1. Classification of Genetic Disorders
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2. Fig. 13.3 ©Scion Publishing Ltd
Etiology of diseases.
For any condition the overall balance of genetic and environmental determinants can be represented by a point somewhere within the triangle.
Fig ©Scion Publishing Ltd

3. Classification of genetic disorders
+ environment
Multifactorial
Single gene
Chromosomal
Mitochondrial
Somatic mutations (cancer)
Male

4. Fig. 1.15 ©Scion Publishing Ltd
Continuum of penetrance.
There is a continuum of penetrance from fully penetrant conditions, where other genes and environmental factors have no effect, through to low-penetrance genes that simply play a small part, along with other genetic and environmental factors, in determining a person’s susceptibility to a disease.
Multiple sclerosis is used as an example of a multifactorial condition where genetic factors play a major part in determining susceptibility, but current research suggests that each individual factor has a very low penetrance.
Fig ©Scion Publishing Ltd

5. Genetic factors
Male
Mutations in single genes (often causing loss of function)
Variants in genes causing alteration of function
Chromosomal imbalance causes alteration in gene dosage

6. Classification of genetic disorders
Single Gene Disorders
Male
Mutations in single genes
Multifactorial diseases
+ environment
Variants in genes
Chromosome disorders
Chromosomal imbalance

7. Heterozygotes with one copy of the altered gene are affected
Dominant
Heterozygotes with one copy of the altered gene are affected
Recessive
Homozygotes with two copies of the altered gene are affected
Male
X-linked recessive
Males with one copy of the altered gene on the X-chromosome are affected

8. Genetic disorders Multifactorial (common)
- “Environmental” influences act on a genetic predisposition to produce a liability to a disease.
- One organ system affected.
- Person affected if liability above a threshold.
Single gene (1% liveborn)
- Dominant/recessive pedigree patterns (Mendelian inheritance). - Can affect structural proteins, enzymes, receptors, transcription factors.
Chromosomal (0.6% liveborn)
- Thousands of genes may be involved. - Multiple organ systems affected at multiple stages in gestation. - Usually de novo (trisomies, deletions, duplications) but can be inherited (translocations).

9. The contributions of genetic and environmental
factors to human diseases
GENETIC ENVIRONMENTAL
Duchenne
muscular dystrophy
Haemophilia
Osteogenesis imperfecta
Club foot Pyloric stenosis Dislocation of hip
Peptic ulcer Diabetes
Tuberculosis
Phenylketonuria Galactosaemia
Spina bifida Ischaemic heart disease Ankylosing spondylitis
Scurvy
Rare Genetics simple Unifactorial High recurrence rate
Common Genetics complex Multifactorial Low recurrence rate

10. Multifactorial “Environmental” influences act on a genetic predisposition One organ system affected
Single gene Dominant/recessive pedigree patterns Structural proteins, enzymes, receptors, transcription factors
Chromosomal Multiple organ systems affected Inherited or de novo
Environmental Drugs, infections

11. Single gene disorders High risks to relatives
Dominant/recessive pedigree patterns
Some isolated cases due to new dominant mutations
Structural proteins, enzymes, receptors, transcription factors
I:1 AA
I:2 AB
II:1
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II:3 BB ?
III:1
Tom
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