13. Pedigree Method & Monogenic Inheritance: Sex-Linked, Sex-Limited, and Sex-Influenced
Explains how sex-linked traits are traced through pedigrees, covering X-linked recessive, X-linked dominant and Y-linked patterns with their features and examples. It then turns to sex-limited and sex-influenced inheritance, using a comparison table of the five patterns and closing with clinical relevance for genetic counselling and management.
Topic 13 of 65 in the MU Sofia syllabus for Genetics (Medicine, Year 3). The free AceMed note covers it in 7 sections, about 382 words (3 min of reading), with 14 flashcards.
In this note
Pedigree Method & Monogenic Inheritance: Sex-Linked, Sex-Limited, and Sex-Influenced
I. Clinical Genealogical (Pedigree) Method – Recap
II. Sex-Linked Inheritance
III. Sex-Limited Inheritance
IV. Sex-Influenced Inheritance
V. Pedigree Recognition – Key Differences
VI. Clinical Relevance
I. Clinical Genealogical (Pedigree) Method – Recap
Pedigree method = studying inheritance by tracing traits across generations using standardized symbols.
Allows recognition of sex-related inheritance patterns, which differ from autosomal traits due to involvement of sex chromosomes or sex-dependent expression.
II. Sex-Linked Inheritance
Definition
Traits caused by mutations in genes located on sex chromosomes (X or Y).
1. X-Linked Recessive (XLR)
Features
More common in males (hemizygous: only one X).
Females usually carriers; affected if homozygous mutant.
No male-to-male transmission (father passes Y to sons).
Affected father → all daughters carriers.
Pedigree: diagonal inheritance (skipping generations, seen in males).
Examples
Hemophilia A & B (F8, F9 genes).
Duchenne/Becker muscular dystrophy (DMD gene).
G6PD deficiency.
2. X-Linked Dominant (XLD)
Features
Both males and females affected, but females more frequent (XX).
No male-to-male transmission.
Affected father → all daughters affected, no affected sons.
Pedigree: disease appears in every generation (vertical).
Examples
Rett syndrome (MECP2 mutation, usually lethal in males).
Hypophosphatemic rickets.
3. Y-Linked (Holandric)
Features
Only males affected (Y chromosome).
Father → son transmission only (100%).
Rare, since Y carries few genes.
Examples
Y-linked infertility (AZF microdeletions).
Hypertrichosis pinnae auris (hairy ears).
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