Below is a short summary and detailed review of this video written by FutureFactual:
Non Mendelian Genetics Explained: Incomplete Dominance, Codominance, Polygenic Traits and Epistasis
Introduction to Non Mendelian Traits
The Amoeba Sisters begin by explaining that some genetic traits do not follow Mendel's classic dominant-recessive rules. They distinguish Mendelian traits from non Mendelian ones and introduce four major concepts: incomplete dominance, codominance, polygenic traits, and epistasis. The video also notes that height and skin color are influenced by many genes and environmental factors, illustrating complexity beyond a single gene pair.
Incomplete Dominance in Snapdragons
Snapdragons can display three phenotypes: red, white, and in-between pink. In incomplete dominance the dominant allele is not fully expressed when the recessive allele is present, producing a blended pink phenotype in a cross between red (RR) and white (rr) flowers. When two pink flowers (Rr) are crossed, offspring can be red, pink, or white, showing a blend rather than a strictly dominant phenotype. The video also mentions that classrooms vary in how they format incomplete dominance, but the core idea remains the same.
"In incomplete dominance, one allele is not completely dominant over the other." - Amoeba-Sisters
Codominance and Speckled Chickens
Codominance is described as both alleles being expressed in the phenotype. The video presents a cross between a black chicken (BB) and a white chicken (ww), yielding offspring with the genotype Bw that are speckled with black and white. This mixed expression contrasts with simple dominance and demonstrates how Punnett squares can yield organisms with both traits visible.
"In codominance, both alleles are expressed." - Amoeba-Sisters
Polygenic Traits and Environment
The video argues that height and skin color are polygenic traits, involving many genes that contribute to a single phenotype. An individual’s height reflects multiple gene loci, with one allele inherited from each parent at each locus. This multilocus contribution results in a continuous distribution of heights rather than discrete categories. Environmental factors such as nutrition and sun exposure influence the final outcome, underscoring that environment interacts with genetics without changing the genetic code itself.
Epistasis: When One Gene Masks Another
Epistasis is introduced as a situation where the effect of one gene depends on another gene. The llamas example shows a pigment gene B that can produce black wool in BB or Bb genotypes, while a second gene C can block pigment expression. If the llama has little C little C, pigment cannot be expressed, producing albino wool regardless of B. Because two genes are involved, solving such problems often uses a dihybrid cross, which can yield a 16-square Punnett grid in theory. This illustrates how gene interactions shape phenotype beyond single-gene effects.
"Epistasis is when one gene really depends on another gene for it to even be expressed." - Amoeba-Sisters
Practical Takeaways and Problem Solving
The video closes by advising that non Mendelian patterns exist, but you should not assume them unless a problem provides clues. It also mentions other non Mendelian topics such as multiple alleles and sex-linked traits that are explored in other videos. The overarching message is to stay curious and to look for clues when solving genetics problems rather than applying non Mendelian patterns by default.
Conclusion
Overall, non Mendelian genetics reveals how biology uses blending, co-expression, multiple genes, and genetic interactions to shape traits, offering a richer view of inheritance beyond Mendel's original model.
