Peas in a Pod
Episode 1 of Cracking The Code: The Continuing Saga Of Genetics

Summary of the Video (with stopping and discussion points)
Activity Ideas
Connections with History
Song Lyrics
Timeline
Further Reading
Websites

Summary of the Video

Peas In A Pod is introduced by the series theme song, which refers to the genetic secrets hidden within our DNA, which are now being revealed by genetic science. It also refers to what this DNA can tell us about our future health and the traits we inherit from our parents and pass on to our children. It also hints at the fact that we now can alter this DNA through genetic engineering.

This song is sung by Moxy Früvous, who are dressed like detectives, in trench coats, within the setting of a warehouse size archive, suggesting hidden information, then a lab.

The theme of the series is then expanded upon through narration and animation. Our DNA is written in a 4 letter alphabet, which we can now read, exploit and even rewrite. All of which amounts to a revolution in science, medicine and agriculture.

The narration then asks the question - how did we arrive at this point? We now begin the story of the history of genetics, another central theme of the series.

Heredity was always a great puzzle because physical traits are not passed down from parent to child in a straightforward way. We see a large three-generation family who illustrate the idea that a child might bear little resemblance to either parent or to its siblings, or more of a resemblance to a grandparent than a parent. There seemed to be no obvious pattern of inheritance.

Ancient philosophers, like Aristotle, tried to reason their way to an answer, which led to some wildly wrong guesses, some of which survived into the modern era. Using an animated Greek vase we examine one such mistaken notion - that heredity was determined by the male only. That theory had an obvious flaw - half of all children turn out to be girls. That led to another wrongheaded theory about the origin of gender - that it was determined by which testicle the sperm came from. This absurd theory actually survived until the 19th century discovery of the mammalian egg, which is a demonstration of how far astray intelligent thinkers can go using speculation alone.

First stopping point (4:50) This section is intended to stimulate thought among students about how how difficult it is to understand a complex natural phenomenon like heredity by simply staring at it or using theory (ie speculation) alone. And the kind of wrong turns and blind alleys this can lead to. To set up the next section, students can be asked - what are some better alternatives to speculation?

The first such alternative is  observation and the first good observers were farmers and shepherds. They gradually learned how to select out and cross breed those animals with the most desirable traits. The cross-breeding of plants came much later, when it was discovered that plants have sex organs and the ability to cross fertilize.

Carl Linnaeus, the 18th century Swedish scientist and physician who invented a method, still in use, to classify all living things, categorized plants on the basis of their sex organs. Unlike other botanists of the time, he disagreed with the biblical teaching that species were fixed at creation. He was sure that cross breeding two different plant species would produce a new intermediate species, but he didn’t put his theory to the test.

The first to do so was a contemporary of his, the German botanist Joseph Koelreuter, who did the first true experiments in genetics. Through animation we see how he discovered that some plant traits skip generations but failed to understand the significance of this. We also see how his gardener tried to sabotage his experiments, to comic effect.

Second stopping point (8:23) There can be discussion at this point as to why Koelreuter failed to appreciate his experimental findings and the important role that theory plays in the scientific method. Theory that comes after observation and that is tested by experiment is essential to scientific progress. This can be contrasted with the kind of unsupported theorizing the ancients did.

The next section focuses on Charles Darwin, the great English biologist of the 19th century, who was convinced that species can evolve over time. Over shots of his house and garden, we hear a brief explanation of how he arrived at his theory of evolution through natural selection and how its publication in 1859 made him into an immediate  world celebrity. But Darwin was far less successful in trying to explain the mechanics of heredity. Over shots of two different paints blending into a single intermediate colour, we see that his theory of reproduction as a blending process would eventually eliminate variation and therefore bring evolution to a halt. His escape from this dilemma was another mistaken theory called ‘acquired inheritance’. Over animation of a weight lifter building up his muscles, we see how this theory confuses appearance with genetic endowment. Then over animation of the word ‘heredity’ dissolving into chaos, then into the name of Gregor Mendel, we learn that at this very time, someone was busy solving this great puzzle, laying the groundwork for the new science of genetics.

