Topic 7—A: genetics- 6. The chi-squared test

Question 1

What is the chi squared test?

Answer 1

• its a statistical test that’s used to see if the results of an experiment support a theory

Question 2

What is the expected result?

Answer 2

Theory is used to predict a result

Question 3

What is the observed result?

Answer 3

Experiment is carried out and the actual result is recorded

Question 4

To see if the results support the theory what do you have to make?

Answer 4

A hypothesis called a null hypothesis

Question 5

What is a null hypothesis?

Answer 5

• it’s always that there is no significant difference between the observed and expected results
• Your experimental result will usually be a bit different from what you expect but you need to know if the difference is due to chance or because your theory is wrong

Question 6

Why is the x squared test then carried out?

Answer 6

• to compare the goodness of fit of the observed and expected results
• the outcome either supports or rejects the null hypothesis

Question 7

When can you use the x squared test

Answer 7

• in genetics to test theories about the inheritance of characteristics

Question 8

Formula of chi squared (x2)

Answer 8

X^2= sum of (O-E)^2\ E
O= observed result
E= expected result

Question 9

How do you work put expected result?

Answer 9

Total number of offspring/ ratio total x predicted ratio

Question 10

What results are used to work out x^2?

Answer 10

Observed result
- O-E is calculated for each phenotype (expected result is substrates from the observed result)
- resulting numbers are squared
- these figures are divided by the expected results
- the numbers are added together to get x^2

Question 11

How do you determine whether there is a significant difference between the observed and expected results from an experiment?

Answer 11

• You need to compare the x^2 value to a critical value
• the critical value is the value of x^2 that corresponds to a 0.05 (5%) level of probability that the difference between the observed and expected results is due to chance

Question 12

How is the degree of freedom worked out?

Answer 12

Number of phenotypes - 1

Question 13

If your x^2 value is larger than (or equal to) the critical value what does this mean?

Answer 13

There is a significant difference between the observed and expected results i.e. something other than chance is causing the difference
- null hypothesis can be rejected

Question 14

If your x^ value is smaller than the critical value what does this mean?

Answer 14

• there is no significant difference between the observed and expected results
• Null hypothesis can’t be rejected