Mendel's Experiments — But Actually Grow the Generations
Most biology students can draw a Punnett square.
They know that Gregor Mendel worked with pea plants. They have probably learned the words dominant, recessive, homozygous, heterozygous, genotype and phenotype.
Many can confidently predict a 3:1 ratio.
But there is something slightly strange about the way Mendelian genetics is usually taught.
Very few students ever do anything resembling Mendel's actual experiment.
Instead, genetics can become a paper exercise:
Parent A has genotype AA.
Parent B has genotype aa.
What proportion of the offspring will show the dominant characteristic?
The answer is useful, but something important has disappeared.
Mendel did not begin with a Punnett square.
He began with plants.
He grew them. He selected parents. He controlled pollination. He waited for seeds. He planted the next generation. He counted hundreds and sometimes thousands of offspring.
Most importantly, he collected real biological data.
That suggests a wonderful experiment for students who want to explore biology beyond the normal school practical syllabus:
repeat a small version of Mendel's investigation and actually grow the generations.
Genetics Before Genetics Had a Name
Gregor Mendel carried out his famous pea experiments during the nineteenth century, long before anybody knew about DNA, chromosomes or genes in the modern sense.
He selected pea plants because they offered several useful features.
They could be grown relatively easily.
They produced large numbers of offspring.
Their flowers normally self-pollinated, but pollination could also be controlled experimentally.
Most importantly, Mendel identified characteristics that could be separated into clearly recognisable forms.
These included characteristics associated with:
seed shape;
seed colour;
flower colour;
pod shape;
pod colour;
flower position;
plant height.
Mendel began with plants that reliably produced the same characteristic generation after generation.
Today we would describe these as true-breeding lines.
He then crossed contrasting plants and followed what happened through several generations.
This is the part students often hear about.
It is also the part worth actually doing.
The Three Generations That Matter
The terminology initially sounds more complicated than the experiment.
P generation
The parental generation, or P generation, contains the original parents selected for crossing.
Imagine, for simplicity, that we have a characteristic controlled by two alleles.
A = dominant allele
a = recessive allele
Suppose our original parents are:
AA x aa
One parent is homozygous dominant and the other homozygous recessive.
F1 generation
Their offspring form the first filial generation, usually written F1.
Every offspring receives:
A from one parent
a from the other
Therefore all the offspring are:
Aa
If A is completely dominant, all the F1 plants show the dominant phenotype.
This result alone is interesting.
The recessive characteristic appears to have disappeared.
But it has not disappeared genetically.
The allele is still there.
F2 generation
Now allow F1 individuals to produce another generation.
The cross becomes:
Aa x Aa
The possible offspring genotypes are:
AA
Aa
Aa
aa
The predicted genotype ratio is therefore:
1 AA : 2 Aa : 1 aa
But because AA and Aa show the same dominant phenotype, the predicted phenotype ratio becomes:
3 dominant : 1 recessive
That familiar classroom ratio suddenly becomes much more interesting when the four possibilities are replaced by 100, 200 or 500 living organisms.
The Practical Challenge — Can We Actually Grow It?
Using traditional garden peas is possible, but it is not necessarily the best choice for a teaching investigation.
Mendel had patience.
School students generally have timetables.
A better approach is to look for fast-growing plants with clearly identifiable inherited characteristics.
Certain varieties of fast-growing Brassica, for example, have been developed specifically for education and can complete their life cycles surprisingly quickly.
Depending upon the variety being used, potential characteristics might include differences in:
pigmentation;
stem characteristics;
leaf characteristics;
hairiness;
colour;
other easily scored phenotypes.
The essential requirement is not that the organism happens to be a pea.
The important thing is that:
the characteristic is genetically determined;
the alternative phenotypes can be distinguished reliably;
the inheritance pattern is known;
generation time is reasonably short;
enough offspring can be produced to make meaningful comparisons.
That creates a genuine experimental genetics project rather than simply a demonstration.
Start with the Parental Generation
The first stage is careful observation.
Students should photograph and describe the parental plants.
For each parent record:
plant identification number;
phenotype;
assumed genotype if known;
date planted;
date flowering began;
height;
relevant physical characteristics.
Labelling is extremely important.
A surprisingly useful lesson from multiday biology experiments is that memory is a terrible laboratory notebook.
Plant P1 may seem unmistakable today.
Three weeks later, surrounded by twenty similar plants, it may not be quite so obvious.
Every plant should therefore have an identification code from the beginning.
Controlling Pollination
This is where the experiment starts feeling much more like real biology.
Rather than simply allowing random pollination, selected parents can be crossed.
The exact method depends upon the species being used, but students may transfer pollen between chosen flowers using a small brush or another suitable technique.
Flowers can then be labelled so that the resulting seeds can be traced to a particular cross.
With some species it may also be necessary to prevent unwanted pollination.
The objective is simple:
Know who the parents were.
That is fundamental to any breeding experiment.
