Download Results section conventions

Biology 100B
Mount Holyoke College
Spring 2001
Results section conventions
plus hints for using Word and Excel to your advantage
and some thoughts on Fast Plants and trichome genetics
Figures
A drawing, graph, diagram, or photograph (in short, any kind of picture) is a Figure. Put
its label, number, and title (which should be as explicit as you can make it) beneath it, as
shown in Figure 11. Refer to it as Figure __ at the appropriate place in your written
account.
mg of Vitamin C per 100 mL juice
14.00
fresh OJ
Trop OJ
12.00
10.00
8.00
6.00
4.00
2.00
0.00
0
1
2
3
4
5
6
7
Days at room temp
Figure 1. Decline in vitamin C content (as mean plus or minus standard deviation)
of fresh squeezed orange juice and Tropicana orange juice at room
temperature over a one week period
Color is used primarily to distinguish between two or more sets of data. For example, to
put two sets of data (e.g., fresh and carton orange juice in Figure 1, or hairiness of first
and second fast plant generation in our experiment) on the same graph in order to show
the differences between them, you must give the reader an easy way to distinguish
between them. (Shades of gray or hatch marks may stand in for color in a black-andwhite printed sheet.)
You can copy and paste an Excel “chart” into a Word file, as I have done here. Then you
can place your illustration wherever you think it best fits into your paper.
1
These data are from a Unity of Science independent group project, spring 2000.
Results sections, p 2
Excel doesn’t like to put titles below the graph, so I usually type them with Word. This
allows me to use Word’s automatic numbering capability. (Insert/caption lets you put in
figure and table numbers; Insert/cross-reference lets you refer to them later.) If you use
this automatic numbering, you can change the order of your figures and tables as you edit
your paper and they will renumber themselves, as well as all references to them. (You
may have to save, close, and reopen the file to see the renumbering occur, however.)
Tables
A list of numbers or words is a table, and gets numbered separately from the figures.
Table labels and titles go above the table, as illustrated in Table 12. Refer to tables by
number as you do figures.
Table 1.
Comparison of the three principal types of plant pigments, with respect
to color, anatomical location, solubility, and biological function.
pigment type
color
Chlorophyll
green
Carotene
[xanthophylls
(oxidized
carotenes)]
Anthocyanin
(sugar +
“principle”)
location in cell
chloroplasts inside
mesophyll cells
solubility
function
alcohol
photosynthesis
yellow, orange plastids
(like a chloroplast,
with or without
(reddishchlorophyll)
)orange
alcohol
accessory to
photosynthesis,
pigmentation
red, magenta,
purple, blue
(red in acid;
blue in base)
water
pigmentation in
flowers; fall colors
vacuole
Numbers
Reading a long list of numbers isn’t very interesting, so you need to come up with a way
to point out the salient features of a graph or table without telling every single value.
Think about what you find striking or instructive about the data, and point it out.
Avoid unnecessary decimal places. For example, it seems more efficient (and more
emphatic) to say that none of the plants selected for smooth petioles had any trichomes
than to say they had an average of 0.00 trichomes.
If you weigh it, or measure its volume, you have an amount; if you count them, you
have a number, e.g., an amount of dirt; a number of trichomes.
Our plants
I ordered 800 seeds from Wisconsin Fast Plants, all from the same stock. Students
planted 6 seeds in each of 120 pots. The labels “hairy” and “smooth” were put on the
pots not because the pots contained different seeds at planting, but because the plants
2
This table comes from “Light and Color in Leaves”, Lab 10 from Unity of Science Laboratory Manual,
Fall 2000.
Results sections, p. 3
were going to be selected one way or the other. Only the particular plants chosen to be
the parents of the second generation can be called “hairy” or “smooth.”
Explicit descriptions
Be sure to give the reader information that cannot be obtained otherwise, e.g., which
trichomes you counted. You can leave out details about the organization of your data
sheet and the numbering of pots, unless you think this bears directly on the data or your
interpretation.
When you say something like “increased hairiness in the second generation”, be sure it is
clear to the reader whether you mean more hairs per plant, or more plants with hairs.
Distinguish between “gene”, “allele”, and “trait”.
Allele dominance and frequency
An allele is dominant if it takes only one to express a trait. Dominant alleles are
generally those that do something: specify a working version of an enzyme, an antigen, a
hormone, a transcription factor, a receptor, etc. Recessive alleles, on the other hand, are
generally those that represent a loss-of-function. That is, they do not specify a working
version of the protein in question.
It seems more likely that alleles that result in
production of trichomes are dominant.
Allele frequency depends not on dominance, but on current prevalence and usefulness.
Alleles that reduce the reproductive capability of their bearers tend to become less
common, while those that enhance reproductive capability tend to become more
common.
The population genetics exercise we did in class on Thursday March 1 shows
mathematically why in the absence of selection, allele frequencies do not change. Allele
frequencies change when there is selection for one allele over the other.
When you read The cost of defense against herbivores, by Agren and Schemske, you
should be thinking about how herbivores act as agents of selection on their food plants.
If herbivorous insects prefer to eat smooth stemmed plants, then the smooth stemmed
plants will, on average, produce fewer offspring. Therefore, if hairiness is a heritable
trait, the frequency of alleles for hairiness should increase in the population, and the
frequency of alleles for smooth stems should decline3.
