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Pure Breeding:
The Whole, the Parts,
and the Consistency of the Material


The whole or the parts?

"I must confess," Brother Gregory said sadly, "In two years of hard work I have not been able to repeat any of Herr Professor Gartner's work with plant hybrids. Not in a single case."

"Why is that?" Brother Joseph asked him. He had frequently seen Mendel struggling in his garden and this year had offered to help.

"I suspect," his friend replied, "that he did not diagnose his hybrid types sufficiently. Doktor Gartner characterized his plant hybrids using the habitus, or form, of the whole plant. He placed them into one of three groups, depending on the total outward form of the hybrid plant; gemischte, or blended forms, gemengte, or comingled forms showing some parts of the male and female parent, and decidirte, or biased forms with the prevailing habitus or form of just one parent."

"What was wrong with that?" Brother Joseph asked.

this concept of treating an organims holistically was universal before Mendel "His thinking is wrong," said Mendel, shaking his head in frustration, "Doktor Gartner is treating the plant as a whole entity all of whose parts are fixed and unchanging. He is saying that the total form of the species is indivisible, holistic, and all its features must be considered together when the hybrid is formed."

"That would agree with the ideas of Linnaeus," nodded Brother Joseph, who had read and studied the work of the Swedish botanist and his work on classification. "But you think otherwise?"

"Perhaps it is my training in physics," Mendel grinned, "but I like repeatable observations and experiments. In physics we like the phenomena we are studying to be stable, and measurements taken on one day to be repeatable on the second day."

"But physics is not botany," Brother Joseph objected, "Our pea plants cannot be expected to behave the same way as Herr Joule's tiny units of energy, work and heat."

one of the reasons why Mendel was successful! "Why not!" Brother Gregory almost shouted, "there is no reason why we should not apply the principles of consistency and reproducibility to our determinations in plant hybridization, just as we do in physics."

"So what do you suggest," Brother Joseph said hurriedly, not wanting to upset his friend. "If you don't want to study plant hybridization using the whole plant, what are you going to study?"

"In physics," Mendel told him, "we use the principle of reducing all complex machines, or phenomena, to their simplest parts. Parts so small they cannot be cut down in size any further."

the idea of an "atom" was an important breakthough in physics and chemistry "A-tomos," interjected Brother Joseph, and then seeing the look of incomprehension on his friend's face, he translated the two Latin words. "A-tomos, it means "without-cut". I understand it is the term used by those scientists studying the smallest particles that make up physical matter or substance. They are calling them "atoms"."

"Hummph," grunted Mendel, "very well. If we want to use that term. Herr Docktor Gartner studies hybridizations holistically - the whole plant at once with it's entire form indivisible - where as I want to study plant hybridization - er - atomistically - that is 'one small part at a time'. Do you see?"

"I thinks so," said Brother Joeseph slowly, working his mind around this revolutionary new idea. "Instead of following what happens to the whole plant when a hybrid is formed, you just want to study what happens to a tiny part of the plant - an 'atom' of the plant, if you like."

"Exactly!" said Brother Gregory, triumphantly.

"So how are we going to do that?" Brother Joseph wanted to know.

Mendel did a lot of work making sure that his peas always gave consistent results "For two years I have been working with Pisum sativum, the common pea plant. I have repeatedly crossed 34 different types of plant with each other and studied the viability and consistency of the hybrid forms. From these I have chosen 22 that always give fertile hybrids, and among these I have noted seven factors, or small particles of their form - "

"The atoms!" Brother Joseph interrupted.

"I don't like that term," replied Brother Gregory, "let's not use Latin for everything, how do you say 'a feature' in French?

"I'm not sure, but trait, means a 'line or a stroke', I think."

"That will do," said Brother Gregory, who was famous for not being very good at languages, even Czech. Let's call them - er - traits - how's that?"

"Fine," said his friend, "by all means let us call the smallest part of a plant a trait. But how does that help us? What are the traits you have discovered in these Pisum plants?

"Well," said Brother Gregory slowly, "a - er - trait - had to be a feature you could see easily on each plant, not something that was hidden. It also had to be very clear and not open to question, and it had to have two, clearly distiguishable alternative forms. For example ...

the three traits used in these experiments on plant hybridization, and their alternative forms

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Difference in Color of Unripe Pod
also, coloration of stalks, leaf-veins, and calyx
Alternate forms, or characteristics of the unripe seed pods.
green pods yellow pods
light to dark green vivid yellow

Difference in Stem Length
height to which plants grow at full maturity
Alternate forms, or characteristics of the stem length trait.
tall plant short plant
stem length
1.9 - 2.2 meters
stem length
0.24 - 0.46 meters

Difference in Flower Color
pigmentation in petals of flowers
Alternate forms, or characteristics of the flower color trait.
purple flowers white flowers
purple flowers white flowers

[Historical Note from the Author: Gregor Mendel never used pea plants with different colored flowers in his famous work. I decided to use such an invented trait in this simulation to maintain consistency with examples used in the text book].

the start of the investigation

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- consistency of the material

B efore you can start helping Brother Gregory with his experiments in plant hybridization, you must first investigate the Pisum sativum, (Pea plants) that he has given you. These plants are your starting material. You must find out which of these plants, and which of their traits, you can trust.

the question Which traits will give consistent results during all genetic crosses?

Plant Hybridization
---click here to start the simulation ---

Select "tall plants" from the Traits menu. Click on "TRAIT ONE" of Parent One. The image of that version of the trait will appear. If it does not, check your choice and try again.

Carry out exactly the same proceedure for Parent Two.

You should now have two parent plants that have the trait "tall plants". Check.

If both plants are showing this trait, it is now time to use them as parents. Transfer the pollen from one to the stigma (part of the female structures of the flower), and carry out the fertilization by clicking on the "Collect Peas" box. The embryos of the next generation of pea plants will appear in a pod.

It is now the next spring and you must plant the pea seeds you have collected and see what type of offspring they grow into.

Click on the "Plant Peas" button. The new peas will grow and number and type of offspring will appear in the boxes underneath.

record your results Write down, and record

  1. what you did, and
  2. what results you obtained.

Now click the "Start Again" button, and you will be able to start another genetic cross experiment.

collect more data Still using the "tall plants" trait, repeat this genetic cross many times (at least 10 - 20 times). Each time record the results.

answer these questions What did you find? Were "tall" plants consistent? Did they always give offspring that were 100% tall? Can you depend on this trait?

now try a different trait Make sure you have clicked the "Start Again" button.

Choose the alternative version of this trait, "short plants", and create two parents, both of which are short.

Carry out a series of genetic crosses (10 - 20), as before, record the results and then answer the same series of questions. Were the "short plants" consistent?

Brother Gregory calls any parent plant(s) that consistently give only one type of offspring

Pure Breeding

Which traits of these pea plants are consistently "pure breeding"?

special peas Before you joined the investigation, Brother Gregory performed some genetic crosses and produced some "Special Peas". These are at the bottom of the simulation. You will find them useful at various times in your own investigations.

There are two "pure breeding" seeds, including a "pure breeding tall plant". Use this plant in a series of genetic crosses with other tall plants, and see what kind of results you get.

Make sure you have clicked on the "Start Again" button, then click on the chosen "special pea". It's traits will appear as one of the parents. If you click once, only one parent recieves these traits, but if you click twice, both parents have this trait.

Were the results consistent when pure breeding tall plants were one, or both parents?

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Science at a Distance
© 1999 Professor John Blamire