Inheritance Patterns
Overview
Inheritance patterns describe how genetic traits and disorders are transmitted from one generation to the next. This chapter is foundational to genetic counseling practice: understanding inheritance is essential for constructing accurate pedigrees, calculating recurrence risks, and communicating genetic information to patients. Inheritance patterns are foundational to pedigree interpretation and to the risk-assessment calculations they support.
The chapter covers both Mendelian and non-Mendelian inheritance. Mendelian patterns (autosomal dominant, autosomal recessive, and X-linked) follow predictable rules first described by Gregor Mendel and remain the starting point for most clinical genetics discussions. Non-Mendelian patterns, including mitochondrial inheritance, trinucleotide repeat disorders, mosaicism, and multifactorial inheritance, introduce additional complexity that is critical for accurate counseling.
A strong grasp of inheritance patterns allows you to move fluidly between pedigree interpretation, risk calculation, and patient communication. Many clinical scenarios begin with a pedigree and ask you to identify the most likely inheritance pattern, which then determines the recurrence risk and appropriate testing strategy.
Key Concepts
- Pedigree analysis: recognizing which inheritance pattern best fits a given family structure
- Recurrence risk calculation: applying the rules of each inheritance pattern to determine risk for future pregnancies or at-risk relatives
- Exceptions to classic patterns: reduced penetrance, variable expressivity, anticipation, germline mosaicism, and sex-limited expression
- Distinguishing Mendelian from non-Mendelian: recognizing when a pattern does not fit a simple Mendelian model and what alternatives to consider
Mendelian Basics
The three Mendelian-basics leaves establish the core inheritance framework. Autosomal Dominant Inheritance and Autosomal Recessive Inheritance cover vertical-transmission vs. carrier-couple patterns, along with the concepts that surround them (penetrance, expressivity, carrier status, new mutations). X-Linked Inheritance treats both recessive and dominant X-linked conditions, with particular attention to manifesting female carriers and the diagonal/skipping-generation patterns that distinguish X-linked from autosomal pedigrees.
Mendelian Extensions
The "extensions" are concepts that complicate clean Mendelian predictions without breaking the underlying model. Penetrance and Variable Expressivity explain why an obligate carrier may be unaffected (incomplete penetrance) and why two carriers of the same variant can present very differently (variable expressivity). Pleiotropy describes one gene affecting multiple organ systems. Genetic Heterogeneity describes the inverse: one phenotype caused by variants in many different genes. Haploinsufficiency vs Dominant-Negative distinguishes the two main mechanisms by which dominant variants act, which matters for predicting severity and for choosing the right functional assay.
Non-Mendelian
The non-Mendelian leaves describe inheritance that does not follow Mendel's rules. Mitochondrial Inheritance covers maternal transmission, heteroplasmy, and the threshold effect. Repeat Expansion Disorders introduces anticipation and the molecular basis for conditions like Huntington disease, fragile X, and myotonic dystrophy. Mosaicism explains how post-zygotic mutations create mixed cell populations, affecting both phenotype and recurrence risk (germline mosaicism is the most frequently miscounseled scenario in this category). Multifactorial Inheritance covers the threshold/liability model, heritability, and empiric recurrence risks for common conditions like neural tube defects and congenital heart disease.
Together, these topics provide the genetic framework that underpins risk assessment, laboratory interpretation, and counseling communication.