Third stopping point (11:35) At this point there can be further discussion about Charles Darwin - his life, his scientific work, his worldwide celebrity after the 1859 publication of  ‘The Origin Of Species’ and his mistaken notions about heredity. The final animation showing a chaotic picture resolving into the name ‘Gregor Mendel’ introduces the scientist who finally solved the puzzle of heredity.  At this point, students can be asked about their prior knowledge or guesses about how he was able to do reduce the apparent chaos and complexity of heredity to its underlying rules. The answer as we will shortly see is through some highly focussed experiments and controlled plant cross-breeding experiments.)

We now delve into the surprising personal story of Gregor Mendel, a contemporary of Darwin’s, who was as obscure as Darwin was famous. Over footage shot in the Czech Republic and in Vienna, we learn how he became a monk and then how he ended up at the U. of Vienna, where he studied statistics. This proved critical to his later success at sorting out heredity. Over animation, we learn that he decided to cross breed different varieties within the same plant species and to count his results, the first time this had ever been done. We also learn why he chose the common pea plant as his experimental model. Then we see how he conducted his cross breeding experiments, focussing on his crosses between two contrasting varieties - one that always grew purple flowers and one that always grew white ones. We also see the great lengths he went to to prevent contamination with foreign pollen. This was the parent or P generation. The next or F1 generation grew only purple flowers, which Mendel termed the ‘dominant’ trait. The missing white flowers he called the ‘recessive’ trait. When these hybrids self-fertilized, their seeds produced the F2 generation, which showed a mixture of white and purple plants. Mendel counted them out and found that the ratio of purple to white was 3:1. This was also true for the other contrasting traits he studied in a similar fashion. This was the first pattern of inheritance every discovered.

Over humorous animation of Mendel in his monastery library (which is also seen on film), we see how Mendel conceived his particle theory of heredity in which each trait is determined by pairs of particles or genes. Phenotype and genotype are defined and all their possible variations are detailed. We then see how Mendel’s theory of random segregation led to his first law of genetics. Then we see how Mendel used the Law of Random Segregation to explain his experimental results. ‘homozygous’ and ‘heterozygous’ genotypes are defined. Then using various visual metaphors, we see that for any given fertilization event in the F1 generation, there are four equally likely outcomes in terms of genotype. When these four genotypes are translated into their corresponding phenotypes the result is 3 purple plants and 1 white one, the same 3:1 ratio that Mendel found in his various F2 generations.

Fourth stopping point (23:00). At this point, the following concepts can be reviewed - ‘dominant’ and ‘recessive’ traits and genes; ‘genotype’ and ‘phenotype’; ‘homozygous’ and ‘heterozygous’ genotypes; P, F1 and F2 generations; random segregation. Mendel and his personal story can also be further explored. There could be an interesting discussion how his family backgound, his entering the monastery as a monk, his studies at the U. of Vienna and his interest in plant breeding all came together to lead him to his brilliant and timeless discoveries.

Then using the visual metaphor of a slot machine in a casino, we see how Mendel tested for whether the seven contrasting traits he studied segregated independently or were somehow linked together. This led to his second law of genetics - The Law of Independent Assortment, which is introduced in true Las Vegas style.

Fifth stopping point (24:51) It can be pointed out here that Mendel’s Second Law is actually not true in many cases and it is only because of ‘Mendel’s luck’ that it worked out for the seven contrasting traits he studied. Independent assortment is always true at the chromosome level. All chromosome pairs segregate randomly, so there is independent assortment of chromosomes. But there are far more genes than chromosomes, so groups of genes must be located on the same chromosome.

