Medical Genetics · MU Plovdiv

16. Non-Mendelian type of inheritance – types. Dynamic mutations. Triplet repeat expansion – essence, examples, clinical-genetic characteristics. MGC.

Inside: 14 sections · 15 labelled pictures · 3 tables · 27 flashcards · 9 min

Topic 16 of 33 in the MU Plovdiv syllabus for Medical Genetics (Medicine, Year 4). The free AceMed note covers it in 14 sections, about 1,177 words (9 min of reading), with 27 flashcards.

In this note

  1. Introduction
  2. I. Introduction
  3. II. Genomic imprinting
  4. III. Uniparental disomy (UPD)
  5. IV. Dynamic mutations (repeat expansion disorders)
  6. V. Huntington disease (HD)
  7. VI. Myotonic dystrophy (DM)
  8. VII. Fragile X (Martin–Bell) syndrome
  9. VIII. Gonadal mosaicism
  10. IX. Incomplete penetrance
  11. X. Variable expressivity
  12. XI. Pleiotropy
  13. XII. De novo mutations
  14. XIII. Clinical importance and genetic counseling

Introduction

This topic covers the types of non-traditional (non-Mendelian) inheritance, with emphasis on dynamic mutations (trinucleotide repeat expansion): their essence, examples (Huntington disease, myotonic dystrophy, fragile X syndrome), clinical-genetic characteristics and medical genetic counseling.

I. Introduction

  • Mendelian inheritance explains most single-gene disorders (AD, AR, X-linked).
  • However, many traits and diseases deviate from these simple patterns.
  • These deviations are grouped as non-traditional (non-Mendelian) inheritance mechanisms.
  • Understanding them is crucial for genetic counseling, diagnosis, and predicting disease course.

II. Genomic imprinting

Definition

  • Epigenetic phenomenon where the expression of a gene depends on whether it is inherited from the mother or father.
  • One parental allele is silenced by DNA methylation and histone modifications, leaving only the other allele expressed.

Features

  • Parent-of-origin effect → phenotype depends on which parent transmitted the mutation.
  • Imprints are erased and reset in the germline according to the sex of the parent.

Examples

  • Prader–Willi syndrome: deletion of 15q11–q13 on the paternal chromosome. The maternal allele is normally imprinted (silenced). → Hypotonia, obesity, intellectual disability.
  • Angelman syndrome: deletion of the same region, but on the maternal chromosome (UBE3A gene active only from the maternal allele). → Severe intellectual disability, ataxia, “happy puppet” behavior.

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