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Factors Affecting Pedigree Analysis

Factor Definition Effect on Pedigree Analysis
Incomplete Penetrance
(不完全外显) 		Proportion of individuals with a pathogenic genotype who show clinical symptoms is less than 100%. Causes skipped generations, mimicking recessive inheritance or new mutations; unaffected carriers may be misclassified as non-carriers.
Variable Expressivity
(变异性表现度) 		Variation in phenotype severity among individuals with the same genotype. Mildly affected individuals may be overlooked or considered unaffected, distorting segregation ratios.
New (De Novo) Mutation
(新突变) 		Mutation arising in germline or zygote without prior family history. Isolated affected individual in a family; for dominant disorders, may appear as a sporadic case; recurrence risk is low but not zero (germline mosaicism in parent).
Phenocopy
(拟表型) 		Environmental or acquired condition mimicking a genetic disease. Sporadic cases without family history may be mistaken for inherited disease; overestimation of genetic contribution; incorrect inheritance pattern assignment.
Nonpaternity
(非父权) 		Legal father is not the biological father. Breaks expected genetic links; can falsely exclude or confirm genetic transmission patterns; linkage analysis becomes invalid.
Consanguinity
(近亲婚配) 		Mating between individuals sharing a common ancestor. Increases probability of recessive disorders; presence of multiple affected siblings with healthy parents points to recessive inheritance; may cause pseudodominance.
Genetic Heterogeneity
(遗传异质性) 		Mutations in different genes producing similar phenotypes (locus heterogeneity) or different mutations in same gene (allelic heterogeneity). Affected individuals from different families may not share the same gene; within a family, atypical segregation can occur; pooling families for linkage analysis may fail.
X-inactivation
(X染色体失活) 		Random inactivation of one X chromosome in female cells (Lyonization). Skewed inactivation can cause variable expression in females with X‑linked disorders, mimicking autosomal dominant or recessive patterns; female carriers may be symptomatic.
Pleiotropy
(基因多效性) 		One gene influencing multiple seemingly unrelated phenotypic traits. Multiple system involvement may be misattributed to separate diseases; affected individuals with only one manifestation may be missed.
Onset Age
(发病年龄) 		Age at which disease symptoms first appear. Late‑onset diseases lead to young carriers being recorded as healthy; early‑onset lethal diseases may eliminate affected individuals before ascertainment, reducing observed affected count.
Sex‑influenced
(性别影响) 		Autosomal gene whose expression is modulated by sex hormones; one sex more severely or more frequently affected. Imbalance between sexes may be mistaken for X‑linked inheritance; correct pattern only revealed by family data across both sexes.
Sex‑limited
(性别限制) 		Autosomal gene expressed only in one sex (e.g., uterus, prostate). The non‑expressing sex carries the gene without showing symptoms, creating apparent skipping of generations; transmission appears “through” unaffected individuals.
Mitochondrial Inheritance
(线粒体遗传) 		Transmission of mitochondrial DNA (mtDNA) mutations exclusively through the mother. Affected females transmit to all offspring; affected males never transmit; pattern is non‑Mendelian and may be mistaken for dominant or recessive without maternal link.
Genomic Imprinting
(遗传印记) 		Gene expression depends on parent‑of‑origin (maternal vs. paternal allele). Disease appears only when inherited from a specific parent; non‑Mendelian transmission; can mimic dominant or recessive depending on imprinting status.
Mosaicism
(嵌合体) 		Presence of ≥2 genetically distinct cell lines in one individual (somatic or germline).
Anticipation
(遗传早现) 		Increasing severity and/or earlier onset in successive generations. Observed in trinucleotide repeat disorders (e.g., Huntington, myotonic dystrophy); may be misinterpreted as different diseases if not recognized; suggests dynamic mutation.
Pseudodominance
(假性显性) 		Apparent dominant transmission of a recessive disorder due to consanguinity or high population carrier frequency. Pedigree shows affected individuals in consecutive generations, mimicking autosomal dominant inheritance; correct interpretation requires knowledge of consanguinity.
