PPT 10

Non-Mendelian Genetics Overview

Different Types of Genetic Patterns

• Simple Autosomal Recessive Inheritance

  • Trait determined by one gene on an autosome.

  • Only expressed by homozygous recessive individuals.

  • Certain diseases are common in specific populations.

Characteristics of Simple Autosomal Recessive Traits

• Traits can skip generations.

• Equal chance of expression in males and females.

• Carrier: An individual who carries the recessive allele but does not exhibit the disease.

• Both parents must be carriers to pass the disease to their children, often found in closely related individuals.

• Homozygous recessive individuals (aa) are not considered carriers.

Cystic Fibrosis

• Mutation in the gene controlling salt and water balance in cells.

• Symptoms: Thickened mucus causing respiratory and digestive issues.

• Carrier Statistics: 1 in 20 White North Americans are carriers (genotype Cc).

• Homozygous recessive (cc) individuals cannot be carriers as CF is deadly; they do not reproduce.

• Genetic analysis and medical symptoms (e.g., sweat test) are used for diagnosis.

• Mandatory screening for newborns in North America, including Quebec.

Tay-Sach's Disease

• Common among Eastern European Jews and French Canadians.

• Caused by enzyme malfunction leading to brain swelling and potentially death by age 5.

• Genetic mutation leading to a frameshift in the HEXA gene.

Sickle Cell Anemia

• Epidemiology: 1 in 4 Black people in Central Africa are carriers.

• Carriers of the sickle cell trait may have a survival advantage in malaria-prone regions.

• Symptoms include susceptibility to infections, severe pain crises, and complications with overexertion and altitude.

• Crystallization of hemoglobin under low oxygen levels deforms red blood cells into a sickle shape, reducing their ability to carry oxygen.

Simple Autosomal Dominant Inheritance

• Affected individuals have at least one dominant allele ( H) and usually have an affected parent.

• Traits produced by one gene site on an autosome, independent of gender.

• Major conditions: Dwarfism and Huntington's Disease.

  • Dwarfism: Caused by double dominance; lethal in some cases.

  • Huntington’s Disease: Neuromuscular degeneration that manifests in mid-adulthood (40s-50s); inheriting one allele is sufficient for disease expression.

Comparing Recessive and Dominant Inheritance

• Recessive traits require both parents to carry the allele for expression.

• Dominant traits require only one affected parent, leading to potential purging of harmful alleles over generations.

Sex-Linked Recessive Inheritance

• Traits located on the X chromosome.

• Males have a higher chance of expressing recessive traits on the X chromosome; examples include color blindness and hemophilia.

Pedigree Analysis

• Pedigrees track inheritance patterns within families, identifying carriers and affected individuals.

• Useful for understanding genetic diseases, as patterns can be analyzed across generations.

X-Inactivation

• In female mammals, one X chromosome becomes inactive (Barr body) early in embryonic development, ensuring dosage compensation between sexes.

• This random inactivation results in females being mosaics for X-linked traits.

Incomplete Dominance and Co-Dominance

• Incomplete Dominance: Offspring exhibit a phenotype that is a blend of the parents' traits (e.g., Snapdragons).

  • Example: Red (CR CR) x White (CW CW) = Pink (CR CW).

• Co-Dominance: Both alleles contribute to the phenotype without blending.

  • Example: Blood types A (IA) and B (IB) are co-dominant.

Polygenic and Multifactorial Inheritance

• Polygenic Inheritance: Multiple genes influence a single trait (e.g., human height).

• Multifactorial Inheritance: Genetic and environmental factors influence traits, leading to conditions that can run in families.

Extranuclear Inheritance

• Involves DNA from mitochondria and chloroplasts, inherited maternally.

• Useful in genetic studies and tracing ancestry or species lineage.