HL Bio - Unit 2: Gene Linkage and Chi-Squared test

Autosomal Gene Linkage

Genes located (locus) near each other on the same chromosome tend to be inherited together in genetic crosses - They would deviate from Mendel’s theory (will not get the 9:3;3:1 ratio)

Representation of gene linkage

• Two vertical lines represent homologous chromosomes to show gene linkage between alleles and how they are inherited together 

  • So when the 9:3:3:1 ratio isn’t achieved assume the gene is linked

Recombination

• Crossing over can cause recombinations within the gametes

• Unlinked genes will have gametes with 50% of the parents phenotypes and then 50% of the parents non expressed phenotypes 

Recombinants

Genotypes expressing a new combination than observed by the parents

Chi-Squared Test:

used to determine if there is a good fit between the expected ratios and what was actually obtained in a genetics experiment - is it statistically significant 

Null hypothesis (H_o)

• There is not a significant difference between the observed and expected results. Any variation between what is expected and what is observed could be due to chance alone.

  • assumes that any kind of difference between the chosen characteristics that you see in a set of data is due to chance

  • the traits fit the predicted ratio

Alternative hypothesis (H₁):

• There is a significant difference between the observed and expected results. Something more than chance is likely causing the difference between observed and expected results.

  • Meaning that the null hypothesis is wrong

  • the traits do not fit the predicted ratio.

Chi-Squared test Steps

1. Determine the null and alternative hypothesis 

2. Determine the expected ratios

3. Determine the expected results based on ratios using the total 

4. Determine the difference between the observed and the expected (O-E) (negative numbers are allowed)

5. Then find the difference squared 

6. Last divide the difference squared by the expected result 

7. Add up all difference squared over expected results together to get chi-squared 

8. Once chi-square has been calculated we must determine the degrees of freedom (DF)

   • Degrees of freedom is determine db the number of independent pieces of information we used to calculate the statistic minus 1

9. Then use DF to determine the critical value from the table (usually for biology the standard is 0.05)

When would a null hypothesis be accepted

• The null hypothesis would be accepted if it was 5% within the expected results if not the alternative hypothesis is accepted