BIO1000F: Genetic Inheritance

Definition: Incomplete dominance occurs when the phenotype of heterozygotes is intermediate between the phenotypes of the homozygotes.
Example: Variation of colors (e.g., white to pink) due to differences in the extent of incomplete dominance.

Definition: Epistasis is the interaction between two or more genes where the presence of one gene can mask or inhibit the expression of another gene's phenotype.
- Derived from Greek: "stasis" means to stand over or affect.
Mechanism: One gene controls the expression of another; both genes can be functional, but the observed phenotype may not present both traits based on genetic makeup.
Example: In mice, a gene that inhibits melanin deposition can lead to color variations in fur.
- Types of Epistasis:
  - Dominant epistasis: The dominant allele of one gene prevents the expression of another gene.
  - Recessive epistasis: The recessive alleles inhibit the expression of another gene.
- Phenotypic Expression: Different combinations of alleles lead to different observable characteristics, showing how a dominant gene can overshadow others.
Melanin Expression:
- Mice without melanin due to recessive alleles appear white even if they should have black or brown fur based on other genetic factors.

Cause of Red Eyes: Mice that lack melanin have red eyes because light reflects off the blood vessels in the retina.
Tyrosine Pathway: The production of melanin is dependent on the TYR gene, which involves multiple enzymatic steps in metabolizing the amino acid tyrosine.
- A mutation in this pathway can lead to lack of functional melanin, resulting in conditions such as albinism.

Definition: Pleiotropy occurs when one gene influences multiple phenotypic traits.
- Key Aspects: One allele can affect various aspects of an organism’s physiology, behavior, and metabolism.
Examples:
- Chickens with a mutation causing frizzled feathers also show a higher metabolic rate, illustrating how one change can have multiple effects.
- Sickle cell anemia is another example where a single point mutation affects both blood cell shape and organism health.

Concept: Linkage refers to the tendency of genes located close to each other on a chromosome to be inherited together.
Mendelian Genetics: The physical basis of inheritance follows Mendel's laws of segregation and independent assortment, explaining how traits are inherited.
Human Chromosome 1 (Karyotype):
- Largest chromosome in the human genome, home to many genes influencing diseases.
Gene Mapping:
- The process of determining the locations of genes on a chromosome, akin to GPS mapping.
- Significant advances have made sequencing chromosomes easier, but mapping (determining the exact locations and functions of genes) remains complex.
- Human Chromosome 1 Statistics:
  - Comprises approximately 249,000,000 base pairs and holds about 8% of the entire genome size.

Thomas Hunt Morgan's Work:
- Won the Nobel Prize in 1933 for ground-breaking research on linkage and sex-linked inheritance using Drosophila (fruit flies).
- His experiments were critical in understanding how genes are mapped to chromosomes and observe inheritance patterns.
- The rapid life cycle and ease of breeding of fruit flies made them an ideal subject for genetic studies.

The ongoing quest to identify the numerous genes influencing traits, such as eye color, indicates that our understanding of genetic interactions is still evolving. Many genes are identified, yet their specific functions remain largely unknown, demonstrating the complexities of genetics.