Deck 18: Genetics of Behavior

Question 1

Why do neurotransmitters have to be removed from the synapse?

Answer 1

During transmission of a nerve impulse, neurotransmitters are released by the presynaptic neurons. They bind to receptors on postsynaptic neurons, and bring about a change in the membrane potential of the postsynaptic neuron. Thus, the nerve impulse is conducted from one neuron to another.
However, once neurotransmitters are recognized by the postsynaptic receptors, they are released back into the synaptic cleft. Once released into synapse, they must be either removed immediately or inactivated. The removal of neurotransmitter is essential to prevent constant stimulation of the postsynaptic cell and continuous propagation of action potential.
Body has several mechanisms to prevent continuous propagation or stimulation by neurotransmitters, as follows:
• Recycling : Transporter proteins on the membranes of presynaptic cells carry the neurotransmitters back into the same cell. The chemicals are either degraded away or re-packaged into a vesicle for release
• Diffusion : Peptide neurotransmitters quickly diffuse away into the surrounding medium, once they are released back into the synapses
• Hydrolytic enzymes present at the synapses inactivate the neurotransmitters through hydrolysis

Question 2

What type of mutation causes Huntington disease? How does this mutation result in neurodegeneration?

Answer 2

Huntington disease (HD) is a neurological disease where too many glutamines are produced within a specific protein. The protein is produced by the HD gene, and Huntington disease is formed when a repeat of three nucleotides are included within the gene.
When three nucleosides of CAG are present and repeated in the HD gene, many glutamine residues are produced in the HD protein. Overexpression of glutamine residues within the HD protein is toxic to the neurological cell, and the cells that produce the mutated HD protein become toxic and die.

Question 3

Perfect pitch is the ability to identify a note when it is sounded. In a study of this behavior, perfect pitch was found to predominate in females (24 out of 35 in one group). In one group of seven families, individuals in each family had perfect pitch. In two of those families, the affected individuals included a parent and a child. In another group of three families, three or more members (up to five) of each had perfect pitch, and in all three families, two generations were involved. Given this information, what, if any, conclusions can you draw about whether this behavioral trait might be genetic? How would you test your conclusion? What further evidence would be needed to confirm your conclusion?

Answer 3

The fact that the perfect pitch trait was found across generations of the tested families suggests that this trait may be genetic. Using the permission of the families involved, we could map out the gene using genetic markers to try and isolate the location of the gene that creates the trait of perfect pitch.
If the perfect pitch gene was found, we could introduce a drug to stop its expression, and knock out the trait in individuals. This could be evidence that the perfect pitch trait originates from one gene.

Question 4

The opposite of perfect pitch is tone deafness-the inability to identify musical notes. In one study, a bimodal distribution in populations was found, with frequent segregation in families and sibling pairs. The author of the study concluded that the trait might be dominant. In a family study, segregation analysis suggested an autosomal dominant inheritance of tone deafness with imperfect penetrance. One of the pedigrees is presented here. Based on the results, do you agree with this conclusion? Could perfect pitch and tone deafness be alleles of a gene for musical ability?

Question 5

Name three genes whose mutations lead to an altered behavioral phenotype. Briefly describe the normal function of the mutated gene as well as the altered phenotype.

Question 6

Mutations in the gene encoding monoamine oxidase type A (MAOA) have been linked to aggressive and sometimes violent behavior. Based on this finding, it is conceivable that a genetic test could be developed that could identify individuals likely to exhibit such behaviors. Do you think such a test would be a good idea? What would some of the ethical and societal implications of the test be?

Question 7

What are the advantages of using Drosophila in the study of behavior genetics? Can this organism serve as a model for human behavior genetics? Why or why not?

Question 8

You are a researcher studying an autosomal dominant neurodegenerative disorder. You have cloned the gene underlying the disorder and have found that it encodes an enzyme that is overexpressed in the neurons of individuals who have the disorder. To better understand how this enzyme causes neurodegeneration in humans, you make a strain of transgenic Drosophila whose nerve cells overexpress the enzyme.
a. How might you use these transgenic flies to try to gain insight into the disease or identify drugs that might be useful in the treatment of the disease?
b. Can you think of any potential limitations of this approach?

