some genes show heterozygote superiority, but these cases are relatively few. Moreover, these instances may be influenced by other genetic phenomena such as linkage in the repulsion phase or epistatic interactions.
• Prevalence of Dominance: Current evidence generally supports that dominance gene action accounts for a substantial part of heterosis. However, epistasis and overdominance also contribute to the overall phenomenon of hybrid vigor.
• Role of Epistasis and Linkage: Overdominance is often difficult to distinguish from epistasis and linkage effects. Recent studies, particularly in maize, suggest that overdominance might not be the primary cause of heterosis, with many reported cases potentially reflecting epistatic interactions or linkage effects rather than true overdominance.