BISC219/F13: Gene Linkage

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Wellesley College BISC 219 Genetics Fall 2013


LAB 1 Worm Boot Camp

Complete the Lab Entry Survey
Please complete two brief surveys about interest and concept familiarity with genetics. These surveys are found at: Entry Survey and Genetics Pre-Test.
The password for the Pre-Test survey is posted to the welcome message on the lab Sakai site. Thank you for your participation. You will get a few points for completing the entry and the exit versions of these surveys but the surveys must be done before the end of the first week of classes.

Becoming a "Worm Wrangler"

Today we will:

  2. Learn to recognize: males and hermaphrodite larval stages.
  3. Practice moving worms from one plate to another.

The Worm Pick

You will acquire the most important tool of worm wrangling, the worm pick, from your instructor. The worm pick consists of a pasteur pipette and a ridiculously expensive piece of platinum wire. You must keep track of your worm pick and mend it yourself if it breaks. If you return the pick to us at the end of the semester, you will NOT be charged $10 (to help defray the cost of replacing the wire).

Directions for Making and Repairing a Worm Pick

  1. Find your platinum wire and obtain a glass Pasteur pipette (there should be a box in the back of lab where our extra supplies are kept).
  2. Using tweezers, insert about 3-4 millimeters of wire into the small hole at the end of the pipette.
  3. Hold the region where the wire and pipette meet over open flame until the glass melts to the wire and seals the open end completely. This should take less than a minute. WARNING - the wire and glass are VERY HOT at this point.
  4. After a minute or so gently tug the wire with tweezers to be sure it is attached securely to the pipette.
  5. Wrap a piece of your team's tape around the top end of your new worm pick.
  6. Write your name and lab day on the tape.
  8. Flame the end of your pick
  9. Happy worm farming!

If your wire happens to fall out of the pick or become loose during the semester, which is likely, just repeat the above steps with a new glass pipette and the same piece of wire.

FAQ: Why does the wire have to be platinum? Platinum heats up and cools off very quickly which means you sterilize the tip easily between picking and you don't kill the worms when you pick the next ones after sterilizing because your pick is too hot!

To Do Today: Worm Wrangling

Each of you needs to become facile worm wranglers. The more you do today to acquire the skills needed to quickly and successfully recognize and manipulate the appropriate stage and class of worm, the more successful your experiments will be. Recognize that there is a significant learning curve to acquiring these skills ; therefore, please, don't be frustrated if you are slow or aren't 100% successful today at the tasks we have described below. By the end of the semester, you will be astonished at how fast and facile you are at these manipulations! The more you practice, the better your crosses will be in the final step of your work today. So take your time with this part of today's work. It is crucial that you get confident and comfortable with recognizing worm stages (particularly L3 and L4's), differentiating males from hermaphrodites, and knowing which are wild type worms and which are single or double mutants. You can do it!

After you have made your worm pick, find a plate of N2 (wild type) worms for you and your partner to share.

  • Identify all the stages of worm development: egg, L1, L2, L3, L4, young adult, mature adult. Pay particular attention to the L3 and L4 stages and BE SURE that you can differentiate those from more immature and from adult worms. Use the pictures of worm stages in the Worm Info section of the wiki to guide you.
  • Within the adult and L4 worm populations, identify males and differentiate them from hermaphrodites. Use the pictures of them in the Worm Info section of the wiki as a guide. BE SURE you can pick out males and L3 and L4 worms from a mixed population before you move on.
  • Get a new plate of media seeded with E. coli strain OP50 bacteria (worm food). Practice picking worms from one plate to this new plate. It is very important not to gouge the media as the worms will burrow into it and not be easy to find later for scoring or to use for other crosses. The best way to do transfer (pick) worms is to flame your worm pick, touch the bottom of the shovel end lightly to the bacterial food (it acts as glue), and then touch your pick to a worm you want to select. Move the pick over to the new media and touch lightly in the center of the new plate to remove the worm. Observe to see if the worm is on the new plate and moving. FLAME your worm pick!
  • Acquire a plate of unc and dpy mutant worms for you and your partner to share. Differentiate Unc & Dpy (phenotype) single and double mutants from wild type in the adult and the late larval stages.

