Investigation
Mendel

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Pea
Do Traits Blend?
pangenesis, blending or dominance


Pea

Inheritance by Pangenesis?

Brother Joseph walked across the monastery garden towards his friend, who was sitting with a pile of books on a shaky bench by the greenhouse. As he got closer he saw that a worried frown was spread across the face of his fellow monk.

"What's the problem?" he asked, sitting down and picking up one of the volumes Mendel was reading.

"We are living in the second half of the nineteenth century," Brother Gregory said with some exasperation, taking off his glasses and wiping them with a cloth, "and yet we still have no idea how the shape and form of a living organism is determined. Look here!" He pointed to a book on his knees.

the writings of the Hippocratic corpus in the 5th century BC describes sexual reproduction in this way Brother Joseph saw that Mendel was reading the collected works of Hippocrates, published in the 5th century BC.
"Its quite clear from this that the Pre-Socratics considered pangenesis, to be the main theory of inheritance. They thought that all parts of an adult organism produce gemmules, which collect together in the male semen and then contribute to the formation of the next organism."

"Here," and he quoted from the Hippocratic corpus, "The offspring resembles its parent because the particles of the semen come from every part of the body."

'soma' is the body
'germ' is the gametes
"Yes, I remember reading that once," said Brother Joseph, "if they are right, then the bodies of adult organisms - the somatoplasm - gives rise, through the process of pangenesis, to the specialized germplasm of the semen, which, when placed in the female organism, grows by morphogenesis into more somatoplasm"

Mendel was shaking his head. "I know that pangenesis is a popular theory of inheritance," he said, "but, like Aristotle, I feel that there is something wrong there."

"But what? Even the great Aristotle had great difficulty in refuting the idea that the adult form contributes significantly to the form of the next generation."

Aristotle argued against pangenesis, but could not devise a more convincing theory "Yes, his arguments were at best weak ones, but I just feel that he was right. For one thing, both parents, male and female, contribute aspects of their form to that of their children, not just the male. Also, the way I see it, the reproductive organs don't just passively collect particles or gemmules from around the body and then transmit them into the offspring."

"What happens then?"

"I don't know," Mendel said in frustration, "but I have in mind an experiment that will help answer part of the question - at least!"

"What question?" asked Brother Joseph, suspecting that another investigation into the mechanism of inheritance was about to begin.

this is new
no one had considered this idea before


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"I suspect," Mendel said, "that each of the traits seen in my pea plants and in the plant hybrids, is produced through the action of a "transmission element", which, somehow controls the development of the form of that trait in the somatoplasm, or body of the offspring."

"Very well," said Brother Joseph slowly, "and you would argue that both parents contribute one of these "transmission elements" to their offspring."

"Yes."

"But what happens to them once they are in the body of the offspring? Do they both contribute equally? Is the male 'element' stronger than the female 'element'? Do they fight one another? Are their contributions separate, or do they blend together to produce a mixture of the two factors?"

"Stop, stop," laughed Brother Gregory, "these are all good questions, but let us address them one at a time, starting with the last question first. Do the controlling transmission elements blend together or remain separate in the body of the offspring?"

"How can a genetic cross tell you that?"

"Let's find out"

the start of the investigation


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Pea
Investigation
- do traits blend or not?


B rother Gregory wants you to investigate the properties of the Pisum sativum, (Pea plants) he has given you. He wants you to find out if alternative versions of a trait blend together in an offspring, or if the different forms of a trait remain independent and distinct.

If he is right, and the form of a trait is controled by a 'transmission element', an offspring recieves one 'element' from its male parent and a second 'element' from its female parent.

Once in the body of the offspring, these 'elements' direct the development of the traits they control. There are two possibilities for what happens next.

  • the male and female elements blend together like two cans of paint, and the trait they produce is a mixture, or 'blend', of both.
    This could be called the theory of blending inheritance.

    Or,

  • the male and female elements remain separate, and the trait they produce is the product of only one of the elements - the dominant, or 'stronger', one of the two.
    This could be called the theory of non-blending inheritance.

Are the 'transmission elements' proposed by Brother Gregory fluids, in which case they can mix themselves together, like two different colored liquids in a cup, or are they solid particles that always remain separate and distinct from one another?

the question Do different versions of a trait blend together or remain separate during genetic crosses?

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


Select "purple flowers" 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.

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

You should now have two parent plants, one that has the 'purple flower' version of the trait, and one that has the 'white flower' version of the same trait. Check.

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 "purple flower" and "white flower" versions of this trait, repeat the genetic cross many times (at least 10 - 20 times). Each time record the results.

answer these questions What did you find? Did the flowers of the offspring ever show a 'blended' color (such as pink)? What kind of results did you get; were flowers of the offspring always all one color? How do you explain your results?


now try using some of the other traits Make sure you have clicked the "Start Again" button.

Choose the version of a trait called "green pods" as one of the parents in a genetic cross. Parent Two should be the other version of the same trait, yellow pods.

Carry out a series of genetic crosses (10 - 20), as before, record the results and then compare the results you obtain with those from the first version of these crosses. Were the consistent? Were the results of the two sets of crosses the same or different?

What patterns of inheritance did you see for these two trait?

Which theory (blending inheritance or non-belnding inheritance) do your results support? Why?



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Science at a Distance
© 1999 Professor John Blamire