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SAQ marking: Genetics

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  • SAQ marking: Genetics

Below you will find four sample SAQs for the question: Explain one study that investigates the role of one or more genes in one human behaviour.

For each of the samples, refer to the rubric to award marks 1 - 9. After each sample, there is a predicted grade as well as feedback on the strengths and limitations of the sample.

SAQ rubric

SAQ Sample 1

One study of the role of genetics in depression is the study by Kendler of monozygotic (MZ) and dizygotic (DZ) twins. Researchers study MZ twins because they have identical DNA, having come from a single fertilized egg (zygote).  DZ twins are from two different fertilized eggs. They are born at the same time but their DNA is as different as any other set of siblings.  Psychologists argue that if the concordance rate of MZ twins for a behaviour is significantly higher than the concordance rate for DZ twins, then there is a genetic component to the behaviour. Psychologists also know that although one may have a certain genetic makeup (genotype), not all of the genes that are inherited may be expressed. The idea of gene expression is that sometimes an individual may have a predisposition to a behaviour as a result of inheriting the gene from a parent, but until a stressor from the environment causes the gene to be expressed, the person will not show that behaviour. Hence, genes alone cannot cause a behaviour - but it is the interaction of genes and the environment that leads to behaviour.

Kendler carried out a study of 42000 MZ and DZ twins to find out whether depression might be inherited.  He predicted that the MZ twins would have a greater concordance rate for depression than DZ twins. The researchers found that the MZ had a concordance rate of 0.44, whereas the concordance rate for the DZ twins was only about 0.17. However, what was interesting to the researchers was that even though MZ twins shared the same genotype, their concordance rate was not 100%. 

The difference in concordance rates indicates that depression may be inherited. The study also supports the theory of gene expression.  Even though the MZ twins had exactly the same genes, they had different life experiences and so would have a different epigenome - that is, different genes would be expressed.  This could potentially explain why the concordance rate is not 100%.

334 words

The student demonstrates a good understanding of genetic inheritance. The study is relevant, well described and linked back to the question. 9 marks.

SAQ Sample 2

Based on the Human Genome Project and gene mapping, research suggests that certain genes are responsible for certain behaviours and disorders. A study of the 5HTT gene has shown a correlation between the gene and depressive symptoms. A study done by Caspi looked at the role of this gene on participants and attempted to draw a correlation between the gene and depression. The study consisted of the researchers identifying participants with the gene and seeing whether it made them depressed.

The study was an observation as no IV was manipulated by the researchers and the participants were not randomly allocated to any conditions.  However, results showed that there was a correlation between the gene and depression. Participants that had the 5HTT genes displayed signs of depression.  The researchers concluded that participants who inherited the gene had a higher likelihood of becoming depressed.

The study demonstrated the role of the 5HTT gene.  This further adds information to gene mapping and the role of genetics in behaviour.

165 words

The response demonstrates some understanding of the role of genetic inheritance on behaviour. The actual role of the 5HTT gene - that is, a serotonin transporter gene that regulates reuptake of serotonin in the synapse - is not mentioned.  This would be an important link to the behaviour of depression, assuming that serotonin deficit plays a role in depression. The study could be considered a natural experiment (born with a mutation and the life events are not manipulated by the researcher), but it is not an observation. The procedure and the results are not well described. Everyone has a 5HTT gene, it is the short-allele mutation that was actually studied. 3 marks

SAQ Sample 3

One example of the role of genetics in behaviour is the role of genetic mutation in human depression, as shown in a study by Caspi et al. 

The aim of the study done by Caspi et al was to determine if there is evidence of a gene-environment interaction for a mutation of the serotonin transporter gene. This gene is involved in the reuptake of serotonin from the synapses in the brain. Psychologists believe that serotonin plays a role in mood and thus also has a role in depression. The gene is the 5HTT gene. The short allele of the 5HTT gene is a mutation of the long allele, which is the norm.  Caspi wanted to test if people who inherited the two-short-allele version of the gene would have a higher chance of developing depression after stressful life events, than those that had the normal two long alleles of the 5HTT gene.

The study was longitudinal and included a sample of 800 26-year old New Zealanders. They were split into three groups. In the first group were the people with 2 short alleles, in the second group were those with one short and one long allele and the final group had two long alleles. The participants were assessed for depression and asked to complete a "Stressful life events" questionnaire. 

The results showed that people with two short alleles and three or more stressful life events had a higher chance of developing depression.  The study reveals that the mutation of the serotonin transporter gene does play a role in human depression.

260 words

The study is well described, but the findings and the link back to the question could be more developed. 6 marks

SAQ Sample 4

Genetics plays a role in many human behaviours. Genes have evolved over time, so psychologists argue that natural selection can be used to explain human behaviour. One such behaviour is mating behaviour.

One study of the role of genes in behaviour was carried out by Wedekind.  It is called the "Sweaty T-shirt" study. In this study, a sample of university students was used to see if the smell of a person's sweaty t-shirt would be attractive.  The study was based on the study of MHC - major histocompatibility complex.  This is a set of proteins that are the building blocks of the immune system.  MHC is inherited from both parents and codominantly expressed. Wedekind argues that having a more diverse set of MHC increases the chance of the offspring surviving.  Therefore, it is important for a person to choose a mate with a different MHC to their own.  MHC affects the smell of our sweat.

Wedekind had men with different MHC sleep in a t-shirt for four nights.  They were instructed not to do anything to change their body odour - for example, no spicy food, no perfumes and no sex.  They were also told to use a special unscented soap for showering. The t-shirts were then put into plastic tubes and the women were asked to smell the shirts and rank them for "pleasantness." The study was a double-blind experiment. 

The researchers found that the women ranked the MHC combination that was most different from their own as the most pleasant, supporting the idea that MHC plays a role in how we choose a partner.

266 words

This response is problematic.  It is not a study of "genetic inheritance."  Instead, it is a study of evolutionary theory.  Evolutionary theorists base their arguments on the assumption that behaviour is inherited.  They do not actually study the inheritance of behaviour.  What is inherited in this study is the immune system - not the mating behaviour.  This response would earn no marks.