This would link them together together during segregation ie those gene variations (alleles) grouped together on the same chromosome would end up in the same germ cell (assuming no other reshuffling of genes occurs in the meantime, which of course it does, through ‘crossing-over’, which is covered in the next episode, ‘Microscopes and Mutants’). Pea plants have only seven pairs of chromosomes. So for Mendel to get the results he did, each of the seven gene pairs he studied would theoretically have to be on a separate chromosome. The odds against that are about 63 to 1, hence ‘Mendel’s Luck’. In actual fact, only 2 of his contrasting traits were on separate chromosomes. 2 shared a different chromosome but were so far apart that there was no linkage between them. And 3 shared a different one altogether. Of the three possible combinations on this chromosome, two of them are far enough apart and the one combination that isn’t and that might have shown linkage was never studied by Mendel. 

Sadly, Mendel’s momentous discoveries were totally ignored in his lifetime. In contrast, Darwin’s discoveries immediately brought him great fame. Through humourous animation, we learn about the piece of mail that might have changed all that. Mendel mailed a copy of his paper to Darwin, but Darwin never opened the letter. Mendel died in 1884 at age 62. It took another 16 years for the world to finally learn about him. This is followed by a Moxy Früvous song about Mendel and his discoveries, sung in true ‘Monkish’ style.

Activity Themes and Suggestions

1) Misconceptions about genetics Genetics was a late arrival as a science, in part because there were so many deep rooted misconceptions and myths about reproduction and heredity to overcome. Some of these wrong-headed ideas are discussed in the video. Another such idea was called ‘pre-formation’. It proposed that at conception, a very tiny but fully formed human being was implanted in the mother. Where it came from in the first place was less clear and quite controversial. Find out about the mighty 17th battle between the ‘ovists’ and their rivals the ‘spermists’. It centred in the Dutch city of Delft and  involved two famous names in the history of science - Régnier de Graaf and Antony van Leeuwenhoeck.
2) Family photos/family traits  Look at old family photos and see if traits persist, don’t persist or skip generations
3) Classify this Find out more about the Linnaean system of classifying living things. Then make up your own Linnaean-style system of classification to apply to non-living things, like songs or vehicles or clothing.	Carl Linnaeus
4) Selection - natural or otherwise Darwin’s is celebrated for his theory of evolution through natural selection. How is it related to non-natural (breeder) selection? Is the theory always true? Think of counter examples of some human traits or physical characteristics that are non-useful but still survive. What are some possible explanations (such a neutral effect, hidden benefit, group benefit, etc.)?	Charles Darwin
5) Mendel flunks out Gregor Mendel was a great scientist but not so great at passing exams. He flunked his teacher certification exam twice and ended up as a substitute teacher. Find out about Mendel’s chequered career as a teacher and the personal reasons behind it. 6) Mendel’s other scientific activities Mendel was interested in everything to do with Nature. Find out about his other careers as bee-keeper, apple grower and student of the weather. Look into his theory of how tornados begin (published in 1871) and how well it has stood the test of time. He also did animal breeding experiments. Which animal?	Gregor Mendel
7) Back-crossing Mendel used a different type of cross breeding, called back-crossing, to confirm his results. Find out what a back-cross is and why plant breeders still use it today.
8) Sexual vs. asexual reproduction What is the difference between sexual and asexual reproduction? Could Mendel have made his discoveries using plants that reproduce asexually. In fact, he did try to reproduce his pea plant results using an asexually reproducing plant called Hieracium or hawkweed. Find out why he chose this plant and what happened as a result.
9) Mendel vs. Darwin on heredity Darwin’s mistaken theory of heredity was called ‘pangenesis’. Find out what that is and how it differed from Mendel’s Laws of Genetics. A major problem for Darwin’s theory was that it could not explain how new variation (the raw material for natural selection) is created. How did Mendel’s Laws explain the creation of new variations? Why are they also better able to explain variation that skips generations?	Charles Darwin
10) Exceptions to Mendel’s Laws There are many cases where Mendel’s Laws don’t seem to work in practice. An example that is covered in the next episode is that genes on the same chromosome can be linked to each other and hence they don’t always obey his Second Law of Independent Assortment. There are other reasons why his Laws don’t always pan out in real life situations. Find out what the following terms mean are and why they might cause these exceptions - 1) multiple alleles, 3) incomplete penetrance, 3) multi-genic traits, 4) X and Y chromosomes, 5) epistasis (control of one gene by another gene), 6) imprinting (the inactivation of a gene from one parent only). What do these exceptions to Mendel’s Laws say about them. Are they wrong or simply incomplete?
11) Heterozygous vs. homozygous Find out which type of gene pair is more common in humans, heterozygous or homozygous. What about in other species like pea plants, insects, etc.?
12) Polymorphic genes In his pea plant experiments, Mendel focussed on traits that come in two forms only, such as purple or white for the color of the flower. He theorized that the underlying ‘factors’ or genes also come in two forms only. However many genes are polymorphic, that is they come in multiple forms or alleles.  it is estimated that the average human gene has 4 - 26 alleles. Find out how our genes compare to those in other species in terms of polymorphism.
13) The great debate - Continuous vs. Discontinuous Darwin believed that variation is continuous, that it occurs imperceptibly along a smooth curve. Mendel believed that variation is discontinuous, because it is due to different combinations of discrete particles. Continuous vs. discontinuous variation (and therefore evolution) was the great debate in biology in the second half of the 19th century. Find out who William Bateson was and what role he played in this debate and also in promoting Mendel’s Laws of Genetics after their rediscovery in 1900.	William Bateson
14) When Mendel almost met Darwin Mendel visited England in 1862. Find out how close he came to where Darwin lived and speculate on what might have happened had they met.	Charles Darwin

