Lecture 10 - Patterns of Inheritance II

Disease genes can be categorized as:
- Recessive: Conditions such as Cystic Fibrosis are due to recessive alleles.
- Dominant: Conditions like Huntington disease arise from dominant alleles.
- Autosomal: Genes located on one of the 22 pairs of autosomes, e.g., Huntington disease.
- Sex-linked: Genes found on sex chromosomes.

Genetic Characteristics:
- Autosomal dominant inheritance pattern.
- Located on the short arm of chromosome 4.
Genetic Mechanism:
- Involves transcription from DNA to an mRNA strand.
- The normal gene contains fewer than 36 CAG repeats coding for glutamine, whereas the mutated gene has more than 36 repeats, leading to production of mutant huntingtin proteins (mHtt) with excessive glutamine residues.

Simple Mendelian Inheritance:
- Traits determined by alleles with dominant/recessive relationships located on autosomes.
- Dominant alleles mask the presence of recessive alleles.
- Often recessive alleles are nonfunctional leading to a reliance on functional dominant alleles.
X-linked Inheritance:
- Traits determined by X chromosome genes; males are hemizygous for these genes, making X-linked recessive disorders more common in males.
- Female heterozygotes can express the dominant trait due to the presence of one dominant allele.
Incomplete Dominance:
- Heterozygotes show intermediate phenotypes, as seen in pink four-o’clocks from red and white parents.
Codominance:
- Both alleles are expressed, as in ABO blood types; an AB individual expresses both A and B antigens.
Epistasis:
- One gene's alleles mask another gene’s alleles, indicating complex interactions in phenotype expression.
Continuous Variation:
- Offspring display a range of phenotypes due to the cumulative effects of multiple genes and environmental factors.

Various systems, including:
- X-Y System: Males are XY, females are XX.
- X-O System: Males can be either X or XO; females are XX.
- Z-W System: Males are ZZ, females are ZW.

Larger X chromosome carries more genes compared to Y; thus, X-linked traits are more prevalent.
Males have only one X, making them more susceptible to X-linked disorders, such as Hemophilia A, which is caused by a recessive X-linked gene that encodes a defective clotting protein.

Thomas H. Morgan’s research linked genetic traits, like white eye color in Drosophila, to sex chromosomes.
His test cross illustrated that certain allele traits were correlated with the inheritance of sex chromosomes, establishing a foundation for the field of genetics.

Pleiotropy:
- A single gene mutation can affect multiple traits and phenotypes.
- Example: Cystic Fibrosis affects chloride ion balance, leading to various symptoms like thick mucus and salty sweat.
Incomplete Dominance vs. Codominance:
- Incomplete dominance: Heterozygotes exhibit intermediate expressions.
- Codominance: Both alleles are fully expressed simultaneously.

The norm of reaction reflects how environmental variations can affect phenotypic outcomes.
Example: Identical plants may grow to different heights based on the temperature and environmental factors present.

Traits often arise from multiple genes, indicating that real-world genetic interactions are complex, as seen in examples like the inheritance of flower color in sweet peas.
Epistasis: Gene interactions can mask phenotypic expressions, requiring multiple genes to achieve specific traits.

Probability calculations are fundamental in genetics; events are often subject to sampling errors based on the size of the population.
Understanding the product rule aids in predicting the likelihood of genetic outcomes, exemplified when assessing the probabilities in Punnett squares for offspring traits.