Design an experiment to test the role of thyroxine in helping the body adjust to cold temperatures. Describe the overall investigation to be done, the variables to be changed, the variables to be kept constant, the subjects to be used, the control and experimental conditions to be used, the type of data collected, and how the data will be collected. Explain how these data can be used to justify the conclusion that thyroxine is involved in adjusting body temperature in response to cold. (9 points)

- Hypothesis: thyroxine gene is responsive to low temperatures, thereby its transcript level is higher in cold climates

- Subjects of study: gene expression data collected from individuals from countries with different climates  

- Experiment: to determine thyroxin gene expression level by Real-Time PCR (RT-PCR) in individuals living in cold climates (e.g., Iceland) versus individuals living in warmer climates (e.g., Puerto Rico)

- Independent variable: temperature

- Dependent variable: thyroxin transcript level  

- N sample: 500 individuals of each population  

- Control group: individuals living in the same climate conditions but from different countries. It can be done by collecting gene expression data from individuals living in another country. For example, both Fairbanks (Alaska) in the USA and Reykjavik in Iceland are at 64 degrees N latitude, and thereby these cities have similar climates

- Constant variable: all data collected from individuals of adult age (age range: 20-30 years)  

Explanation:

The scientific method is a sequential process that consists of making hypotheses in order to explain observations from the real world, make predictions based on the working hypotheses, carrying out experiments based on those predictions, test the predictions and draw conclusions based on the results obtained. In an experiment, the independent variable isn't changed by the other variables, while the dependent variable is the variable being measured in the experiment. A constant variable does not change during the experiment. Moreover, a control group is a particular group to which comparisons are made during the experiment (the control group isn't exposed to the independent variable, and therefore any change in the experimental groups can be compared with the control). In this case, it is expected to measure thyroxine expression levels in 500 individuals from each country by RT-PCR (RT-PCR technique is used to measure the amount of mRNA of a target gene of interest). Subsequently, it is possible to determine if the differences in the thyroxin gene expression levels between populations (i.e., individuals living in Iceland vs individuals living in Puerto Rico) are statistically significant to confirm that thyroxin gene activity is increased in response to cold exposure. Finally, the control group can be used to determine if observed differences in gene expression are irrefutably correlated with cold conditions.