Posted by: arrogantscientist | January 12, 2009

Balancer Chromosomes –

As a running commentary to some pictures, I thought I would give some information on why Dmel is such an amazing genetic tool.

In this post I will introduce Balancer Chromosomes, and why they are so important in genetics. To maintain picture-mediated interest, here’s a picture:


Heterozygous FM7i females (FM7i/X, above) have heart shaped, red eyes. FM7i/FM7i females and FM7i/Y males have bar shaped red eyes.

If you look at the wild-type fly a couple of posts down, you’ll notice the fly above has different shaped eyes. This change in eye shape is a dominant mutation in the gene Bar, and is the most prominent marker of first chromosome balancer, FM7, in adult flies.

Dmel have four chromosomes, one (X/Y), two and three being important, four being rather small. There are two copies of each per normal cell. Females have two X chromosomes (X/X), males have one X, one Y (X/Y). A fly with a wild-type genome would be expressed as +/Y ; +/+ ; +/+ ; +/+ on paper (if the fly was male). + just indicates a wild-type chromosome, in the order, 1, 2 ,3, 4.

During meiosis, i.e during the production of eggs and sperm (gametes), two processes ensure no two gametes are the same.
The first is how the chromosomes line up and segregate – one of each chromosome goes to each gamete, but whether X or Y ends up with 2a or 2b, 3a or 3b and 4a or 4b is random, but predictable.
The second process, chromosome crossover, is more subtle and unpredictable. While the chromatids are lined up together, it is possible for sections to swap places with each other. This is particularly an issue if your gene of interest were to move to the other chromatid, when all your predictions assume it doesn’t.

Balancer chromosomes typically contain multiple inverted sections and one or more marker genes. The inverted regions entirely prevent crossover, and if it were to occur, the markers splitting apart would highlight there’s a problem. The markers also make it very clear where the the balancer is, which also tells you the location of the other chromosome, even if it is phenotypeless and visually indistinguishable from wild-type.

All this conspires to make genetic crosses with balancers totally predictable and verifiable.

But that’s not all balancer chromosomes are good for. Mutations are often detrimental, and if left in a stock of flies with a wild-type counterpart, after a number of generations they will be lost. Some can be kept on their own, if they are viable enough, but some mutations are poorly viable, or totally lethal. In this case, in order for the stock of flies to persist, a wild-type copy of the gene must be present to rescue the lethality. If present on a wild-type chromosome, the mutation will be lost from the stock as the wild-type chromosome takes over. However, if the wild-type copy is present on a balancer, the balancer cannot take over as they are homozygous lethal. In the case of a recessive lethal mutation, this creates a situation where they need each other – only files containing the lethal chromosome and the balancer can survive.

There are numerous different balancers (with many variations) for each chromosome , named for the chromosome they stabilise. For example, in FM7i the F stands for first and the indicates it is multiply-inverted. Each carrying dominant mutations to make them obvious. Here are a couple of examples of markers, one for each chromosome.



Bar-eyed FM7i male, full genotype FM7i/Y ; +/+ ; +/+ ; +/+.

The coolest thing about this variant of FM7 (i) is these flies actually glow green under UV light, thanks to the virtues of GFP.This allows you to tell larvae apart.



Curly winged female, full genotype: w1118/w1118 ; +/CyO ; +/+ ; +/+. (w1118 is not a balancer, it indicates there is a mutation on each X chromosome, which gives the fly white eyes).

Curly is often found on SM balancers.



Stubble male (look at the hairs), full genotype: X/Y ; +/+ ; +/TM3, Sb ; +/+.

You may notice the third picture here looks different. It’s taken on the same camera, but is lit with an LED light, which is much less yellow then usual.

For my next post, I may do something on the range of eye-colours you can get in flies, and how they’re useful.



  1. thank you for this picture . I also want a shaven bristle drosophila fly

  2. I don’t have any sv mutants unfortunately, and it seems pretty hard to find a picture online.

    Best I could find was in this paper: . It’s a SEM picture though, so no colour.

  3. Thanks so much for this. I’m a postbac premed volunteer in a fly lab, learning a lot of this, uh, on the wing. (Sorry.)

    A lot of these basic genes and traits (FMZ, CyO) seem to fall into that space between general knowledge posted on Wikipedia and specialized stuff that Googles only impenetrable papers.

    I’m finding blog posts like these enormously helpful. I also can’t imagine I’m the only one.

    Yours gratefully,

  4. Dear Distracted PhD student,

    I am having to learn some Dros genetics, and you have helped me a little, “boot camp” keep it up

    Where are you (Ph.D lab) and what is your thesis going to be about?

    tom sturgill

  5. thnx a lot

  6. Nice work indeed!!

    But I have to say you wrote the wrong genotype of the so called “FM7i” fly. FM7i fly have heart shape eye indeed, but it is white. Check the eye color of male fly in this stock “BL5263”, the female eye is red because of double X, but male is white. I guess the pic of your “FM7i” is acturally FM7i-ACT-GFP, like in this stock “BL4559”.

    I am working on P-element mutagenesis of two genes on X-chromosome now, so I have study the eye color of X balancer, haha.

    Anyway, great work. There is little pictures of balancer phenotype in the website.

  7. Yep, you’re entirely correct. I didn’t know until I came to write up a report and found there was more than one “FM7i” when I went to FlyBase to find the full genotype. My lab obviously got hold of FM7i-Act-GFP at some point and started calling it “FM7i”. Just goes to show how important it is to keep track of full genotypes.

  8. i think every biologist feels grateful for this unselfish assistance!

  9. i am grateful to your blog and comments for helping my project

  10. why do they call it “balancer”?
    When I think of the word balancer, I think of homeostasis, equilibrium or something.

  11. […] URL: […]

  12. Hi,
    I want to ask for your permission to use three photos of the wild/bar and heterozygous female for my biology assignment please?

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