Conclusion

Variations on Mendelian monohybrid and dihybrid ratios can result from incomplete dominance, codominance, multiple alleles in a population, epistasis, and gene interaction.

In incomplete dominance, heterozygotes for single gene traits display a novel phenotype that is not clearly the phenotype of either homozygote. Flower color in snapdragons is an example of incomplete dominance where the novel phenotype is pink flowers, a phenotype intermediate between the red flowers and white flowers of the two homozygous classes.

Codominance is similar to incomplete dominance in that the heterozygotes have a phenotype distinct from either homozygous phenotype, resulting in monohybrid phenotypic ratios that are the same as the genotypic ratios. However, the phenotype of individuals heterozygous for codominant alleles is the phenotype of both homozygous classes. The MN blood group system in humans is an example of codominance.

A population can have more than two alleles segregating at a locus, and the alleles may have different relationships in determining phenotype. The ABO blood group system is an example of multiple alleles. In the ABO blood group system, the alleles for A and B antigen types are codominant and the allele for type O is recessive.

The Bombay phenotype in the ABO blood group system is an example of epistasis, where an allele of one gene can mask the expression of alleles at another locus. Individuals homozygous for a recessive allele of a gene involved in the synthesis of an AB antigen precursor do not produce the precursor. If such individuals have A and/or B antigen alleles, they are still functionally type O because the precursor is not synthesized.

Many phenotypes are the result of alleles at more than one locus. In cases involving two genes, the expected dihybrid genotypic ratios are obtained when the genes are not linked. However, gene epistasis or gene interaction can result in novel phenotypes and modifiied dihybrid phenotypic ratios.

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