Mendelian Inheritance

Mendelian genetics is based on three primary laws: the Law of Segregation (each individual has two alleles for a trait which segregate during gamete formation), the Law of Independent Assortment (genes for different traits segregate independently), and the Law of Dominance (one allele can mask the presence of another). In humans, Mendelian disorders are caused by mutations in a single gene (monogenic). A pedigree chart is used to visualize these patterns: squares represent males, circles represent females, and shaded shapes represent affected individuals. Autosomal traits affect both sexes equally. Dominant traits typically appear in every generation, while recessive traits often skip generations and require two copies of the mutant allele. X-linked traits show a sex-bias, usually affecting males more severely as they are hemizygous. While Mendelian laws provide a framework, real-world genetics is often complicated by factors such as incomplete penetrance (carrying the gene but not showing the phenotype), variable expressivity (different degrees of severity), and pleiotropy (one gene affecting multiple organ systems). Understanding these nuances is essential for accurate risk assessment and diagnosis in clinical practice.

The biological basis of Mendelian inheritance lies in meiosis and the behavior of chromosomes. Genes are located at specific loci on chromosomes. During Meiosis I, homologous chromosomes pair up and then separate. This ensures that each gamete receives only one allele for each gene, fulfilling the Law of Segregation. Because chromosomes align randomly at the metaphase plate, genes on different chromosomes are distributed into gametes independently of one another (Independent Assortment). However, genes located very close together on the same chromosome may be inherited together, a phenomenon known as 'linkage,' which is an exception to Mendel's second law. Dominance occurs when the protein product of one allele is sufficient to produce the phenotype or when a mutant allele produces a 'gain-of-function' or 'dominant-negative' effect. Recessive traits usually involve 'loss-of-function' mutations where 50% of the protein product (as in a carrier) is sufficient for health, and disease only occurs when both alleles are non-functional.

Mendelian inheritance governs over 10,000 human disorders. Clinicians use pedigree analysis to identify inheritance patterns and calculate the probability of offspring being affected. This is crucial in preconception counseling for conditions like Cystic Fibrosis (AR) or Huntington's Disease (AD). Differential diagnosis often hinges on inheritance; for example, a male with a bleeding disorder and an affected maternal uncle strongly suggests Hemophilia (X-linked). Recognizing these patterns also guides the selection of genetic tests, moving from targeted single-gene sequencing to broader panels if the inheritance suggests a specific pathway.

1. Pedigree Chart: Drawing at least a three-generation family history. 2. Punnett Square: Used to calculate theoretical risk for offspring. 3. Target Gene Sequencing: Based on the suspected Mendelian pattern. 4. Segregation Analysis: Testing other family members to confirm if a genetic variant tracks with the disease phenotype.

Management involves genetic counseling to discuss recurrence risks (e.g., 25% for AR, 50% for AD). Refer to Clinical Genetics for detailed risk modeling. For many Mendelian disorders, management is supportive and focused on the specific organ systems affected, though gene therapy is an emerging field for some (e.g., Spinal Muscular Atrophy).