Detailed Notes on Human Genetics and Laws of Inheritance

Human Genetics IV

• Focus on Laws of Inheritance
• Presented by Silvana Gaudieri, BSc (Hons), PhD, School of Human Sciences, UWA

Key Topics Recap:

• Understanding of replication: mitosis and meiosis
• Transfer of information related to DNA structure, transcription, and translation as per Chapter 1, 9.1-9.2, 10, and 11 of "Human Genetics" (Lewis, 12th Ed)

Genetic Lectures Themes:

• Structure and function of genomes, chromosomes, DNA, and genes
• Mechanisms of replication and mutations
• Reproductive processes: meiosis, recombination, chromosome number variation
• Laws of inheritance focusing on monogenic disorders and pedigrees
• Genetic variation reflecting evolutionary diversity
• Application of genetic principles in society including personalized medicine

Genotype and Phenotype Dynamics:

• Genotype: genetic constitution impacting traits
• Monogenic disorders (simple traits)
• Complex disorders (polygenic traits)
• Neutral mutations (no effect on phenotype)
• Phenotype: observable characteristics influenced by genotype and environment
• Mutations can be:
• Fully penetrant (all carriers show traits)
• Partially penetrant (some carriers show traits)
• Dominant and recessive categories determined based on expression

Mendel’s Study of Inheritance:

• Traits acquired through genetic transmission from parents
• Gregor Mendel (1822-1884) is acknowledged as the "Father of Genetics"
• Focused on traits in pea plants and developed predictive models for inheritance
• Mendel's Laws:
• Law of Dominance: Dominant traits mask recessive ones
• Law of Segregation: Alleles segregate independently during gamete formation
• Law of Independent Assortment: Genes independently assort into gametes

Dominance and Recessive Alleles:

• Observed traits in plant hybrids (Aa) showed dominance (Tall = T, Short = t)
• Homozygotes (TT, tt) lead to clear dominance or recessiveness
• Genetic crosses analyzed through Punnett Squares to predict offspring genotype and phenotype proportions

Punnett Squares:

• Used to visualize possible offspring genotypes from parental alleles
• Example: Cross of Aa x Aa reveals ratios:
• Genotypes: AA (1/4), Aa (1/2), aa (1/4)
• Phenotypes: Dominant (3/4) vs Recessive (1/4)

Mendelian Segregation Ratios:

• Derived from various genetic crosses involving homozygotes and heterozygotes
• Ratios showcase predictable inheritance outcomes under Mendelian rules

Monogenic Disorders:

• Caused by mutations in a single gene, with diseases classified based on inheritance patterns:
• Autosomal Dominant: Needs one affected allele (e.g., Huntington’s disease)
• Autosomal Recessive: Affected only if at least two recessives are inherited (e.g., Cystic fibrosis)
• X-linked Dominant: Father’s affected allele always passed to daughter (e.g., Rett syndrome)
• X-linked Recessive: Affected males display disorder; daughter may be carriers (e.g., Hemophilia A)

Pedigrees in Monogenic Disorders:

• Visual representation of inheritance patterns in families
• Carriers are often indicated in pedigrees without showing symptoms

Example Pedigrees:

• Analyze risk in family lines for dominant and recessive traits
• Consideration of history of traits can inform healthcare approaches

Factors Affecting Expression:

• Variability in phenotype expression includes environmental influences and genetic modifiers
• Understanding incomplete penetrance can help guide genetic counseling

Recent Advances:

• Genome sequencing assists in identifying mutations and understanding inheritance mechanisms
• Increased knowledge of gene-phenotype relationships enhances clinical applications.