Final answer:
Monoallelic expression is when only one allele of a gene is expressed, while the other is inactive. It occurs in genetic mechanisms like imprinting and in males for genes on the X chromosome, contrasting with codominance and incomplete dominance where both alleles affect the phenotype.
Step-by-step explanation:
Monoallelic Expression, Monoallelic expression refers to a genetic scenario in which an offspring expresses only one allele of a gene, instead of exhibiting both alleles inherited from each parent. This can occur due to various genetic mechanisms, one of the most well-known being imprinting, where only one allele of a gene is active while the other is silenced due to epigenetic marks.
Another example of monoallelic expression is exhibited by genes on the X chromosome in males (hemizygosity), where only one X chromosome is present, hence only one allele is expressed. Furthermore, certain immune system genes are monoallelically expressed to ensure that each cell produces a single type of antigen receptor, which is important for the diversity of the immune response.
These instances contrast with cases of codominance and incomplete dominance, where both alleles influence the phenotype, as seen in the MN blood group or flower color in some plants.
In summary, monoallelic expression is quite distinct from the Mendelian inheritance patterns taught in basic genetics, where typically both alleles contribute to an individual's phenotype -- either through dominant-recessive relationships or through more complex interactions like codominance.