Biology Forums - Study Force

Are two genes linked or unlinked?
Genes that are in close proximity on the same chromosome will result in the linked alleles being inherited together more often than not. But how can you tell if certain alleles are inherited together due to linkage or due to chance?
If genes are unlinked and therefore assort independently, the phenotypic ratio of offspring from an F1 testcross is expected to be 1:1:1:1. If the two genes are linked, however, the observed phenotypic ratio of the offspring will not match the expected ratio.
Given random fluctuations in the data, how much must the observed numbers deviate from the expected numbers for us to conclude that the genes are not assorting independently but may instead be linked? To answer this question, scientists use a statistical test called a chi-square (χ2) test. This test compares an observed data set to an expected data set predicted by a hypothesis (here, that the genes are unlinked) and measures the discrepancy between the two, thus determining the “goodness of fit.”
If the difference between the observed and expected data sets is so large that it is unlikely to have occurred by random fluctuation, we say there is statistically significant evidence against the hypothesis (or, more specifically, evidence for the genes being linked). If the difference is small, then our observations are well explained by random variation alone. In this case, we say the observed data are consistent with our hypothesis, or that the difference is statistically insignificant. Note, however, that consistency with our hypothesis is not the same as proof of our hypothesis.

The χ2 value means nothing on its own--it is used to find the probability that, assuming the hypothesis is true, the observed data set could have resulted from random fluctuations. A low probability suggests the observed data is not consistent with the hypothesis, and thus the hypothesis should be rejected.
What is the hypothesis that you are testing?

Answer Choices:
A)The two genes are unlinked and are assorting independently, leading to a 1:1:1:1 ratio of phenotypes in the offspring.
B)The two genes are linked and are assorting together, leading to a 1:1:1:1 ratio of phenotypes in the offspring.
C)The two genes are linked and are assorting together, leading to a ratio of phenotypes in the offspring that deviates significantly from 1:1:1:1.
D)The two genes are unlinked and are assorting independently, leading to a 1:1:0:0 ratio of phenotypes in the offspring.

Hi, welcome to the forum:

Part C: Correct option: a. The two genes are unlinked and are assorting independently, leading to a 1:1:1:1 ratio of phenotypes in the offspring.

From the number of offspring given in the table, option 1 fulfills 1:1:1:1 phenotype ration as mentioned in the expected phenotype values. 1:1:1:1 phenotype ration can be obtained only if the genes are unlinked and assort independently.

Hence, option A is the most appropriate one.

Note: 1. As mentioned in the question X2means nothing on its own; it is the p value (probability) that let you decide to accept or reject the null hypothesis.

Assuming the 95% CI, the p value obtained is, p = 0.7832

For option A: p value is insignificant (is less than 0.95 i.e. 95%), thus the null hypothesis is rejected. It means that any one or all of the three criteria (underlined words/ phrases) mentioned in the ‘null hypothesis’ is not valid for the given case.

It further means that one or more point in the null hypothesis is NOT valid- i.e.

                I. genes are NOT unlinked

                II. genes are NOT assorted independently

                III. phenotype ration is NOT 1:1:1:1

TY :)