If there are no predatory insects to maim or kill the smooth-stemmed plants, then the
smooth-stemmed plants may actually have an advantage – they can spend their energy on
making babies instead of making trichomes. In this case, the allele for smooth stems
should increase in frequency in the population.
3
Of course, the plants also act as agents of selection on the insects. If all the plants are hairy, the individual
insects that cannot abide trichomes do not breed as much (because they don’t get enough to eat) as those
that find a way to eat hairy plants. Allele frequencies will change in the insects, producing a population
more willing to eat hairy plants. This will likely result in further selection by the insects against the
relatively less hairy plants, and an arms race of sorts will ensue.
Results sections, p 4
For neither scenario (selection against smooth plants by herbivorous insects or selection
for smooth plants in the absence of herbivores) does it matter which allele is dominant.
Whichever allele is most useful under the current conditions will become more common
in the population.
We have acted as very severe agents of selection in our population of plants. We have
chosen two subsets of the original heterogeneous population to be parents, and have
killed all the rest! We reduced the reproductive capability of the non-chosen plants all
the way to zero! In one subset, only smooth-stemmed plants were allowed to breed. In
the other subset, only those in the hairiest 10% or so of the population were allowed to
breed.
If hairiness is an inherited trait, then we should be able to create two populations by this
technique, one in which the hairy allele(s) is(are) common, and one in which the smooth
allele(s) is(are) common. No matter which allele is dominant, we can change its
frequency in the population. This should further reinforce the idea that either the
dominant or the recessive allele can be more common in the population, depending on the
circumstances.
Polygenic traits
Quantitative traits, such as number of trichomes per petiole, often are influenced by more
than one gene, which is why we don’t see a clear either/or pattern in the phenotypes. To
think about how this works, let’s start by imagining that there is only one gene affecting
the number of trichomes, and there are only two alleles of that gene. Let’s call it the “A”
gene. Because making trichomes seems like doing something and not making trichomes
seems like not doing something, let’s say the hairy allele is dominant and the smooth
allele is recessive, but let’s imagine the dominance is incomplete, so that the heterozygote
has a phenotype intermediate between those of the two homozygotes.
There are only 3 possible genotypes in the population: AA, Aa, aa. There should also
only be three phenotypes: very hairy, somewhat hairy, not hairy. We don’t actually see a
such a clustering of phenotypes in our plant population, so maybe more than one gene is
involved.
Suppose there are two genes, the “A” gene, and the “B” gene. Suppose further, for the
sake of simplicity, that each dominant allele (of either gene) adds the same number of
trichomes to the plant. We should have 5 phenotypic categories, then, as shown in Table
2.
Table 2.
Genotypic and phenotypic categories for a 2-gene model of
trichome number inheritance.
number of dominant alleles
phenotype
genotype(s)
4
hairiest
AABB
3
2
1
0
smooth
AaBB AAbb Aabb aabb
AABb AaBb aaBb
aabb
Two parents from the second hairiest category might have the genotypes AaBB and
AABb, and could produce offspring in the hairiest category (AABB), the second hairiest
Results sections, p. 5
category (AABb or AaBB), or the middle category (AaBb). Because they cannot
produce offspring in the two least hairy categories, though, if we chose parents only from
the second hairiest category, we’d have less of a spread (a smaller standard deviation)
among their offspring than in the original population.
Two parents from the middle category might both have the genotype AaBb, and they
could produce offspring in any of the 5 phenotypic categories!
So if we had to include some parents from the middle category, the same range of
trichome numbers would be possible in our hairy second generation as in the first
generation as a whole. However, we can expect a different distribution, since only one
pair of parental genotypes (AaBb x AaBb) can produce smooth stemmed offspring
(aabb), and even they can only do so 1/16 of the time.
If there were three genes, “A”, “B”, and “C”, we’d have 7 phenotypic categories, as
shown in Table 3.
Table 3.
Genotypic and phenotypic categories for a 3-gene model of trichome
number inheritance.
number of
dominant
alleles
6
5
4
3
2
1
0
phenotype
hairiest
smooth
genotype(s) AABBCC AaBBCC AABBcc AABbcc AAbbcc Aabbcc aabbcc
AABbCC AABbCc AAbbCc AaBbcc aaBbcc
AABBCc AaBBCc AaBBcc AabbCc aabbCc
AaBbCC AaBbCc aaBBcc
AAbbCC AabbCC aaBbCc
aaBBCC aaBBCc aabbCC
aaBbCC
The more genes involved in adding to the number of trichomes, the more phenotypic
categories there are, and the more closely the theoretical distribution can match the real
distribution, which after all, had 24 different numbers of trichomes on their petioles.
Another factor is how many trichomes on average are added by each dominant allele.
Biological things being the way they are, we should expect a rather loose correlation
between the number of dominant alleles and the number of trichomes, as opposed to a
precise one-to-one correspondence.
What happens in the second generation depends on the nature of the inheritance of
trichome number in fast plants. What if, for example, the hairiest individuals in our first
generation did not happen to have all possible trichome-generating alleles? (I.e., What if
our hairiest individuals were in fact heterozygous for one or more trichome genes?) How
would that affect the possible range of trichome number in their offspring?
What if there is more than one genotype that results in a smooth petiole, so that some of
the smooth individuals were heterozygous? What might we see in their offspring?