A Punnett square assumes we know the parental genotypes.
The practical investigation shows how much work may be required before we are justified in making that assumption.
Grow the F1 Generation
Seeds produced by the selected cross can then be planted.
This produces the F1 generation.
Now comes the first prediction.
If the parental generation consisted of true-breeding contrasting forms under a simple dominant-recessive inheritance model, students might predict that all F1 offspring will show the dominant phenotype.
But instead of simply writing:
100% dominant
they can test it.
Suppose 36 F1 seedlings germinate.
Students might find:
Dominant phenotype = 36
Recessive phenotype = 0
That is certainly consistent with the prediction.
But suppose they find:
Dominant phenotype = 35
Recessive phenotype = 1
Now the experiment becomes more interesting.
Was the plant classified incorrectly?
Was one parent not actually true-breeding?
Was there accidental pollination?
Was the supposed characteristic more complicated than expected?
Good experiments do not merely confirm theories.
They make us ask better questions when observations do not agree with predictions.
Then Produce the F2 Generation
The next step is the really satisfying one.
Cross suitable F1 plants, or allow self-pollination where appropriate, and collect the next group of seeds.
Grow those seeds.
Now look for the characteristic that apparently vanished during the F1 generation.
If the simple Mendelian model applies, the recessive phenotype should reappear.
The theoretical expectation is:
3 dominant : 1 recessive
But there is an important word in that statement.
Expectation.
It does not mean every four plants will consist of exactly three dominant plants and one recessive plant.
Mendelian Ratios Are Probabilities, Not Instructions
This is one of the most valuable lessons in the entire experiment.
Consider tossing a coin.
The probability of heads is 1/2.
If I toss the coin four times, that does not guarantee:
2 heads
2 tails
I might obtain:
3 heads
1 tail
or even:
4 heads
0 tails
The same principle applies to inheritance.
For an Aa x Aa cross, each offspring independently has a 3/4 probability of showing the dominant phenotype and a 1/4 probability of showing the recessive phenotype.
With only eight plants, the observed ratio might look rather unlike 3:1.
With 200 plants, it is likely to be considerably closer.
This gives us an immediate connection between genetics and statistics.
A Numerical Example
Suppose we grow 160 F2 plants.
The expected numbers for a 3:1 ratio are:
Dominant phenotype:
160 x 3/4 = 120
Recessive phenotype:
160 x 1/4 = 40
But perhaps our actual results are:
Dominant = 118
Recessive = 42
The observed ratio is:
118 : 42
or approximately:
2.81 : 1
Does that mean Mendelian genetics has failed?
Of course not.
The result is extremely close to the expected pattern.
Biological data contains variation.
That is precisely why collecting the data is more educational than simply completing a Punnett square.
The Faster Version — Genetic Maize
There is another excellent way to investigate Mendelian ratios that requires much less waiting.
Count maize kernels.
Specially prepared genetic maize ears can contain kernels displaying easily distinguished inherited phenotypes.
Depending upon the particular educational material being used, kernels may differ in characteristics such as colour or texture.
Instead of growing two generations of plants, students can inspect hundreds of individual kernels.
That turns an ear of maize into a remarkably compact genetics experiment.
Imagine counting 200 kernels and finding:
Dominant phenotype = 146
Recessive phenotype = 54
The theoretical 3:1 expectation would be:
Dominant = 150
Recessive = 50
Again, the experimental result is not exactly 3:1.
Nor should we necessarily expect it to be.
The interesting question becomes:
Is the difference small enough to be explained by chance?
That takes us into another extremely important area of biology.
Adding a Chi-Squared Test
For A-level students, the investigation can be extended using a chi-squared test.
The basic calculation is:
X^2 = sum((O - E)^2 / E)
where:
O = observed frequency
E = expected frequency
Take an example with 160 individuals:
Observed dominant = 118
Expected dominant = 120
Observed recessive = 42
Expected recessive = 40
For the dominant phenotype:
(118 - 120)^2 / 120 = 4 / 120
For the recessive phenotype:
(42 - 40)^2 / 40 = 4 / 40
Therefore:
X^2 = 4/120 + 4/40
X^2 = approximately 0.133
Students can then compare their value with an appropriate critical value.
Suddenly several areas of the biology course have come together:
genetics;
probability;
experimental design;
sampling;
mathematical analysis;
hypothesis testing.
And all because we counted real organisms rather than simply filling four boxes in a Punnett square.
Try Changing the Sample Size
There is another experiment hidden inside the experiment.
Suppose you have an ear containing several hundred kernels.
Count only 20 randomly selected kernels.
Calculate the ratio.
Then count 50.
Then 100.
Then 200.
Finally count as many as practical.
You will probably find that the estimated ratio jumps around dramatically with small samples and tends to become more stable as the sample becomes larger.
That is a powerful demonstration of sampling error.
Students often encounter the instruction:
"Use a large sample size to improve reliability."