Now you are ready to set up some crosses and start your first investigation!

Autosomal vs. X-linked Genes

Series 1 is a brief investigation into patterns of inheritance. You will start it today and finish it in Lab 2. You will perform a number of crosses to illustrate important genetic concepts in inheritance. Recessive X-linked mutations are expressed differently in males (C. elegans males are XO) versus females or hermaphrodites (XX). WHY? Think about hemizygosity. In the case of C. elegans, when a hermaphrodite is mated with wild type males, the male progeny receive their single X chromosome from the maternal parent. If the X chromosome carries a mutation, all male progeny will express the maternal mutant phenotype, while the crossprogeny hermaphrodites will be phenotypically wild type. Recessive autosomal mutations, on the other hand, are not expressed in any of the F1 progeny. Why not?

Series 1 will also show that linked genes segregate differently than unlinked genes; that is, unlinked genes should give us expected ratios for dihybrid inheritance (such as 9 wild type: 3 mutant A: 3 mutant B: 1 double mutant or testcross 1 wild type: 1 mutant A: 1 mutant B: 1 double mutant), whereas linked genes will produce ratios that depart from those of dihybrid inheritance.

To test these concepts, you will cross wild-type males with three different mutant strains, all showing dumpy and uncoordinated phenotype (Dpy Unc).

The dpy and unc mutations could be inherited in several different ways. Possible outcomes are:

  1. Autosomal and linked (both mutations on the same somatic chromosome)
  2. Autosomal and unlinked (mutations on different somatic chromosomes)
  3. One of the mutations is autosomal and the other is on the sex chromosome (unlinked)
  4. Both mutations are X-linked

Your task is to determine the inheritance pattern present in each strain. This task will be accomplished experimentally by making a series of double heterozygotes and examining the progeny from those heterozygotes (self-progeny).

Background Information on Genetic Manipulations

General Instructions for Setting up and Scoring Matings:
To start a mating, you will place 4 to 5 young adult (or L4) males on a plate with 4 young adult (or L4) hermaphrodites. We will use plates seeded with a very small drop of OP50 bacteria for our crosses. These seeded plates have been prepared for you by our hard working lab specialists. If you were working as an investigator in a worm lab, you would be doing this preparatory work yourself, including growing the bacteria to be used as "worm food", pouring the agar into sterile plates, and seeding those plates with the bacteria. Yes, you should appreciate that this has been done for you! When setting up a mating, it is important to use only a small central area of the plate. Doing so helps our worms find each other quickly and helps insure a successful mating of the parental generation. In the F1 progeny, any self-cross progeny produced by the parental hermaphrodite must be distinguished from outcross progeny. Generally, the hermaphrodite is homozygous for a visible marker and the progeny from self-crossing will show the same phenotype as the hermaphrodite, while progeny from a cross with a wild type male (outcross) will be heterozygous for the maternal marker and thus appear wild type if the mutation is recessive. When no other way is available to distinguish selfcross from outcross progeny, only males are scored. It is the outcross progeny of a mating that are of interest; essentially, all male progeny are crossprogeny. For this reason it is critical than the male transfer not be contaminated with any eggs or hermaphrodites, as they will be confused with crossprogeny.




Scoring Phenotypic Ratios

To Do in Lab Today (per group)

  3. Label the cross plate using your GREEN Sharpie. Make sure the labeling on each plate is on the side (NOT THE LID) with the cross scheme: wild type ♂ X (strain name) hermaphrodite (H). DO NOT USE TAPE - this will prevent you from being able to observe your worms.
  4. Add 4 or 5 L4 mutant hermaphrodites to the cross plate.(These worms should still have the "clearing" at the center where the vulva is developing.) Because these mutant strains of worms do not move well, they tend to form piles of worms. GENTLY spread the piles out with your worm pick to find L4 worms to transfer.
  5. Repeat for each of the other 2 mutant strains.
  6. Put an elastic around the three plates.


