Final answer:
Underdominance in genetics refers to a condition where the heterozygote has lower fitness than either homozygote, resulting in a disadvantage for survival or reproduction compared to individuals who are either homozygous dominant or homozygous recessive.
Step-by-step explanation:
Definition of Underdominance
In genetics, underdominance is a situation where the heterozygote has lower fitness than either of the homozygotes. This means that individuals carrying two different alleles (heterozygotes) are at a disadvantage compared to those with two copies of the same allele (homozygotes). In other words, underdominance occurs when the phenotype of the heterozygote is less fit for survival or reproduction than the phenotypes of both homozygotes.
This genetic phenomenon stands in opposition to other inheritance patterns such as complete dominance, where the dominant allele completely masks the effect of the recessive allele in the phenotypic expression, and heterozygote advantage or overdominance, where the heterozygote has a higher fitness than either homozygote. Underdominance can lead to decreased variety within a population, as the less fit heterozygotes are selected against.
For example, if a plant species has alleles 'A' and 'a' for a certain trait with 'AA' and 'aa' both expressing favorable adaptations but 'Aa' expressing an unfavorable adaptation, underdominance would be at play. Hence, in this scenario, the heterozygote with genotype 'Aa' is less adapted to the environment compared to both homozygous forms.