IB Psych - Genetics Studies

Kendler et al (2006):Aim

To investigate the heritability of depression

Kendler et al (2006):Method

Correlation (interviews; twin study)

Kendler et al (2006):Participants

42000 twins from Sweden's twin registry

Kendler et al (2006):Procedure

The participants were interviewed and assessed for major depressive disorder in accordance with DSM-IV

Kendler et al (2006):Results

The researchers estimated from the concordance rates in mono- & dizygotic twins that the heritability of depression is ≈ 38%- The rate was higher in females. (MZ male - 31%: MZ female - 44%: DZ males - 11% DZ females - 16%)

Kendler et al (2006):Conclusion

Depression is somewhat heritable, but environment & stress play at least as big a role. (Predisposition & stress: diathesis stress theory)

Weissmann et al (2005): Aim

To study the potential genetic nature of Major Depressive Disorder over 3 generations

Weissmann et al (2005): Method

· Sample of 161 grandchildren and their parents and grandparents· Took place over a twenty year period, looking at families at high and low risk for depression· The original sample of depressed patients (now, the grandparents) was selected from an outpatient clinic with a specialization in the treatment of mood disorders.· The non-depressed participants were selected from the same local community. (environment but less able to generalise)· The original sample of parents and children were interviewed four times during this period.· The children are now adults and have children of their own - allowing for study of the third generation.· Data was collected from clinicians, blind to past diagnosis of depression or to data collected in previous interviews.In order to establish credibility, researcher triangulation (reliability) was used. Children were evaluated by two experienced clinicians - with one being a child psychiatrist and the other a psychologist.

Weissmann et al (2005): Results

· The inter-rater reliability of their diagnoses were 0.82 for MDD, 0.65 for anxiety disorders and 0.94 for alcohol dependency (more quantifiable).· High rates of psychiatric disorders in the grandchildren with two generations of major depression. By 12-years-old, 59.2% of the grandchildren were already showing signs of a psychiatric disorder - most commonly anxiety disorders.· Children had an increased risk of any disorder if depression was observed in both the grandparents and the parents, compared to children where their parents were not depressed. In addition, the severity of a parent's depression was correlated with an increased rate of a mood disorder in the children.On the other hand, if a parent was depressed but there was no history of depression in the grandparents, there was no significant effect of parental depression on the grandchildren. (one parent has depression)

Weissmann et al (2005): Conclusions

· Association between parental MDD and child diagnosis is moderated by grandparental MDD status· Rates of psychopathology are highest in grandchildren of parents and grandparents with a moderately to severely impairing depressionAnxiety disorders are the early sign of psychopathology in the young grandchildren

Kendler et al (2018): Aim

To investigate the role of the environment in the origin of major depression with a sample of Swedish siblings

Kendler et al (2018): Method (Sample)

2,596 Swedish siblings (at high risk for major depression)- one sibling raised with the biological family and one with an adoptive family

Kendler et al (2018): Method (Procedure)

National medical registries used to assess history of major depression. - high-risk = having at least one biological parent with MDD

Kendler et al (2018): Results

MDD risk was reduced by 23% when a child from a high-risk family was adopted.- This difference disappeared when a step-sibling or adoptive parent had major depression or the adoptive home was disrupted by divorce or parental death.

Kendler et al (2018): Conclusion

The environment in which a child grows up in influences the development of MDD

Caspi et al (2003): Background

The 5-HTT gene = serotonin transporter gene, involved in reuptake of serotonin and influences the level of serotonin----- comes in two different versions, alleles, a long one and a short oneEvery human individual inherits two 5-HTT(Two long, two short, or one of each)

Caspi et al (2003): Aim

to investigate whether the short allele in combination with stressful events increases the risk of depression

Caspi et al (2003): Sample

847 Caucasian non-Maori children from New Zealand- opportunity sampling(originally taking part in a longitudinal study on health and development)

Caspi et al (2003): Procedure

Gene mapping was used- 17% had two short 5-HTT alleles- 51% had one short allele- 32% had two long allelesAfter 21st birthday and before 26 years: they were interviewed about 14 different kinds of stressful life events that had occurred since(included employment, financial, housing, health, and relationship stressors)- 30% had no experience of a stressful life event- 25% had experienced one- 20% had experienced two- 11% had experienced three- 15% had experienced four or moreThere was no significant difference between the three genotype groupsALSO AT 21-26: interviewed about symptoms of MDD over the last year17% met criteria for a major depressive episode (42% male and 58% female)

Caspi et al (2003): Results

1) The likelihood of a depressive episode during the last year was the same for both genotypes in case there had been no stressful event during the last five years2) The number of negative life events did not affect carriers of two LONG ALLELES3) There was a clear correlation between numbers of stressful events and likelihood of depressive episodes among carriers of SHORT ALLELES----- these also were more likely to have thought about committing suicide or to have attempted suicide

Caspi et al (2003): Conclusion

- there is no direct relation between short alleles on the 5HTT gene and depression----- there is a relationship between these and incidences of stress and subsequent depression- the long alleles seem to protect against suffering depression as a result of stress- the effects of the gene adaptation are dependent on environmental exposure to stress