4.7 bio notes

Mendelian genetics is based on specific assumptions that may not always hold true for all traits.

Assumption 1: Two Alleles per Gene
- Mendelian genetics assumes each gene has only two alleles (traits).
- Counterpoint:
- Genes can have multiple dominant and recessive alleles.
- Example: ABO blood types exhibit multiple alleles (IA, IB, i).
Assumption 2: Single Gene for Each Trait
- Assumes that each trait is determined solely by one gene.
- Counterpoint:
- Complex traits may result from multiple genes or environmental interactions affecting development.

Example: ABO Blood Types
- The ABO blood group system features three alleles:
- IA: Produces type A sugar chain.
- IB: Produces type B sugar chain.
- i: Produces type O sugar chain (no additional sugars).
- Genetic Properties:
- Co-dominance: Both IA and IB are expressed equally in type AB blood (IA IB).
- Typical blood type phenotypes result from various genotype combinations:
- Type A: Genotypes IAIA or IAi
- Type B: Genotypes IBIB or IBi
- Type AB: Genotype IAIB
- Type O: Genotype ii

Traits can blend together under certain circumstances, such as in incomplete dominance.
Example: Sickle Cell Anemia
- Caused by mutations in the hemoglobin gene.
- Two alleles:
- HA: Normal hemoglobin.
- HS: Sickle cell hemoglobin, resulting in sickled shape of red blood cells.
- Possible genotypes include:
- HAHA: Normal hemoglobin, homozygous dominant.
- HSHS: Sickle cell anemia, homozygous recessive.
- HAHS: Sickle cell trait, heterozygous; symptoms can vary.
- Sickle cells cause health complications such as anemia and pain due to blockages in blood vessels.

Epistasis: When a gene's effect is hidden or altered by another gene.
Example: ABO Blood Groups
- The H gene regulates the expression of blood type through its enzyme's interaction with the I gene.
- The H gene has two alleles (H, h):
- H: Functions normally, allows addition of H sugar.
- h: No functional enzyme, prevents addition of H sugar, leading to type O blood regardless of I genotype.

Pleiotropy: One gene affects multiple traits.
- Example: Sickle Cell Disease affects blood health, skin, eyes, and may cause blood clots or high blood pressure.
Polygenic Traits: Multiple genes influence a single trait.
- Example: Human Height
- Influenced by numerous alleles (approximately 10,000), resulting in a normal distribution (bell curve).
- Only about 80% of height variation is genetically determined, the rest is due to environmental factors (nutrition, physical activity).

Environmental factors can also influence simple traits.
Example: Siamese Cats
- Color variation due to tyrosinase enzyme activity affected by temperature.
- Cooler body parts (ears, paws) exhibit dark coloration, while warmer parts remain light.

Co-dominance of ABO blood types and corresponding genotypes.
Incomplete dominance of sickle cell anemia with examples.
Understand epistasis using ABO blood types mechanics.
Describe pleiotropy via sickle cell disease impacts.
Grasp polygenic traits and their normal distribution characteristics.
Explore environmental factors with specific examples (human height, Siamese cats).