Caenorhabditis elegans is a species of worm that is about one millimeter in
length, feeds on different types of bacteria, and can be housed very easily in the lab for
experimentation (1). For these reasons, they are often used in genetic experimentation in
the lab. The first person to begin experimenting with C. elegans was Sydney Brenner in
the 1960s (2). Since then, C. elegans has become a very popular organism in researching
and teaching. There are two different sexes of C. elegans: hermaphrodite and male.
Hermaphrodites have sex chromosomes XX while males have only one sex chromosome
represented by the genotype XO. Hermaphrodites can self-fertilize or mate with males to
reproduce, while males can only mate with hermaphrodites to reproduce. Hermaphrodites
cannot mate with each other. When hermaphrodites self fertilize, the result is an F1
generation that is exactly like the original hermaphrodite. When a male and
hermaphrodite reproduce, half of the F1 generation will be male and the other half will be
hermaphrodite (1).
In the experiment we were assigned to complete, we were to study a certain
mutation in the C. elegans species and determine whether the gene for the mutation was
dominant, recessive, or incomplete dominance, and also whether it was autosomal or sexlinked. Our group was assigned the UNC-76 mutation for study. UNC-76 stands for
uncoordinated because the worms with this mutation tend to curl up and remain
immobile. Under the microscope they look like a swirl or a coiled snake. To determine
the characteristics of the mutation, we needed to set up a cross of two worms on a petri
dish. But which two? We were provided with a dish of male and hermaphrodite “wildtype” worms - or worms the way you would find them in their natural habitat, and one
dish of only hermaphrodites with the UNC-76 mutation. Bearing in mind that the focus of
the experiment was the mutation, we knew that placing a wild-type male and a wild-type
hermaphrodite in the test plate would not teach us anything about the mutation. We also
knew that putting two UNC-76 hermaphrodites in the dish would only produce two F1
generations of clones of the two hermaphrodites placed in the dish. From these
observations we concluded that there was only one match left that would create a
genetically diverse F1 generation and that included a UNC-76 hermaphrodite in the
cross. This would be a cross of the UNC-76 hermaphrodite with a wild-type male.
Once the cross was complete, by observing and counting the F1 generation, we
would be able to determine if the mutation was dominant, recessive, or incomplete
dominance and then whether it was autosomal or sex-linked. The F1 generation produced
by the UNC-76 hermaphrodite and the wild-type male would produce half males and half
hermaphrodites. By observing the phenotypes of the F1 generation of the cross and
counting and calculating ratios, we would be able to determine the characteristics of the
gene that codes for the mutation. In studies of sex-linked gene mutations, males are more
likely to have the mutation than hermaphrodites, even if the mutation is recessive. The
reason behind this is that males only have one X-chromosome. While hermaphrodites
might inherit the chromosome that has the recessive gene for the mutation from one
parent, they might also inherit a second X-chromosome to cover up the recessive gene,
preventing the mutation. Males, however, only inherit one X-chromosome, therefore, if
they inherit the X-chromosome that has the gene for the mutation, the mutation will show
up in their phenotype because they don’t have a second X-chromosome to cover it up.
From this we determined that if the gene for the mutation was recessive and sex-linked,
the F1 generation would have all wild-type hermaphrodites and all UNC-76 mutated
males, as displayed in the Punnett Square below. If the gene was dominant and sexlinked, the F1 generation would produce all mutated hermaphrodites and all mutated
males. The way to tell if the mutation is autosomal is by observing at the phenotypes of
the F1 generation on the plate. If there are some normal males and some normal
hermaphrodites along with some mutated worms, the gene is autosomal. If there is a
majority of mutated individuals in the dish, the mutation is dominant, while if there are
only a few mutated individuals it is recessive. If the dish is full of worms that look like
they are somewhere in-between mutated and dominance, the gene must be incompletely
dominant.
UNC-76 Hermaphrodite crossed with wild-type male
when the mutation is recessive and sex-linked.
x
X
x
Xx
Xx
xO
xO
Xm
O
UNC-76 Hermaphrodite crossed with
wild-type male when the mutation is
recessive and autosomal.
m
M
M
UNC-76 Hermaphrodite crossed with wild-type male
when the mutation is dominant and sex-linked.
m
Mm
Mm
Mm
Mm
Xm
X
XmX
XmX
O
XmO
XmO
After we studied the mutation and predicted the phenotypes of the F1 generation
for each combination of characteristics, it was time to complete the actual cross. We used
a worm spatula to place a wild-type male and UNC-76 hermaphrodite on the petri dish
with a large food supply of E. coli. We left them in the lab to breed for one week. After
one week, we came back to the lab to see the results. To determine the characteristics of
the gene for UNC-76, we observed that F1 generation under the microscope and counted
one quadrant of the dish. Because the C. elegans reproduce so quickly, there were too
many to allow counting the whole plate, so counting one quadrant and multiplying all the
values by 4 gave us a close estimate of what the whole dish’s count would be. We
counted 100 worms and tallied the amount of each category. The table below shows our
counts for one quadrant.
Tally
Total
Mutated
Normal Hermaphrodite
Normal Male
Worm
llllllllllllllllllllllllllllllllllllllllllllllllllllllllllll llllllllllllllllllllllllll llllllllllllll
60
26
14
Based on our counts, we can conclude that the UNC-76 mutation both autosomal and
recessive. We know this because, as stated earlier, if the mutation was sex-linked,
whether dominant or recessive, all males would be mutated; but our plate displayed a
number of normal males. We can conclude that the gene is recessive because only a few
individuals were mutated. The Punnett Square below shows the cross of the male (left)
and hermaphrodite (top) when the gene for UNC-76 is autosomal and recessive. Because
the C. elegans life cycle is so short, we see some mutated individuals because some of the
F2 generation has already appeared on the plate. This Punnett Square is also shown
below. The only difference from the predictions to the results was that the F1 generation
dish showed some mutated individuals because we didn’t take into account to F2
generation already being on the plate. From this experiment, we can conclude that the
UNC-76 mutation of the C. elegans species is an autosomal and recessive gene mutation.
F1- Herm on top
Male on left
M
M
m
m
Mm
Mm
Mm
Mm
F2-Herm on top
Male on left
M
m
MM
Mm
Mm
mm
M
m