Non-Mendelian Genetics: Incomplete Dominance, Codominance, and Multiple Alleles

Alleles as Genetic Sequences: Alleles are specific DNA sequences that serve as instructions to code for different proteins. Variations in these sequences lead to different physical traits.
Complete Dominance in Mendelian Genetics: In Mendel’s initial observations of pea plants, he identified that traits were often either dominant or recessive. Under the principle of complete dominance, the presence of at least one dominant allele fully masks the expression of the recessive allele.
Degrees of Dominance: While Mendel focused on complete dominance, genetic reality often involves varying degrees of dominance where alleles do not simply mask one another in a binary fashion.

Smooth Pea Allele (Dominant): The allele for smooth peas codes for an enzyme that effectively prevents water from entering the seed coat. By maintaining a regulated internal environment, the seed remains smooth.
Wrinkled Pea Allele (Recessive): This allele codes for a different form of the enzyme that allows water to enter the seed. The influx of water causes the seed to swell and then shrink, resulting in a wrinkled appearance.
Genotypic Expressions in Peas: * Homozygous Dominant ( $RR$ ): Produces the enzyme to keep water out; the pea is smooth. * Heterozygous ( $Rr$ ): Even with only one dominant allele, the seed produces a sufficient amount of the enzyme to prevent water entry. Thus, the phenotype remains smooth, demonstrating complete dominance at the organismal level. * Homozygous Recessive ( $rr$ ): Lacks the functional enzyme for water regulation; the pea is wrinkled.

Definition of Incomplete Dominance: This occurs when neither of the two alleles is fully dominant over the other. When passed to offspring in a heterozygous state, the result is an intermediate or "blended" phenotype.
Genetic Persistence: In incomplete dominance, both alleles continue to exist and are passed on genetically; only the physical expression (phenotype) is a blend.
Example: Flower Color (Snapdragons): * Parent Generation ( $P$ ): A cross between a red flower ( $RR$ ) and a white flower ( $rr$ ). * Offspring ( $F_1$ ): All offspring are heterozygotes ( $Rr$ ) and express a pink phenotype, which is a blend of red and white.
Cross of Two Pink Flowers ( $Rr \times Rr$ ): * Possible Genotypes: $1 \times RR$ (Red), $2 \times Rr$ (Pink), and $1 \times rr$ (White). * Probability Analysis: In a cross between two pink flowers, there is a $2/4$ or $50\%$ probability that the offspring will also be pink.

Definition of Codominance: In this condition, the alleles for a trait coexist without blending. Instead, the heterozygous phenotype expresses both alleles fully and equally. Neither allele masks the other.
Example: Camellia Flowers: * When a red flower and a white flower are crossed, the resulting heterozygous flower is spotted with both red and white patches. * Genotype Notation: Dominant alleles are represented by capital letters. For red, the genotype is $C^R C^R$ ; for white, it is $C^W C^W$ . * Heterozygote ( $C^R C^W$ ): This genotype results in a flower that exhibits both parental colors distinctly.
Cross Analysis of Blended (Spotted) Flowers ( $C^R C^W \times C^R C^W$ ): * Probability for White Offspring: To produce a white offspring ( $C^W C^W$ ), one allele must come from each parent. Based on the Punnett square, the probability is $1/4$ or $25\%$ .

Definition of Multiple Alleles: A single trait that is determined by more than two versions of alleles within a population. This increases the possible diversity of phenotypes.
The ABO Blood Group System: Human blood type is determined by three versions of alleles: $I^A$ , $I^B$ , and $i$ . These alleles code for enzymes that add specific carbohydrates to the cell membranes of red blood cells. * Allele $I^A$ : Adds A-type carbohydrates. * Allele $I^B$ : Adds B-type carbohydrates. * Allele $i$ : Does not add any surface carbohydrates (often referred to as type O).
Genotypes and Resulting Phenotypes: * Type A: Can be homozygous dominant ( $I^A I^A$ ) or heterozygous with the recessive O allele ( $I^A i$ ). * Type B: Can be homozygous dominant ( $I^B I^B$ ) or heterozygous with the recessive O allele ( $I^B i$ ). * Type AB: The genotype is $I^A I^B$ . This is a codominant condition where both A and B carbohydrates are present on the cell surface. * Type O: The genotype is $ii$ (homozygous recessive), resulting in no surface carbohydrates.

Cross Scenario: A parent with Type AB blood ( $I^A I^B$ ) and a parent with Type O blood ( $ii$ ).
Offspring Distribution: * The possible combinations are $I^A i$ and $I^B i$ . * Two out of the four possibilities result in the $I^A i$ genotype, which expresses the Type A phenotype.
Results: * The probability of having a child with Type A blood is $2/4$ or $50\%$ . * The probability of having a child with Type B blood is $2/4$ or $50\%$ . * The probability of having Type AB or Type O offspring in this specific cross is $0\%$ .

Population Diversity: Incomplete dominance, codominance, and multiple allele inheritance provide the greatest amount of genetic diversity within a population compared to simple dominant/recessive patterns.
Complex Interactions: These patterns demonstrate that the relationship between genotype and phenotype can involve intermediate states or the simultaneous expression of multiple traits.