Mendelian Patterns of Genetic Inheritance

Introduction to Mendelian Inheritance

Mendelian inheritance denotes genetic transmission patterns based on single genes located on chromosomes within the nucleus.
Transmission follows three fundamental laws defined by Mendel: Law of Segregation, Law of Independent Assortment, and Law of Dominance.

Demonstrates vertical transmission, appearing in multiple successive generations without skipping.
Affected individuals possess a 50% chance of passing the deleterious gene to each offspring.
Males and females are equally likely to be affected and transmit the trait.
Presence of male-to-male transmission confirms autosomal dominant inheritance and differentiates it from X-linked inheritance.

De novo mutations frequently account for isolated cases and are often associated with advanced paternal age.
Variable expressivity causes differing phenotypic severity among individuals harboring the identical genotype within the same family.
Incomplete penetrance occurs when an individual inherits the pathogenic variant but fails to manifest clinical symptoms, leading to skipped generations in pedigrees.
Pleiotropy involves a single gene defect resulting in multiple, apparently unrelated phenotypic anomalies across different organ systems.

Achondroplasia, Neurofibromatosis Type 1, Marfan syndrome, and Tuberous Sclerosis.

Exhibits horizontal transmission, typically restricting phenotypic manifestation to a single sibship within one generation.
Carrier parents face a 25% risk for an affected child, 50% risk for a carrier child, and 25% risk for a genetically normal child per pregnancy.
Males and females are affected in equal proportions.
Increased prevalence is noted in consanguineous unions due to a higher probability of both parents inheriting the identical mutant allele from a common ancestor.

Frequently associated with enzymatic defects and inborn errors of metabolism, since 50% enzyme activity in carriers is generally sufficient for normal function.
Pseudodominance occurs when a homozygous affected individual mates with a heterozygous carrier, producing a dominant-like vertical transmission pattern.

Incidence is significantly higher in males than in females.
Affected males transmit the mutant gene to 100% of their daughters, making them obligate carriers, and to 0% of their sons, demonstrating an absence of male-to-male transmission.
Carrier females face a 50% chance of transmitting the mutant allele to their sons, who will be affected, and to their daughters, who will be carriers.
Follows a criss-cross inheritance pattern from affected grandfather through carrier daughter to affected grandson.

Heterozygous females may occasionally display symptoms due to skewed X-inactivation, known as Lyonization.
Females with Turner syndrome can express X-linked recessive traits due to the presence of only a single X chromosome.

Affects both sexes, but females are affected more frequently, often presenting with a milder phenotype compared to hemizygous males.
Affected males transmit the trait to 100% of their daughters and 0% of their sons.
Affected females carry a 50% transmission risk for both sons and daughters.

Several conditions exhibit male lethality in utero, leading to pedigrees exclusively featuring affected females and an increased incidence of spontaneous abortions.

Inheritance Pattern	Affected Parent Status	Recurrence Risk to Offspring
Autosomal Dominant	One parent is heterozygous	50% risk for offspring of either sex.
Autosomal Recessive	Both parents are carriers	25% affected, 50% carrier, 25% unaffected.
X-Linked Recessive	Mother is a carrier	50% sons affected, 50% daughters carriers.
X-Linked Recessive	Father is affected	0% sons affected, 100% daughters carriers.
X-Linked Dominant	Father is affected	0% sons affected, 100% daughters affected.
Y-Linked	Father is affected	100% sons affected, 0% daughters affected.