The Chi-Square and The Fly Lab

hey everyone! This week in class we learned about the chi-square and we conducted a lab that demonstrated the genetics of flies and using a chi-square to determine information about certain traits.

The Chi-square is a statistical hypothesis test that is used to examine the difference between experimental data and the data that was expected.The Chi-square is a major part of genetics. Just as the test can be used in genetics, it can also be used in everyday situations:

• difference between observed and expected data
  
• to evaluate data from experimental crosses to see if expected explanation is supported by results
  
• to test for a hypothesis to see if the difference is caused by chance alone or other factors


• test for genetic variability
  
• probability that something will occur
  

The Chi-square is also used to test a null hypothesis which is an hypothesis that may explain a given set of data. A null hypothesis is tested to determine whether the data provides reason to use an alternative hypothesis. Based on the chi-square you would be able to determine if the null hypothesis was accepted or rejected. If the null hypothesis was accepted then it means it was most likely due to chance alone. It is accepted at a p value greater than .05. If the hypothesis was rejected then there may be other factors affecting it. The null hypothesis is rejected if the p value is less than .05.

The formula for the Chi-square is:




Once the Chi-square is calculated using the formula, degrees of freedom has to be found in order to know the p-value which is used to determine if the null hypothesis is accepted or rejected. Degrees of freedom are numbers of a total value that are free to vary. The degree of freedom is always one number less than the total number of possible data. For Example, flipping a coin-there are 3 possibities-so the degrees of freedom is 2. By using a Chi-square distribution table you can make an overall conclusion based on the probability. Probablity is important because it shows the likliness of chance work alone and if you should use a different inheritance method.


LAB:


In the flylab we performed, we were able to connect and relate the chi-square to real life examples of genetics. In the lab, 11 groups were assigned 4 different traits to determine the traits of a male and female fly and their offspring.

We had to cross each trait with a wildtype and analyzed the results. Their offspring were then mated to receive the final results. A punnent square was drawn for each cross. We had to state a hypothesis that we expected and the chi-square was then calculated. Given the chi-square and the degrees of freedom we could determine if the null hypothesis was accepted or rejected.

Also for each monohybrid cross we had to determine if it was X-linked or autosomal. This was determined by the number and phenotype. If there was a great difference in the number of male and female then that would mean it was autosomal. We then had to determine if the cross was dominant or reccessive and this was determined by the phenotype of the offspring and if the numbers were close in value. Then, we had to state if it was non lethal or lethal. In your results, if a great percentage of the total number of flies died then the trait was lethal. Lastly, for the monohybrid cross, we had to state if our chi square supported the inheritance pattern which would have varying results for each group.

In the final part of the lab, a test cross or dihybrid cross was designed. The dihybrid cross includes and F1 and an F2 generation. This allowed us to determine:

• epistasis


• independent assortment


• gene linkage


• recombination frequency


• map units
  

I hoped this helped some but if you are still confused then you ask questions but going back and reading the information in the all the packets is a big help. I hope everyone has a great weekend! P.S. Happy Birthday Cathryn :) and The next sherpa is....Jon!