2. INTRODUCTION
• Genetics is the science of genes, heredity and
the variation of individual (organism).
• Hereditary means derived from parent
• Familial means transmitted in the gametes
through generation
• Congenital means present at birth
3. • GENE:
portion of DNA which code for particular traits or
characteristics. They occur in pairs called alleles.
Gene could be recessive or dominant
• Dominant gene: is the gene that expresses its
character even when a contrasting character is
present.
• Recessive gene: is the gene that will not express
its trait in the presence of another contrasting
trait.
4. Genetic pattern of mental illnesses
A. Mendelian pattern
I. Chromosomal disorder
II. Single gene disease
III. Mitochondrial disorder
IV. Polygenic disorder
B. Non-mendelian or complex
I. Endophenotypes
II. Stress-diathesis hypothesis
6. Chromosomal Numeric Aberration
A. Polyploidy :
Multiple of the haploid number (n=23)
exceeding the diploid number (n=46) e.g 3n, 4n
B. Aneuploidy:
Occur when the chromosome is not exact
multiple of the haploid number. Could be;
a. Trisomy (2n+1)
b. Monosomy (2n-1)
7. Example of Aneuploidy
1. Trisomy 21 (down syndrome)
There is extra copy of chromosome 21 resulting
from meiotic non-disjunction
a. It’s the leading cause of mental
retardation
b. A risk factor for the development of
early onset Alzeheimer’s disease
resulting in irreversible dementia
(occuring usually around 45 years)
8. Sex chromosomal abnormality
A. Klinefelter’s syndrome:
i. 47, XXY
ii. Language delay
iii. Motor impairment
iv. Gender problems
v. Poor self esteem
9. Single gene disease
• These result from mutations in single genes that
cause large effects on an individual.
• Mutated gene could either be ;
i. Autosome or sex chromosome
ii. Dominant or recessive
• Pattern of inheritance can be
i. Autosomal dominant: Huntington disease
ii. Autosomal recessive
iii. X-linked recessive
10. Non-mendelian or complex
• Most psychiatric disorders do not show classic
mendelian pattern of inheritance. It implies the
requires severe gene mutation not just a single
gene.
• Gene does not encode mental illness. It encodes
for proteins, this could include protein that
regulate neurodevelopment.
• It could also include proteins ranging from
enzyme to transporter to signal transduction
molecules
11. • Genes do not directly encode mental
illnesses, behaviour or personalities. Instead,
they encode proteins.
• In some cases, gene may produce genetically
altered proteins that codes for subtle
molecular abnormalities, which in turn may be
linked to the development of psychiatric
symptoms.
12. Endophenotypes Concept
• On the path from gene to behaviour lies
endophenotypes (Intermediate phenotypes)
• Endophenotypes are internal phenotype
13.
14. Examples of Endophenotypes
• Sensory gating anomalies in schizopheria
• Eye tracking abnormalities in schizopheria
• Neuropsychological deficits in bipolar disorder,
schizopheria and possibly obsessive-
compulsive disorder
15. Stress-diathesis Hypothesis
• Genes alone are not necessarily enough to cause
mental illness. Something else generally has to
occur from the environment to make the
inheritance of silent risk become manifest as
illness.
• That something else is called stress
• Environment stressor are often life event such as;
a. childhood abuse
b. divorce
c. viruses and toxin
17. Genetic studies
• The studies are;
i. Family study
ii. Twin study
iii. Adoption study
• The first clue that a disorder has a genetic
component usually comes from studying
aggregation in families.
• In psychiatry, this is often complemented by
adoption studies. However, it is twin studies that
provide the most compelling evidence
18. Family studies
• It establishes that a given disorder runs in the
families.
• However does not show directly whether this is
due to gene or environmental factors
• Affected individuals (probands) are selected and
their disorder are examinee for a given disorder.
• Method;
i. Family history method
ii. Family study methodd
19. Twin studies
• Monozygotic twins show 100% genetic material, while
dizygotic twins share as much as 50%
• Concordance is defined as the possibility that both
members of a twin pair will have the disorder being studied
• It is assumed that monozygotic and dizygotic twins grow up
in similar environment
• If the disorder is determined by environmental factor,
concordance rate in monozygotic and dizygotic twins
should be similar
• On the other hand if it is genetically determined,
concordance should be more in monozygotic than in
digzygotic twins.
20. Examples of twin studies
• Concordance for
a. Bipolar disorder in monozygotic twin is
60% and in dizygotic twin is 20%
b. Schizophrenia in monozygotic twin is
40-50% and in dizygotic twin is 10%
c. Dementia is higher in dizygotic twin
than monozygotic twin
21. Summary
• Gene encode protein, not psychiatric
symptoms or mental illnesses
• The hypothetical path from gene (genotype)
to mental illness (phenotype) passes through
important intermediaries called
ENDOPHENOTYPES
