Comprehensive Genetics: Pleiotropy, Blood Type Codominance, and Epistasis

General Principles of Dominance

• Heterozygous organisms always exhibit the phenotype of the dominant parent.
• Exception: This rule does not apply in cases of incomplete dominance.

Pleiotropy and Polygenic Processes

• Pleiotropy: This occurs when one single gene is involved in multiple physiological processes or affects multiple phenotypic traits.     - Example: The $SILV$ gene.     - The $SILV$ gene impacts the following:         - Fur color.         - Eye color.         - Eye development.         - Ear development.
• Polygenic: This occurs when multiple genes are required to complete or regulate a single process.     - Example: Limb development.     - Such processes require the interaction of many different proteins encoded by various genes.

Blood Type Genetics and Codominance

• Codominance: A genetic scenario where you can "see" or observe the effects of two dominant alleles at the same time.
• Phenotypes: There are four different blood phenotypes in the ABO system: $A$, $B$, $O$, and $AB$.
• Alleles:     - $A$: Dominant allele.     - $B$: Dominant allele.     - $O$: Recessive allele (represents the absence of glycoproteins).
• Genotypes and Phenotypes Table:     - Genotype $AA$ or $AO$ results in Phenotype $A$.     - Genotype $BB$ or $BO$ results in Phenotype $B$.     - Genotype $AB$ results in Phenotype $AB$ (This is the heterozygous state with two dominant alleles).     - Genotype $OO$ results in Phenotype $O$.

Immunology of Blood Types

• Antigens:     - Defined as anything that elicits an immune response.     - Typically, these are proteins, but they can also be sugars or lipids.     - The "key thing" regarding antigens is their shape.     - Specifically, these are different glycoproteins found on the surface of red blood cells.
• Antibodies:     - These recognize foreign cells.     - They are part of the adaptive immune response.     - A person possesses antibodies to the antigens they do not have on their own cells.
• Antibody Distribution and Donor Rules:     - Universal Donor: Blood type $O$ is considered the universal donor.     - Universal Recipient: Blood type $AB$ is considered the universal recipient because it has none of these antibodies.     - Type $A$: Contains Anti-$B$ antibodies.     - Type $B$: Contains Anti-$A$ antibodies.     - Type $O$: Contains both Anti-$A$ and Anti-$B$ antibodies.     - Clinical Rule: If you are Blood Type $A$, you can only receive Type $A$ or Type $O$ blood.

Epistasis and Genetic Linkage

• Independent Inheritance: Generally, two genes are inherited independently; the inheritance of one gene does not affect the inheritance of another. This also applies to the expression of a phenotype.
• There are two primary ways of "breaking" the independent inheritance/expression rules:     1. Chromosome Level (Linkage): This results in more individuals with parental allele combinations than would be expected by chance.     2. Phenotype Level (Epistasis): One gene "steps on" or masks another gene. This results in fewer than 4 phenotype combinations in a dihybrid cross.
• Dominant Epistasis: This occurs when a dominant allele in one gene masks the phenotype of a second gene.     - Example: White cats.     - If a cat has at least one dominant $W$ allele, it makes the fur white regardless of the other color genes present.     - Just one dominant allele is required to mask the phenotype of the second gene.