Epigenetics and Genetic similarity

epigenetics + sexuality + depression + genetic similarity

genetic similarity

Genetic Similarity is the extent to which people share genes. It is based on the observation that biological relatives vary in their degree of genetic overlap, which can be exploited by researchers to examine the role of genes in behavior.

concordance rate:

The probability that the same trait will be present in twins, parents/children, and adoption situations

monozygotic twins (MZ):

The clinical term for identical twins; they share 100% of their DNA.

dizygotic twins (DZ)

The clinical term for fraternal twins, which is when two different eggs are fertilized by two different sperm cells. The genetic similarity of dizygotic twins is similar to that of siblings.

kinship/family studies:

Usually referred to as family studies or pedigree studies, look at how inherited traits or behaviours might spring from genetic similarity. Researchers trace a phenotype over several generations in a family tree to determine the likelihood that a behavior is inherited.

direct family relationships:

Between parents and child 50/50 of the DNA is shared. Siblings share 50% of DNA, grandparents share 25% of DNA, cousins share 12.5% of DNA etc.

prospective studies:

Usually longitudinal, and utilised when a behaviour might be genetic. The sample is selected and observed before examining behaviours. Researchers watch for outcomes.

adoption studies:

Similar to family studies, these observe the environmental influences and genetic influences in an adopted child. Selective placement: refers to how adoption agencies place children in homes that are similar to their birth parents. This is a limitation of adoption studies.

human genome project:

An international project aimed at mapping everything in the human genome. Now, scientists can map heritability of a trait using linkage analysis and look for genetic markers for a behaviour. Association studies look for correlations between genetic markers and behaviour.

diathesis stress model:

Attempts to explain a disorder as the result of an interaction between a predisposition and stress caused by life experiences.

gene expression:

The process by which the instructions in our DNA are converted into a functional product; in psychology, this is when a gene is "turned on" or "off", resulting in a behavior.

genetic vulnerability:

An underlying inherited susceptibility - that is, the theory that you may have genes that may make you more likely to have certain traits if those genes are exposed to the appropriate environmental stressors.

Caspi et al. (2003): aim

To investigate the link between the alleles of the 5-HTT gene and depression.

Caspi et al. (2003): method

Interviews and matched pairs design: group 1-two short alleles, group 2-one short and one long allele, group 3-two long alleles

Caspi et al. (2003): participants

847 participants of age 26

Caspi et al. (2003): procedure

The participants were asked to report any stressful life events that had taken place between their 21st birthday to their 26th birthday. The diagnostic interview schedule was used to assess incidences of depression over the last year. The researchers carried out correlational analyses between the following co-variables: each participant’s stressful life events and incidences of depression, the length of each participant’s alleles and incidences of depression, perceived stress and length of each participant’s alleles.

Caspi et al. (2003): results

The study found that individuals with a specific genotype (the short allele of the 5-HTT gene) reported more symptoms of depression in response to stressful life events. This suggests a genetic vulnerability to depression linked to environmental factors.

Caspi et al. (2003): findings/conclusion

Caspi et al. examined the interaction between genetic predisposition and environmental stressors in mental health. They discovered that individuals carrying one or two copies of the short allele variant of the 5-HTT gene demonstrated a higher likelihood of developing depression when exposed to stressful life events, such as loss or trauma. This research supports the diathesis-stress model, indicating that genetic factors can predispose individuals to depression, but the actual onset is contingent upon environmental influences.

Kendler et al (2006): aim

To investigate the heritability of depression in twins

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): findings/conclusion

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

Weissman et al (2005): aim

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

Weissman et al (2005): method

The original sample of parents and children were interviewed four times during this period. Data was collected from clinicians, blind to past diagnosis of depression or to data collected in previous interviews. In order to establish credibility

Weissman et al (2005): participants

Sample of 161 grandchildren and their parents and grandparents

Weissman et al (2005): procedure

The study 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. 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 the study of the third generation. Data was collected from clinicians, blind to past diagnosis of depression or to data collected in previous interviews. Children were evaluated by two experienced clinicians - a child psychiatrist and a psychologist.

Weissman et al (2005): result

The researchers found 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 mood disorders 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.