Question 9

Rachel asked to see a genetic counselor because she was concerned about developing schizophrenia. Her mother and maternal grandmother both had schizophrenia and were institutionalized for most of their adult lives. Rachel's three maternal aunts are all in their 60s and have not shown any signs of this disease. Rachel's father is alive and healthy, and his family history does not suggest any behavioral or genetic conditions. The genetic counselor discussed the multifactorial nature of schizophrenia and explained that many candidate genes have been identified that may be mutated in individuals with the condition. However, a genetic test is not available for presymptomatic testing. The counselor explained that based on Rachel's family history and her relatedness to individuals who have schizophrenia, her risk of developing it is approximately 13%. If an altered gene is in the family and her mother carries the gene, Rachel has a 50% chance of inheriting it.
Why do you think it has been so difficult to identify genes underlying schizophrenia?

Question 10

Two common behavioral disorders are schizophrenia and bipolar disorder. What are the essential differences and similarities between these disorders?

Question 11

Is Going to Medical School a Genetic Trait?
Many behavioral traits follow a familial, if not Mendelian, pattern of inheritance. This observation, along with twin and adoption studies, reveals a strong genetic component in complex behavioral disorders. In most cases, these phenotypes are not inherited as simple Mendelian traits. Researchers thus are faced with the task of selecting a model to describe how a behavioral trait is inherited. Using this model, further choices are made to select the methods used in genetic analysis of the trait. A common strategy is to find a family in which the behavior appears to be inherited as a recessive or an incompletely penetrant dominant trait controlled by a single gene. Molecular markers are then used in linkage analysis to identify the chromosome that carries the gene controlling the trait.
If researchers are looking for a single gene when the trait is controlled by more than one gene or genes that strongly interact with the environment, the work may produce negative results, even though preliminary findings can be encouraging. To illustrate some of the pitfalls associated with model selection in behavior genetics, researchers deliberately selected attendance at medical school as a behavioral trait and then determined if the inheritance of this trait in families is consistent with a genetic model. They surveyed 249 first- and second-year medical students. Thirteen percent had first-degree relatives (parents, siblings, or children) who also had attended medical school, compared with 0.22% of individuals in the general population with such relatives. Thus, the overall risk factor among first-degree relatives for medicalschool attendance was 61 times higher for medical students than for the general population, indicating a strong familial pattern. To see if selecting the right model would show this behavior as an inherited trait, researchers used statistical analysis that supported inheritance and rejected the model of no inheritance. Pedigree analysis supported a single-gene recessive pattern of inheritance, although other models, including polygenic inheritance, were not excluded. Using a further set of statistical tests, the researchers concluded that the recessive inheritance of this trait was just at the border of statistical acceptance.
Similar results of borderline statistical significance are often found in studies of other behavioral traits, and it is usually argued that another, larger study would confirm the results. Although it is true that genetic factors may partly determine whether one will attend medical school, it is highly unlikely that a single recessive gene controls this decision, regardless of the outcome from this family study and segregation analysis of the results.
The authors of this study were not serious in their claims that a decision to attend medical school is a genetic trait, nor did they intend to cast doubts on the methods used in the genetic analysis of behavior. Rather, their work was intended to point out the folly of accepting simple, single-gene explanations for complex behavioral traits.
What role would twin and adoption studies play in selecting a model for determining whether attendance at medical school is a heritable trait? What heritability values would make you take the study to the next level?

Question 12

A pedigree analysis was performed on the family of a man with schizophrenia. Based on the known concordance statistics, would his MZ twin be at high risk for the disease? Would the twin's risk decrease if he were raised in an environment different from that of his schizophrenic brother?

Question 13

What are the major differences in the methods used to study the behavior genetics of single-gene traits versus polygenic traits?

Question 14

You are a researcher studying bipolar disorder. Your results from genomic SNP analysis show linkage between SNP haplotypes on chromosome 7 and bipolar illness. Later in the study, you find that a number of individuals do not carry this SNP haplotype but still develop the disease. Does this mean that bipolar disorder is not genetic?

Question 15

Rachel asked to see a genetic counselor because she was concerned about developing schizophrenia. Her mother and maternal grandmother both had schizophrenia and were institutionalized for most of their adult lives. Rachel's three maternal aunts are all in their 60s and have not shown any signs of this disease. Rachel's father is alive and healthy, and his family history does not suggest any behavioral or genetic conditions. The genetic counselor discussed the multifactorial nature of schizophrenia and explained that many candidate genes have been identified that may be mutated in individuals with the condition. However, a genetic test is not available for presymptomatic testing. The counselor explained that based on Rachel's family history and her relatedness to individuals who have schizophrenia, her risk of developing it is approximately 13%. If an altered gene is in the family and her mother carries the gene, Rachel has a 50% chance of inheriting it.
If a test were available that could tell you whether you were likely to develop a disorder such as schizophrenia later in life, would you take the test? Why or why not?

