Pedigree Analysis and Modes of Inheritance

Comprehensive vocabulary flashcards reviewing pedigree interpretation rules, inheritance patterns (autosomal dominant, autosomal recessive, X-linked, mitochondrial), penetrance, expressivity, and related clinical genetic disorders.

Vertical Transmission

A pedigree pattern where an affected individual or phenotype is observed in every consecutive generation, strongly indicating a dominant mode of inheritance.

Horizontal Transmission

A pedigree pattern where affected individuals appear primarily within a single generation or skip generations, typically characteristic of a recessive mode of inheritance.

Direct Father-to-Son Transmission

The transmission of an allele or trait directly from a father to his son; this definitively excludes X-linked inheritance (since males pass their Y chromosome to sons) and confirms autosomal inheritance.

Marfan Syndrome

An autosomal dominant connective tissue disorder caused by mutations in $FBN1$ affecting approximately $1/5000$ individuals, displaying variable expressivity across the cardiovascular, ocular, skeletal, pulmonary, skin, and central nervous systems.

Fibrillin ($FBN1$)

An essential glycoprotein encoded by the $FBN1$ gene that is required for the proper structure and assembly of elastic fibers.

Reduced Penetrance

A genetic phenomenon in which an individual possesses a disease-causing genotype (such as an autosomal dominant allele) but fails to express the clinical phenotype, appearing unaffected while remaining capable of transmitting the disorder to offspring.

Variable Expressivity

The phenomenon in which individuals carrying the exact same disease-causing genetic mutation exhibit varying severity, forms, or ranges of clinical phenotypes and morphopathologies.

Polydactyly

A limb morphopathology characterized by the presence of extra digits on the hands or feet, demonstrating variable expressivity among family members sharing identical causative alleles.

Consanguinity

Mating between biologically related individuals (familial mating), which markedly increases the probability of bringing together rare recessive alleles and manifesting autosomal recessive phenotypes in offspring.

De Novo Mutation

A new genetic alteration arising in a germ cell or during early development rather than being inherited from carrier or affected parents; achondroplasia is noted as an example where these mutations are common.

Marrying-In Clinical Assumption

The standard rule of pedigree analysis dictating that any individual marrying into a family is assumed to be homozygous unaffected ($aa$ for dominant traits or $AA$ for recessive traits) unless explicit phenotypic or genealogical evidence indicates otherwise.

Autosomal Dominant Recurrence Risk ($Aa \times aa$)

A recurrence risk of $50\%$ for each offspring to inherit the dominant affected allele and present the phenotype when a heterozygous affected individual ($Aa$) mates with a homozygous unaffected individual ($aa$).

X-Linked Dominant Inheritance

A mode of inheritance characterized by vertical transmission in which an affected male transmits the trait to $100\%$ of his daughters and $0\%$ of his sons, with no direct father-to-son transmission.

X-Linked Recessive Inheritance

A mode of inheritance where carrier mothers transmit the condition to sons; an affected male transmits $0\%$ of affected alleles to sons and has $0\%$ affected sons when partnered with a homozygous unaffected female.

Mitochondrial Inheritance

A non-Mendelian maternal inheritance pattern where an affected female transmits the condition to $100\%$ of her offspring (both sons and daughters), while affected males transmit the condition to $0\%$ of their offspring.

Menorrhagia

Excessively heavy or prolonged menstrual bleeding, which can serve as an early phenotypic indicator of an inherited bleeding or coagulation disorder.