Polygenic Inheritance, Environmental Effects, and Human Genetics

Polygenic inheritance involves the additive effects of two or more genes on a single phenotypic character.
This results in quantitative traits that vary along a continuum.
Examples of polygenic traits:
- Skin color (influenced by at least 3 different genes).
- Various diseases and disorders such as diabetes, cancer, and autism.
Question for consideration: What are other examples of traits influenced by multiple genes?

The phenotype of many traits can be significantly influenced by environmental factors.
Phenylketonuria (PKU) is an example of a recessive disorder shaped by environmental factors:
- This condition arises from the absence of an enzyme that breaks down phenylalanine.
- If phenylalanine builds up, it interferes with development, potentially leading to severe mental retardation.
- The condition can be managed by removing phenylalanine from the diet.
Question for consideration: Can you think of other traits influenced by environmental conditions?

In human genetics, we cannot perform controlled crosses to study inheritance patterns directly (due to ethical concerns).
Instead, we collect historical data about families to analyze genetic inheritance, which is often represented in a pedigree.

Recessively inherited disorders can skip generations, requiring both parents to at least be carriers (if not affected).
Examples of recessively inherited disorders:
- Phenylketonuria (PKU).
- Tay-Sachs disease.
- Sickle-cell anemia.
Pedigree analysis: Identifying carriers by marking individuals in the pedigree.

Cystic fibrosis is the most common lethal genetic disease in the US, affecting approximately 1 in 2,500 people of European descent.
It is estimated that 1 in 25 Caucasians are carriers of the cystic fibrosis allele.
The normal allele codes for a membrane protein that transports chloride ions; the presence of defective channels results in mucus buildup.
The average lifespan of individuals with cystic fibrosis is 37 years.

Unlike recessive traits, dominantly inherited disorders cannot skip generations because a dominant allele will manifest in individuals inheriting it.
Dominant alleles leading to lethal conditions are less common than those which are recessive.
Examples of dominantly inherited disorders:
- Achondroplasia (dwarfism).
- Osteogenesis imperfecta (brittle bone disease).
- Liebenberg Syndrome.
- Huntington’s Disease.

Osteogenesis Imperfecta:
- Caused by mutations in type 1 collagen; no cure exists.
Achondroplasia:
- Occurs in 1 in 25,000 births, involving a point mutation in the receptor gene inherited from the father (homozygous form is lethal).
Huntington’s Disease:
- Symptoms: Involuntary movements, behavioral disturbances, and dementia.
- Average onset occurs between ages 35-45 years.
- Caused by the expansion of a tri-nucleotide repeat (CAG) on chromosome 4, resulting in progressively more severe symptoms across generations.

No. of CAG Repeats vs. Outcome:
- Up to 28: Normal range; individual will not develop Huntington’s Disease (HD).
- 29-34: Individual will not develop HD, but the next generation is at risk.
- 35-39: Some individuals may develop HD; next generation is also at risk.
- 40 or more: Individual will develop HD.

X-linked recessive disorders show an unequal number of affected males.
X-linked dominant disorders indicate that an affected father will pass it to all daughters, while an affected mother has a 50% chance of passing it to offspring.

Genetic testing and counseling can provide valuable insights into hereditary conditions.
Ethical dilemmas arise, particularly concerning:
- Abortion decisions based on genetic information.
- Implications for health insurance and employment.
Considerations: In cases of incurable diseases like Huntington’s, should individuals or families pursue genetic testing?