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Introduction to Inheritance

Introduction to Inheritance. Sara Levene Registered Genetic Counsellor London IDEAS Genetic Knowledge Park. Mendelian Genetics. Genetic conditions are not very common. Inheritance Patterns. Autosomal dominant (AD) Autosomal recessive (AR) X-Linked (X-L). Dominant Inheritance. . .

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Introduction to Inheritance

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  1. Introduction to Inheritance Sara Levene Registered Genetic Counsellor London IDEAS Genetic Knowledge Park

  2. Mendelian Genetics

  3. Genetic conditions are not very common

  4. Inheritance Patterns • Autosomal dominant (AD) • Autosomal recessive (AR) • X-Linked (X-L)

  5. Dominant Inheritance  

  6. Dominant Inheritance

  7. Common Autosomal Dominant Disorders • Huntington Disease • Marfan syndrome • Myotonic dystrophy • Neurofibromatosis • Breast / ovarian cancer susceptibility (BRCA1/2) • Familial Adenomatous polyposis • Hereditary Non-Polyposis Colorectal cancer (HNPCC)

  8. Recessive Inheritance Non carrier Carrier Carrier Affected individual

  9. Recessive Inheritance

  10. Common Autosomal Recessive Disorders • Cystic Fibrosis • Sickle cell anaemia • Thalassaemia • Tay Sachs disease • Spinal muscular atrophy • PKU • Haemachromatosis

  11. X-Linked Inheritance   X X X Y X X X X X X Y Y Non carriergirl Non carrierboy Carriergirl Affected boy    

  12. X-Linked Inheritance

  13. Common X-Linked Disorders • Duchenne muscular dystrophy • Becker muscular dystrophy • Haemophilia • Fragile X syndrome

  14. Multifactorial Inheritance

  15. Multifactorial Inheritance • Familial clustering (does not conform to patterns of Mendelian inheritance). • Recurrence risks ~2–4% (compared to higher risks in Mendelian disorders) • Caused by interaction of genetic and environmental factors • None of these factors are well understood!

  16. Multifactorial Inheritance:Congenital malformations • Cleft lip/palate • Congenital dislocation of the hip • Congenital heart defects • Neural tube defects • Pyloric stenosis • Talipes

  17. Multifactorial Inheritance:Acquired diseases of childhood and adult life • Asthma • Autism • Diabetes mellitus • Epilepsy • Glaucoma • Hypertension • Inflammatory bowel disease (Crohn disease/ulcerative colitis) • Ischaemic heart disease • Ischaemic stroke • Manic depression • Multiple sclerosis • Parkinson disease • Psoriasis • Rheumatoid arthritis • Schizophrenia

  18. Theory of Polygenic inheritance(Continuous inheritance) • Many genes at different loci, with each gene exerting a small additive effect • Effects of the genes are cumulative i.e. no one gene is dominant or recessive • Applies to characteristics that show normal distribution in the population e.g. height, IQ (bell curve) • ? Application of this model to discontinuous multifactorial disorders (affected/not affected)

  19. Liability / Threshold Model • ‘Bad’ genes + adverse environmental factors = liability to disorder • Liability = continuous variable (normal distribution) • Above threshold abnormal phenotype expressed. • General population – beyond the threshold is the population incidence • Relatives of cases – beyond the threshold is the familial incidence • Recurrence risk depends on number of affected relatives and closeness to index case

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