Connections with History

1) Who was Aristotle? What other scientific theories did he come up with?	Aristotle
2) Who was Galen? Aside from his mistaken theory about the origin of gender, what was his important contribution to the history of medicine?	Galen
3) The female sex organ, the ovary, is hidden away in mammals and was not discovered until the 19th century. Find out who discovered it and how, and the impact of this discovery.
4) For a long time the great discoveries of Darwin and Mendel were seen to be at odds with each other. Find out about they were brought together in the 1930s to form a new field of study called population genetics.
5) What was the Austro-Hungarian Empire? What language(s) would Mendel have spoken? What happened to the Austro-Hungarian Empire?
6) Mendel’s monastery, called  still exists in the city of Brno or Brünn, in the Czech Republic. Find out which Catholic order it belongs to and the history of that order. There is a museum about Mendel attached to the monastery. Find their web site and learn more about Mendel and his monastery.
7) When Darwin’s landmark book, ‘On The Origin Of Species’, was published in 1859, it caused a sensation. Why it was so controversial at the time? Report back on the famous debate about it that took place on June 30, 1860 at Oxford University between T.H. Huxley and Bishop Samuel Wilberforce.
8) In 1909 Archibald Garrod, an English physician, discovered the first human disease that is inherited in Mendelian fashion - alkaptonuria. Find out what this disease is and how Garrod made his discovery. The parents of his alkaptonuria patients were almost always first cousins. Why is that relevant?	Archibald Garrod

Song Lyrics

Opening song

The script that’s written in our genes
Directs us from behind the scenes.
The words within it shape life’s destiny.

Hidden in your DNA
Is your genetic dossier.
It tells your future and your history

How traits get passed from parents to a child
Is something that has kept us so beguiled.

CRACKING THE CODE
GENETIC FUTURES WILL BE FORETOLD

CRACKING THE CODE
GENETIC POWER ABOUT TO EXPLODE

CRACKING THE CODE
GENETIC MYSTERIES TO UNFOLD

CRACKING THE CODE
GENETIC SECRETS WILL BE TOLD
CRACKING THE CODE

Mendel Song

Oh why dear God did you make it so complex,
To understand the offspring
That result when there is sex?

