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# Determining the mode of inheritance and gene linkage for drosophila crosses...

7. Conclusion and discussion sections:

a. how many genes are involved in creating the eye pigment
(based on the provided data). Mode of inheritance - are the genes
autosomal or sex-linked and why, recessive or dominant and why,
and are some genes linked and why.

b. Hypothesis statement based on conclusions about linkage from
previous step (7a), and chi square test to accept or reject the hypothesis.

c. You should include punnett squares with genotypes to all generations
of all 4 crosses!

I have connected the data table that we were given, and I have also given my
explanation for crosses 1 and 3. However, I have no idea how 2 and 4 are linked,
i have never seen a 14:1:1:4 ration before.

#### Solution Preview

OK. This is a bit complicated, so try and stay with me. This explanation will start out easy and get more complicated as we go. Read it slowly and carefully, making sure you understand as you go.

As you wrote, experiments 1 and 3 show conclusively that the gene in question in those experiments is located on the X chromosome. That particular gene has two alleles, one dominant allele which leads to a wt phenotype, and one recessive allele which leads to a white phenotype. So, that 1 gene (with 2 alleles) is X-linked (or sex-linked). It is not an autosomal gene.

So far, we've got 1 gene involved in eye color.

However, when we look at experiments 2 and 4, we see something weird going on. What can we learn? First of all, the results for experiments 2 and 4 are identical. The reciprocal crosses gave the same results. So, think. What does that tell us? It tells us that this gene (or genes) being investigated in these experiments must be autosomal. They cannot be located on the X-chromosome. That means that we now know that there are at least two genes involved -- the X-linked gene discovered in experiments 1 and 3, and at least one other gene on one of the autosomal chromosomes, discovered in experiments 2 and 4.

Still with me?

But, notice something else. Experiments 2 and 4 are shown with crosses between wt flies and "white" flies. They are not normal white-eyed flies although they have white eyes. That's why the "white" is in quotation marks. It's a flag to tell you to be on guard. Be alert! There's something more going on here than meets the eye!

In fact, if we look at the F2 from these crosses, we find two new eye colour types appearing out of nowhere. That should confirm to us that something unusual is indeed happening (as you figured out anyway). But, the question is: What's going on?

There is no such ...

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