This experiment shows them why.
Two Groups Can Get Different Answers
An even better activity is to give several groups different samples from the same population.
Imagine four groups each count 40 kernels.
They might obtain:
Group 1 — 32:8
Group 2 — 28:12
Group 3 — 31:9
Group 4 — 29:11
None is exactly the same.
Combine the results, however:
Dominant = 120
Recessive = 40
And suddenly the overall result is exactly 3:1.
That leads naturally to discussions of:
replication;
sample size;
random variation;
pooled data;
reliability.
These are scientific ideas that extend far beyond genetics.
Why Not Every Characteristic Behaves Like Mendel's Peas
There is also an important warning to include.
Students can sometimes leave school believing that every characteristic works like:
A = dominant
a = recessive
Biology is considerably more interesting than that.
Some characteristics involve:
incomplete dominance;
codominance;
multiple alleles;
linked genes;
sex-linked inheritance;
polygenic inheritance;
interactions between different genes;
environmental influences on phenotype.
Human height, for example, cannot sensibly be explained using one simple dominant and one recessive allele.
Neither can intelligence, body mass, skin pigmentation or many other complex characteristics.
Mendel's model is enormously important because it reveals fundamental principles of inheritance.
But it is a starting point, not a description of every biological characteristic.
A real breeding experiment provides an excellent opportunity to make that distinction.
Phenotype Is Not the Same as Genotype
Another useful question is:
If a plant shows the dominant phenotype, can we tell whether it is AA or Aa just by looking at it?
No.
Both genotypes produce the same phenotype under complete dominance.
That creates the possibility of another classic genetic technique: the test cross.
An individual showing the dominant phenotype but having an unknown genotype can be crossed with a homozygous recessive individual.
If the unknown plant is:
AA
all offspring should show the dominant phenotype.
But if it is:
Aa
approximately half the offspring should show the dominant phenotype and half the recessive phenotype.
Again, the genotype is not observed directly.
It is inferred from experimental evidence.
That is an important scientific distinction.
Make the Investigation a Proper Research Project
Rather than treating this as a one-hour practical, I would make it a continuing investigation.
Students could maintain a genetics notebook containing:
Week 1
Plant or examine parental generation.
Week 2 onward
Measure growth and record phenotypes.
Flowering
Carry out selected crosses.
Seed production
Collect and label offspring.
Next generation
Germinate and record F1 phenotypes.
Later
Produce F2 offspring where practical.
Analysis
Compare observed and predicted ratios.
Evaluation
Consider experimental errors and alternative explanations.
Photography can make this particularly effective.
Photograph each generation under similar conditions and build a visual family history:
P -> F1 -> F2
You could even create a simple digital pedigree showing which plants produced which offspring.
That begins to resemble the type of record-keeping required in genuine biological research.
What Could Go Wrong?
Quite a lot.
And that is part of the value of the experiment.
Seeds may fail to germinate.
Plants may die.
Pollination may fail.
Labels may become detached.
Phenotypes may not be as obvious as expected.
A supposedly true-breeding line might not behave as anticipated.
Sample sizes may be too small.
Environmental differences may influence the appearance of plants.
And occasionally the results may simply refuse to give a beautiful textbook ratio.
None of this makes the experiment unsuccessful.
It makes it real biology.
A perfectly clean 3:1 result printed in a textbook teaches Mendelian inheritance.
An experimental ratio of 73:27 that students have actually produced teaches Mendelian inheritance and science.
From Punnett Squares to Evidence
This is why I particularly like experiments of this type.
There is nothing wrong with Punnett squares. They are extremely useful models.
But there is a danger when students spend too long manipulating letters on paper that they forget what those letters represent.
A represents biological information carried by chromosomes inside real cells.
Those cells produce gametes.
Gametes combine.
Seeds develop.
Plants grow.
Phenotypes appear.
And somewhere among a tray of F2 seedlings, a characteristic that apparently disappeared a generation earlier suddenly returns.
That is a far more memorable way to understand the meaning of a recessive allele.
Mendel Was Counting — Not Drawing Squares
One of the most revealing things about Mendel's work is the sheer importance of counting.
He did not simply notice that some offspring had one characteristic and some another.
He recorded how many.
That changed inheritance from a collection of observations into something that could be investigated mathematically.
Modern genetics has travelled an extraordinary distance since then.
Today we can sequence DNA, identify mutations and examine individual genes.
But the fundamental scientific approach remains recognisable:
make a prediction, carry out a cross, observe the offspring, count them and compare the evidence with the prediction.
That is why repeating even a modest version of Mendel's experiment can be so valuable.
Students stop being told that F2 offspring should produce a 3:1 ratio.
They grow them.
They count them.
They discover that nature rarely gives perfectly tidy numbers.
And then they have to decide whether their evidence supports the model.
At that moment, Mendelian genetics stops being a Punnett square.
It becomes experimental biology.
No comments:
Post a Comment