Biol Feb 3

Alleles: Each gene has two alleles; one is dominant while the other is recessive. Understanding how these alleles express in individuals is crucial.
Dominant Allele: An allele that can express its effect even when only one copy is present (heterozygous condition).
Recessive Allele: An allele whose effects are masked in the presence of a dominant allele; expressed only in the homozygous state.

Hypersufficiency: When a wild type allele produces enough protein to manifest the associated phenotype.
- Example: A heterozygous individual may not show a mutant condition because the wild type allele is sufficient to compensate.
Examples of Genes:
- Phenylketonuria (PKU): Mutation leads to metabolic disorder; only the homozygous recessive individuals display symptoms.
- Albinism: Having one functional allele is sufficient for melanin production, preventing phenotypic manifestations.

Haploinsufficiency: Occurs when a single functional allele does not produce sufficient protein, leading to a mutant phenotype.
- Example: Instances where the mutant allele is dominant due to inadequate protein production.

Dominant Negative Mutations: A mutated allele that interferes with the function of the wild type allele, resulting in a dominant mutant phenotype.
- Example: Brittle bone disease, caused by abnormal collagen formation where mutated proteins disrupt the structural integrity of bone.

Neomorphic Alleles: Mutations that impart new functions to a protein not previously present.
- Example: In some cases, gene expression can activate silenced leg-forming genes in unexpected locations (e.g., mutations in head regions).

Incomplete Dominance: Intermediate phenotypes appear in heterozygotes; examples include the color spectrum of snapdragons where crossing red and white results in pink.
- Phenotypic Ratio: Results in a 1:2:1 ratio of phenotypes in offspring.
Codominance: Both alleles express independently and equally without blending; example includes blood types.
- Blood Group Example: Type AB blood expresses both A and B antigens on the cell surface.

Sickle Cell Anemia: Results from mutations in hemoglobin, demonstrating both incomplete dominance and codominance.
- Heterozygous Phenotype: Can appear normal under high oxygen but changes shape under low oxygen, reflecting codominance at the protein level.

Lethal Alleles: Alleles that can cause death when homozygous; useful for understanding gene function in populations.
- Example: Coat color in mice where yellow is dominant but lethality occurs when homozygous leads to death, resulting in a phenotypic ratio of 2:1 in offspring.
- Example of Other Lethal Alleles: Traits like spine formation abnormalities can occur in certain breeds, highlighting the importance of specific alleles for survival.

Complete Dominance: One allele completely masks the effect of another.
Incomplete Dominance: Results in a blending of traits; both alleles are expressed but not completely.
Codominance: Both traits are independently and simultaneously expressed without blending.
Lethal Alleles: Mutation can lead to death, providing clear distinctions in genotype and phenotype ratios during genetic crosses.