This document will provide you with some of the knowledge and resources required to generate
informed hypotheses regarding the patterns of inheritance you will observe from the fruit fly crosses
you have created
Recall, we learned that genes are typically inherited in one three different patterns, which we refer to
as patterns of inheritance:
1. Independent assortment
2. Genetic linkage
3. Sex Linkage
Let’s review, briefly what causes theses patterns of inheritance so we can begin to understand how we
would expect different phenotypes to be passed on in our fruit fly lines.
Independent assortment is a pattern of inheritance in which the alleles, or separate copies of a gene
inherited from each parent, are inherited independently of alleles from other genes. What this means
in simpler terms is that the inheritance of one gene has no affect on the inheritance of another gene.
We would expect this pattern of inheritance to occur when the genes in question are located on
different somatic, or non-sex, chromosomes. Here is a diagram illustrating this concept:
Genetic linkage is another pattern of inheritance in which the alleles are more likely to be inherited
together than would be expected via the law of independent assortment. What this means is that
certain phenotypes/genotypes will be much more common or rare than what Punnet square analysis
would predict to be the case. This is how we recognize cases of genetic linkage. We would expect
genetic linkage to occur to when genes are located close together on the same chromosome. Below
are several helpful diagrams.
The final pattern of inheritance that we will discuss is known as sex linkage. Sex linkage is a pattern
of inheritance where a trait is expressed more frequently or exclusively by members of a certain sex
within a species. We would expect to observe this pattern of inheritance when a trait is encoded for by
a gene located on a sex chromosome (one responsible for determining biological sex). If we expect a
gene to show this pattern of inheritance would expect it to be disproportionately expressed in one sex
over the other. Below you will again find a helpful diagram to further explain this concept.
You may now be wondering how you are supposed to come up with hypotheses for what pattern of
inheritance you would expect to observe with each trait in your mutant lines. It may already be
intuitively obvious to you that in order to make predictions about this we must first know where these
genes lie within the fruit fly genome, particularly on which chromosome they lie. Fortunately, because
D. melanogaster is a model organisms there is a wealth of resources to help you answer this question.
I will show you how to answer this question using the website FlyBase by searching for the location
of a trait we are not working with (ebony body) within the genome of D. melanogaster. I have also
provided a copy of the D. melanogaster karyotype (chromosome diagram) as a reference.
1) Click on the hyperlink above to take you to the FlyBase home page. Alternatively you can
manually enter the website “http://flybase.org/#/” into your browser. It will take you to the page
By following the link listed on the page above you should be taken to a website that looks the same as
the image above. Note that you are not required by the website to pay a yearly access fee to utilize its
services. Scroll down the page until you reach a feature on the page called “QuickSearch”
Once you have reached the “QuickSearch” feature click on the tab labeled “Data Class”. From there
you can enter the name of your gene of interest (which may have to be googled beforehand) into the
text bar indicated by the red arrow and click on the search button
Once you hit the search button you will be taken to a results screen that will look like the picture
above. Your gene of interest should be listed as the first or second result. If not, you have probably not
searched for the proper annotation of that gene. Once you have located a result you believe to be the
correct gene, click on the “Gene Snapshot” feature and read the short description of the gene that will
pop up. This will help you to confirm is this is in fact the gene you are looking for. Once you are sure
the gene you are looking at is the gene you want click on that symbol representing that gene located
within the red box (indicated by a red arrow)
Now that you have successfully located your gene of interest, you can confirm what chromosome this
gene is located on. Ultimately this will allow you to hypothesize the pattern of inheritance you expect
to observe for each gene trait. The picture above is the what screen should appear once you click on
the red box from the last step. First make sure the species listed for this gene is in fact D.
melanogaster. If it is, you will want to locate the box labeled “sequence location” (indicated with a
red arrow). The first two characters in the sequence locations will tell you what chromosome the
sequence is located on. In this case they are 3R, so I know the gene encoding for an ebony body is
located on 3rd right chromosome. This technique can be used to determine the location of every gene
from the mutant fly crosses. It will be up to you to form your hypotheses from this data. Don’t forget
to cite FlyBase as a source if you use this method in developing your hypotheses!
D. Melanogaster chromosome arrangement
Images sourced from