But there is one monk among us who can tell
How it all works, we feel like jerks
Next to Gregor Mendel.

The answer's in my garden
Where I've planted different peas,
And sprinkled pollen as I please,
Then counted out the progenies.

What did you discover
In your garden with your peas
About those factors we cant see, but
Which explain our family trees.

Here's the news. They comes in twos.
They separate. Its up to fate,
If a sperm or an egg
Has a trait that will dominate.

Here's the news. They comes in twos.
They segregate. Its up to fate,
If a sperm or an egg
Has a trait that will dominate.

And when they join together
My forecasting's most impressive.
Betcha three times out of four I'm right,
Unless they're both recessive.

To what do you owe your success?
To counting and my green thumb.
But where these unseen factors are
Well that I cannot fathom.

Here's the news. They comes in twos.
They segregate. Its up to fate,
If a sperm or an egg
Has a trait that will dominate.

Later on the world was awed, at
What he learned from those pods.
But back then no one hurrahed
Gregor Mendel but his God.
Back then no one hurrahed
Gregor Mendel but his God.
Back then no one hurrahed
Gregor Mendel but his God.

Closing song

Now we’re reading from life’s page
How did we get to this new stage,
To solving what was once life’s mystery.

Would you like to know from whence you’ve sprung.
Would you like to stay forever young.
Would you like to shape your own heredity.

We’re learning how to pull upon the strings
To rewrite the script from which life itself springs

CRACKING THE CODE
GENETIC FUTURES WILL BE FORETOLD
CRACKING THE CODE
GENETIC POWER ABOUT TO EXPLODE
CRACKING THE CODE
GENETIC MYSTERIES TO UNFOLD
CRACKING THE CODE
GENETIC SECRETS WILL BE TOLD
CRACKING THE CODE

Timeline

1694 - Joachim Camerarius recognizes plant sexes
1750 - beginning of plant hybridization
1735 - Linnaeus publishes his first system of classification
1761-67 - Koelreuter does the first plant cross-breeding experiments, with tobacco plants
1827 - von Baer discovers the mammalian ovary
1856 - Mendel begins his experiments with pea plants
1865 - Mendel presents his work at a scientific meeting in Brno
1866 - Mendel publishes his work
1902 - Bateson coins the terms ‘homozygote’ and ‘heterozygote’
1906 - William Bateson coins the term ‘genetics’
1909 - Wilhelm Johannsen coins the term ‘gene’ for Mendel’s factors

Further Reading

1) ‘The Monk In The Garden: The Lost and Found Genius of Gregor Mendel, The Founder of Genetics’, Robin Marantz Henig, Houghton Mifflin Books, 2001.
(It has a very comprehensive bibliography of other books about Mendel and the history of genetics, which can be found at
http://www.houghtonmifflinbooks.com/features/monk_garden/bibliography.shtml

2) ‘Gregor Mendel: Planting the seeds of genetics’, Simon Mawer, Harry N. Abrams Inc., 2006

Websites

1) http://www.houghtonmifflinbooks.com/features/monk_garden/other.shtml
2) http://www.mendelweb.org /
3) http://www.pbs.org/wgbh/nova/genome/her_men.html
4) http://learn.genetics.utah.edu/content/begin/tour/
5) http://www.dnai.org /
6) http://www.dnalc.org/home_alternate.html
7) http://www.biology.arizona.edu/mendelian_genetics/mendelian_genetics.html
8) http://www.esp.org/timeline/  - very good timeline
9) http://www.biology.arizona.edu/mendelian_genetics/mendelian_genetics.html
 - Mendelian inheritance explained
10) http://www.bscs.org/search/index.html?ae=%DF&op=or&q=mendel&x=7&y=8
11) http://www.geneticstv.org/peas_in_a_pod/Peas%20in%20a%20Pod.pdf
12) http://www.esp.org/timeline/

BACK TO TOP

On to Episode 2