End of course questions
Explain your reasons for agreeing or disagreeing with each of the following sentences.
(a) If a disease has a genetic basis and someone has the (abnormal) alleles for the disease, then that person will have the symptoms of the disease.
(b) If you have the genes for aggression, then you are an aggressive person.
(c) ‘His father was a good footballer too. It must be genetic.’
(a) Whilst this may be the case for some genetic disorders (e.g. lissencephaly) it is not the case for all. Wilson's disease and phenylketonurea, for example, can be symptomless provided copper and phenylalanine intake, respectively, are controlled.
(b) The wording of the sentence makes two assumptions: first it assumes that there are genes which affect only aggression, and second it assumes that if someone has those genes, they will necessarily be aggressive. Neither of these assumptions is correct.
(c) If a son has good footballing skills like his father then the most that can be said is that the footballing skills are familial. The skills might have a genetic basis, an environmental basis, or both.
Use named examples to show how (a) a gene can directly influence nervous system development and (b) an environmental factor can directly influence nervous system development.
(a) Examples of alleles affecting the nervous system include sli, LIS1, FMR1.
(b) Examples of environmental factors affecting the nervous system include light, testosterone and retinoic acid.
For each of the following statements about the effect of kangaroo care being evaluated after six months, explain why it is true or false.
(a) Six months was the minimum time necessary to demonstrate an effect.
(b) Six months was sufficient time to demonstrate an effect.
(c) If there is an effect after 6 months, then the effect is permanent.
(d) If there is no effect by 6 months, then there is no effect.
(a) The statement is false because some effects may have been evident immediately or soon after kangaroo care finished (e.g. heart rate or responsiveness to physical contact), i.e. before the babies were six months old.
(b) The statement is true, as an effect was demonstrated.
(c) The statement is false; from the information provided, there is no way of knowing whether any effects would be evident at any later time. The only way to find out would be to examine the participants at some later time.
(d) The statement is false; noticeable differences could appear after 9 months or after ten years.
Just as there are different breeds of dog, so there are different strains of rat. To reduce genetic variation, the strains are inbred, meaning simply that offspring are produced only by animals of the same strain. These strains have been bred in captivity for many years and each has particular characteristics. For example, the Fisher 344 strain is highly responsive to novelty and readily secretes large quantities of certain hormones in response to stress (e.g. being restrained). In comparison, Long-Evans rats are considerably less responsive to novelty and secrete far less hormone in response to the same stressor.
(a) Are the differences in characters between the strains familial?
(b) Why might the differences mentioned be thought to have a genetic basis?
(c) What other explanation might there be for the persistence of the differences?
(a) Yes the differences persist through generations and so are familial.
(b) The differences mentioned might be thought to have a genetic basis because the whole point of inbred strains of animals is that they are genetically uniform and genetically different from other strains. Characters that differ between strains are therefore often thought to arise because of the genetic differences between strains.
(c) An alternative explanation for the persistence of the differences might be that differences in maternal behaviour affect the characters in question and affect maternal behaviour. So, for example, any female rat pup reared by an attentive mother rat might become an attentive mother rat in her turn.
What technique would you use to find out whether the differences mentioned in Question 4 were genetic in origin?
The technique to use would be cross fostering.
If a disease has a genetic basis and someone has the (abnormal) alleles for the disease, then that person will have the symptoms of the disease. Give an example where this is the case and an example where this is not the case.
An example where it is the case that someone with the (abnormal) allele(s) for a disease with a genetic basis has the symptoms of the disease is Huntington's disease. Alternatively, you may have mentioned lissencephaly or fragile X syndrome.
An example where it is not the case that someone with the (abnormal) alleles for a disease with a genetic basis has the symptoms of the disease is Wilson's disease. Alternatively, you may have mentioned phenylketonurea.
The following quotation, taken from a leader in Science by Constance Holden, is an accurate summary of part of the results presented in a paper (Caspi et al., 2003) in that issue. Read the quotation and then choose, from the list below, the two correct statements about the results described.
‘Looking back on their records of childhood abuse for the cohort, the researchers found an additional link between 5-HTT gene variants (s-allele and 1-allele ) and depression: Abuse as a child predicted depression after the age of 18 only in people carrying at least one s-allele. Among the 11% who had experienced severe maltreatment, the double s-allele subjects ran a 63% risk of a major depressive episode. The 1-allele participants averaged a 30% risk, regardless of whether they had been abused as children.’
A This result demonstrates that abuse as a child causes depression.
B This result demonstrates that the 5-HTT gene causes depression.
C This result demonstrates a gene/environment interaction.
D This result demonstrates that the s-allele of 5-HTT causes depression.
E This result demonstrates that people with the 1-allele will not suffer from depression.
F This result demonstrates that depression is more likely to occur in people who have the s-allele for 5-HTT and who experienced abuse as a child.
The two correct statements are C and F.
The gene in question is for a chemical transporter called 5 CHTT (5 CHT transporter) that fine-tunes transmission of serotonin, the neurotransmitter affected by the antidepressant Prozac amongst others. The gene comes in two common versions: the long (l) allele and the short (s) allele.
What is the function of transcription factors and why are they important?
Transcription factors are essential to gene transcription; without them mRNA cannot be produced nor proteins synthesized. (Note: there are many transcription factors, each controlling a specific gene or genes.)
Comment on the statement that ‘all growing axons make straight for their target(s) without hesitation or deviation’.
There is abundant evidence that axons do not grow straight to their targets. They constantly extend and retract branches (filopodia), ‘searching’ for the appropriate substrate or chemical cues along which to grow. Thus the movement of a growth cone is hesitant. They also deviate around other cells and and they do not grow through tissue that repels them or is too dense.
Chemotropic and chemotactic factors affect the growth cone. What is the principal difference between chemotropic and chemotactic factors in how they do so?
Chemotropic factors diffuse away from their source (e.g. the target) through the intervening tissue and affect the growth cone at a distance from their source (i.e. distally). Chemotactic factors are attached to the substrate (e.g. other cells) and affect the growth cones when the growth cones contact the source (i.e. proximally).