Uniparental Disomy (UPD)
(单亲二体) 		Both homologues of a chromosome inherited from the same parent. Can cause recessive disease in non‑consanguineous families (parent heterozygous, child homozygous); unexpected segregation ratios; some UPD forms cause imprinting disorders.
Polygenic / Multifactorial Inheritance
(多基因/多因素遗传) 		Interaction of multiple genes + environmental factors causing disease (threshold model). No clear Mendelian segregation; familial aggregation without typical patterns; recurrence risk <10% for first-degree relatives; cannot be traced with single gene analysis.
Age‑dependent Penetrance
(年龄依赖性外显) 		Penetrance increases progressively with age (e.g., BRCA1, Huntington). Young individuals are classified as unaffected but may develop disease later; requires life‑table correction and follow‑up; static pedigree analysis underestimates risk.
Reduced Reproductive Fitness
(生殖适合度降低) 		Decreased ability of affected individuals to have children due to disease severity or infertility. Fewer affected individuals in later generations; may mimic low penetrance or recessive inheritance; ascertainment bias toward milder cases.
Somatic Mosaicism
(体细胞嵌合体) 		Mutation acquired post‑zygotically, present only in a subset of somatic cells (not in germline). Affected individual may have patchy or mild disease; offspring risk is zero unless germline also involved; can mimic acquired/environmental conditions.
Germline Mosaicism
(生殖系嵌合体) 		Mutation present only in a fraction of germ cells, not in somatic cells. Unaffected parents have multiple affected children with a dominant disorder; recurrence risk is >0 but <50% (depends on proportion of affected germ cells).
Balanced Chromosomal Rearrangement (e.g., translocation)
(染色体平衡重排) 		Structural rearrangement without net gain/loss of DNA; carrier is phenotypically normal. Carrier parents have increased risk of unbalanced offspring (deletion/duplication) causing disease; can mimic dominant inheritance with reduced penetrance or new mutations.
Heteroplasmy (mitochondrial)
(异质性（线粒体异质性）) 		Mixture of wild‑type and mutant mtDNA within an individual. Variable expressivity; threshold effect; transmission patterns less predictable than homoplasmy; offspring can inherit different mutation loads.
Digenic Inheritance (double heterozygosity)
(双基因遗传) 		Mutations in two different genes required for disease expression (each inherited separately). Resembles recessive but may show complex segregation; can mimic incomplete penetrance; family members with one mutation are unaffected.
Incomplete Family History
(不完全家系信息) 		Missing medical or demographic information on family members (e.g., deceased, adopted, lost follow‑up). May lead to under‑ascertainment of affected individuals and incorrect inheritance assignment; false-negative family history.
Diagnostic Error / Misclassification
(诊断错误/分类错误) 		Incorrect assignment of disease status (affected vs. unaffected) due to clinical overlap or testing error. False‑positive or false‑negative cases distort segregation ratios and linkage analysis; may suggest wrong inheritance pattern.
Adoption / Unknown Biological Relationships
(收养/未知生物学关系) 		Lack of biological relationship information; individual may not be genetically related to the recorded family. Pedigree structure is incorrect; genetic analysis invalid without correction; nonpaternity is a subset.
Small Family Size
(小家庭规模) 		Few siblings or generations available for analysis (e.g., only child, one generation). Cannot distinguish between recessive and dominant patterns; statistical power very low; impossible to confirm segregation.
Ascertainment Bias
(确认偏倚) 		Non‑random selection of families for study (e.g., through affected probands, large families only). Overestimation of recurrence risks; segregation ratios deviate from Mendelian expectations; linkage distances may be overestimated.
Imprinting + UPD (e.g., Prader‑Willi / Angelman)
(印记合并单亲二体) 		Combined effect of imprinting and uniparental disomy leading to disease. Requires both parent‑of‑origin effect and UPD; not explainable by standard Mendelian or simple imprinting models; specific molecular testing needed.
Epigenetic Modifications (non‑imprinting)
(表观遗传修饰（非印记）) 		Heritable changes in gene expression without DNA sequence change (e.g., DNA methylation, histone modification). Can alter disease expression and transmission patterns across generations; complicates genotype‑phenotype correlation; may cause discordance in monozygotic twins.

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