Episode 146: Mendelian Genetics and Inheritance

Epistasis

Epistasis is a phenomenon where the interaction between different genes affects the expression of traits. James Fodor explains that this can result in fewer phenotypes than expected, as one gene can mask the expression of another. This interaction complicates the classic Mendelian ratios, such as the nine to three to three to one ratio, leading to variations like the nine to three to four ratio 1.

Epistasis occurs when genes are not independent from each other, but one of the genes, or an allele of one of the genes, screens off the phenotypes from one of the other ones.

--- James Fodor

Fodor also notes that real-world traits often show variable penetrance and expressivity due to gene interactions and environmental factors, making Mendelian ratios rare in practice 2.

   

Codominance

Codominance occurs when both alleles in a heterozygote are fully expressed, resulting in a phenotype that shows both traits. James Fodor uses human blood types as an example, where individuals with one A allele and one B allele exhibit both A and B antigens 3. This differs from incomplete dominance, where the heterozygote shows an intermediate phenotype rather than both traits.

In codominance, a heterozygote will actually exhibit both phenotypes, so they'll exhibit the phenotype of the dominant and the recessive.

--- James Fodor

Fodor highlights that codominance can arise from the presence of multiple alleles, adding complexity to genetic inheritance beyond Mendel's initial observations 4.

   

Incomplete Dominance

Incomplete dominance is when one allele is not completely dominant over another, resulting in a blended phenotype. James Fodor explains that this leads to a phenotypic ratio that matches the genotypic ratio, such as one to two to one, instead of the classic three to one ratio 4. Pigmentation in animals is a common example, where heterozygotes show intermediate coloration.

Pigmentation is a common case of incomplete dominance because you'll often see intermediate cases when you're heterozygous.

--- James Fodor

Fodor points out that Mendel's experiments were simpler because they involved traits with complete dominance, making his results more predictable 5.