To Do Outside of Lab on the 3rd day

  1. Before examining your cross progeny, in your lab notebook create a diagram of each cross with predictions for the phenotype expected for the F1 progeny (both hermaphrodites and males) if your mutations are both autosomal or if one is X-linked. (You will make another copy of these cross diagrams to submit as homework and include in this homework the expected F2 generation of the self-cross you will set up on day 3. See the assignment information and a template provided for more explicit instructions.)
  2. Examine the F1 progeny of each cross. Do you have cross progeny (phenotypically wild type) that signifies your cross was successful? If so, look for the presence of males. The hallmark of a successful cross is 30-50% male progeny. If you did not have outcross progeny (determined by the presence of a significant number of males in the F1) it means your hermaphrodite parents self-fertilized rather than mating with the males on the plate. In this case you can not assess the F1 generation to answer our question but all is not lost. Check the progeny of your classmates' crosses to get your answers. IF your classmates have already set up their F2 self-fertilzation plates, you may use their F1 hermaphrodites to make your F2 generation.
  3. Assess your crosses: If you have phenotypically wild type hermaphrodites and about 30% or more males, your crosses are likely to have worked and you have an F1 male generation to assess for x-linkage. Look carefully at the phenotype of each strains F1 males. Remember that X-Linkage is determined from the F1 males only (not the parental males!!! or F1 hermaphrodites )in each strain's cross progeny. If you found that all of the male cross progeny are mutants (either dumpy or uncoordinated in phenotype), you have partially determined the answer to our investigative question: How are the two mutations in mutants MB1, MB2, and MB3 inherited? HINT mutant males that are Unc, if found, will be smaller than the wild type hermaphrodites and look like sticks. If you poke Unc worms in the head they do not reflexively move backwards. Dpy males are shorter and fatter than a male of the same age but if you poke them in the head they will usually be able to move backwards and then forwards (although perhaps not be as graceful and sinusoidal as WT worms). Assess the x-linked mutations AND rule out x-linkage in any strains that have a wild type male F1 generation. If you have ruled out X-linkage in any or all of the three strains, on what type of chromosome are the mutations located?
  4. Take a few photos of representative worms of each strain. Make sure you have at least one good picture of each strain to use as evidence for your conclusions. There are a few digital cameras in the lab that you may use to take photos or you can use the camera on your cell phone or your own camera.
  5. When you have formed your conclusion about whether or not the strains possess X-linked mutation(s), make note of your findings in your notebook and email your lab instructor with your preliminary assessment (dpy or unc or both) and the name of the strain(s) (MB1, MB2, or MB3).
  6. Transfer two wild-type L3 or L4 hermaphrodites from each of the F1 generations to a separate set of new plates. Because you have identified strains with a sex-linked mutation by the phenotype of the F1 male generation, in theory, you need not carry the crosses further to the F2 generation. However, just in case you did not identify correctly sex-linkage, it would be wise to transfer a couple of wild type L3 or L4 hermaphrodites from each of the three strains to new plates to create F2 generations for all three unknowns. (You can always discard x-linked strain F2 progeny later, if you do not need them when we score automosomal strains.)
  7. To guarantee that all F2 progeny are self progeny, make certain that you have transferred only L4 hermaphrodite animals - after you transfer, make certain that there are no other animals on the plate.
  8. Incubate at 23°C until your next class period.


Always prepare for and complete the prelab quiz BEFORE the beginning of lab. Check the | Assignment Section of the wiki for the link to information and directions for the written work you must submit by the beginning of Lab 2.

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