Describe two examples that show deviation or extensions of simple Mendelian genetics.
Example 1: Incomplete Dominance
One example that deviates from simple Mendelian genetics is incomplete dominance, where the expression of a heterozygous genotype results in an intermediate phenotype. For instance, in snapdragons, the cross between homozygous red flowers (RR) and homozygous white flowers (rr) does not result in pink flowers as expected by blending inheritance. Instead, the F1 generation has pink flowers, indicative of incomplete dominance. This occurs because neither the red nor white allele is completely dominant over the other, leading to a blending of the traits, resulting in a new phenotype.
Example 2: Codominance
Another example that extends simple Mendelian genetics is codominance, where both alleles in a heterozygous genotype are fully expressed independently, without any blending. A classic example is the ABO blood group system in humans. In this system, there are three alleles involved: IA, IB, and i. The IA allele encodes the A antigen, the IB allele encodes the B antigen, while the i allele encodes no antigen (O). If an individual inherits an IA allele from one parent and an IB allele from the other, they will have both A and B antigens expressed on the red blood cells, resulting in the AB blood type. This demonstrates that both alleles are equally dominant and both phenotypic traits are visible simultaneously without any blending.