Question 16

A region on chromosome 6 has been linked to schizophrenia, but researchers have not found a specific gene associated with this disease. What steps would be necessary to locate the gene?

Question 17

Is Going to Medical School a Genetic Trait?
Many behavioral traits follow a familial, if not Mendelian, pattern of inheritance. This observation, along with twin and adoption studies, reveals a strong genetic component in complex behavioral disorders. In most cases, these phenotypes are not inherited as simple Mendelian traits. Researchers thus are faced with the task of selecting a model to describe how a behavioral trait is inherited. Using this model, further choices are made to select the methods used in genetic analysis of the trait. A common strategy is to find a family in which the behavior appears to be inherited as a recessive or an incompletely penetrant dominant trait controlled by a single gene. Molecular markers are then used in linkage analysis to identify the chromosome that carries the gene controlling the trait.
If researchers are looking for a single gene when the trait is controlled by more than one gene or genes that strongly interact with the environment, the work may produce negative results, even though preliminary findings can be encouraging. To illustrate some of the pitfalls associated with model selection in behavior genetics, researchers deliberately selected attendance at medical school as a behavioral trait and then determined if the inheritance of this trait in families is consistent with a genetic model. They surveyed 249 first- and second-year medical students. Thirteen percent had first-degree relatives (parents, siblings, or children) who also had attended medical school, compared with 0.22% of individuals in the general population with such relatives. Thus, the overall risk factor among first-degree relatives for medicalschool attendance was 61 times higher for medical students than for the general population, indicating a strong familial pattern. To see if selecting the right model would show this behavior as an inherited trait, researchers used statistical analysis that supported inheritance and rejected the model of no inheritance. Pedigree analysis supported a single-gene recessive pattern of inheritance, although other models, including polygenic inheritance, were not excluded. Using a further set of statistical tests, the researchers concluded that the recessive inheritance of this trait was just at the border of statistical acceptance.
Similar results of borderline statistical significance are often found in studies of other behavioral traits, and it is usually argued that another, larger study would confirm the results. Although it is true that genetic factors may partly determine whether one will attend medical school, it is highly unlikely that a single recessive gene controls this decision, regardless of the outcome from this family study and segregation analysis of the results.
The authors of this study were not serious in their claims that a decision to attend medical school is a genetic trait, nor did they intend to cast doubts on the methods used in the genetic analysis of behavior. Rather, their work was intended to point out the folly of accepting simple, single-gene explanations for complex behavioral traits.
Three generations of the Boone family have been major league baseball players (and All-Stars): Ray, his son, Bob, and grandsons Bret and Aaron. A baseball fan claims this is evidence for the role of genetics in athletic ability. How would you formulate an answer to this, and if asked, how would you approach studying this behavior?

Question 18

Of the following findings, which does not support the idea that alcoholism is genetic?

A) Some strains of mice select alcohol over water 75% of the time, whereas others shun alcohol.
B) The concordance value is 55% for MZ twins and 28% for DZ twins.
C) Biological sons of alcoholic men who have been adopted have a rate of alcoholism more like that of their adoptive fathers.
D) There is a 20% to 25% risk of alcoholism in the sons of alcoholic men.
E) None of these.

Question 19

In human behavior genetics, why is it important that the trait under study be defined accurately?

Question 20

A woman diagnosed with Alzheimer disease wants to know the probability that her children will inherit the disorder. Explain to her the complications of determining heritability for this disease.

Question 21

One of the models for behavioral traits in humans involves a form of interaction known as epistasis. What is epistasis?

Question 22

In July 1996, The Independent , a popular newspaper published in London, England, reported a study conducted by Dr. Aikarakudy Alias, a psychiatrist who had been working on the relationship between body hair and intelligence for 22 years. Dr. Alias told the 8th Congress of the Association of European Psychiatrists that hairy chests are more likely to be found among the most intelligent and highly educated than in the general population. According to this new research, excessive body hair could also mean higher intelligence. Is correlating body hair with intelligence a valid method for studying the genetics of intelligence? Why or why not? What factors are known to contribute to intelligence? Is it logical to assume that individuals with little or no body hair are consistently less intelligent than their hairy counterparts? What type of study could be done to prove or disprove this idea?

Question 23

List the steps involved in transmitting a nerve impulse from one neuron to another.