Investigation
Mendel

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Pea
Patterns of Inheritance
one trait, two generations


Pea

What do the numbers mean?

"I'm convinced I am right," Brother Gregory said firmly as he rubbed ink from his fingers across his chin, "These numbers confirm it. Look!"

Brother Joseph looked, and saw several sheets of the monastery's best note paper spread out on Mendel's desk. On each sheet his friend had written, in neat Germanic handwriting, columns of figures, interspersed with occasional blotches of ink from a faulty pen.

"What are they?" Brother Joseph asked.

"Last year's results on Pisum hybrids," he was told, "I'm doing the numbers. This column is the experiment number, the next is the number of fertilizations, this is the number of plants, and here we see the color of pod results in the first filial generation. I'm calling it the F1 generation."

the word 'filius' means 'son' in Latin Brother Joseph looked over his friend's shoulder and ran his finger down the last column, reading off some of the numbers. "428, 152, 580, 73.79 percent, 26.21 percent - " his voice trailed off. "What does this all mean?"

Mendel looked up at him and rubbed more ink into his face. "You are looking at some of the F2 results," he said, shuffling more papers. "Here are the F1's"

"Wait, wait," Brother Joseph laughed, "wait a moment, I'm getting lost. What are all these numbers and what do they mean?"

raw data often has to be interpreted, Mendel did this very well "Well," said Mendel slowly, "they are only preliminary, and will have to be repeated, but these are the results of my first experiments into plant hybridization and a possible mechanism for inheritance."

"Using your ideas about small particulate "transmission elements" that control the development of the plant body and form?"

"Yes. If I'm right, these numbers confirm that each parent plant in a genetic cross, has ... but why am I telling you all this? We need more results before I can be sure. We also need to test more factors and more traits to see if they all behave in the same way."

"You want us to do more crosses, and gather more data?"

well tested starting material is important in these experiments "Yes, that would be excellent. I've started some hybrids already. These peas," and he picked up a bag from his desk, "are 'pure breeding' forms that I have extensively tested over the last several years. They always give consistent results and they will be our starting material. These," and here he picked up a second bag of seeds, "are the seeds of an F1 hybrid. At least part of our work must be to cross these with at least three types of other plants."

"Three?" Brother Joseph asked, "why three other types?"

this is new
no one had considered this idea before


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"Because it is not always obvious by looking at the form, the soma, of a plant, which transmission elements it contains. It is frustrating, but at least two kinds of plants, both of which look the same and have the same outward form of a trait, may hold different combinations of transmission elements."

"How can that be? If these 'elements' control the form of a trait, shouldn't you be able to tell what 'elements' are inside a plant by what it looks like on the outside?"

"You might think so, and some of the time you can, but I suspect that there may be cases where that would be impossible."

"This makes it difficult."

"More difficult, but not impossible. Let's do the experiments and find out!"

the start of the investigation


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Pea
Investigation
- what are the patterns, and what do they mean?


B rother Gregory wants you to investigate the patterns of inheritance seen in his pea hybrids as the traits are inherited through two generations (called the F1 and F2 generations).

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. It should be possibile to determin what 'elements' each offspring inherits by the numbers, and ratios, of the offspring showing those traits.

Following a 'pattern of inheritance' requires:

  • two parent plants that are 'pure breeding',

  • performing a genetic cross using these plants to produce the F1 hybrids,

  • recording the form(s) of the trait seen in the F1 generation of plants,

  • using some of these F1 plants as parents in a second series of genetic crosses to produce the F2 hybrids,

  • counting the number of times a version of a trait occurs in the F2 hybrids,

  • calculating the ratios of plants showing one form to those plants showing the alternate form of a trait.

the question What different patterns of inheritance can be seen during genetic crosses, and how can these patterns be interpreted?

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



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First Genetic Cross - to produce F1 hybrids

Select "pure breeding tall plant" from the Special Peas menu. This will become "TRAIT ONE" of Parent One (or Parent Two).

Select "short plants" from the Traits menu. Click on "TRAIT ONE" of the other Parent.

You should now have two parent plants. You know that the 'tall' plant is 'pure breeding', but what do you know about the 'short' plant? Is it 'pure breeding'? How would you know? (Hint: haven't you checked this already?).

Carry out the genetic cross by clicking on the "Collect Peas" box, collecting the seeds and then clicking 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.

Repeat this type genetic cross experiment several times, and then use the other 'special pea' that is 'pure breeding for the purple flowered plant'. The other parent in this cross should hold the 'white floweres' trait.

This is now the data for the 'pattern of inheritance' seen as Mendel's transmission elements are passed from the original parent plants into the first generation of hybrids, the F1 hybrids.


Second Genetic Cross - to produce F2 hybrids

genetic crosses involving the F1 hybrids One of the F1 hybrid plants (produced in the first round of genetic crosses) must be one of the parents in the second round of genetic crosses.

To do this, click on the 'special pea' called "an F1 seed from a tall/short cross" and this version of the trait will become one of the parents.

You can now cross this F1 hybrid plant with three other types of plant:

  1. another F1 hybrid plant from a tall/short cross. To do this click again on that 'special seed'. Both parents should now be these F1 hybrids.

  2. a 'tall plant' selected from the Traits menu.

  3. a 'short plant' selected from the Traits menu.

Repeat these genetic cross many times (at least 10 times each). Record all the results.

answer these questions What did you find? Which of the 'F1 crosses' produced consistent results? Which of the "F1 crosses' produced inconsistent results? How do you explain your results?


calculation of ratios Pea The Raw Data

Brother Gregory was able to make sense of his raw data because of the way he interpreted the relationship between the sets of numbers.

In one of his famous experiments he obtained the following results for a cross of two F1 plants to give the F2 offspring:


Total F2 offspring = 1064

Lenght of Stem  -  787 tall,  277 short

What does this raw data mean?


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Interpretation -

  1. Percentages: Mendel calculated the percent of his F2 plants that were tall, e.g.

    percent tall = 787/1064 x 100 = 73.96%

  2. Ratios: Mendel calculated the ratio of tall/short plants, e.g.

    Ratio (tall/short) = 787/277 = 2.84 : 1

When interpreted this way, the variation in the raw numbers seen from one experiment to the next, suddenly vanishes! In the F2 generation, the percentage of tall plants (and the percentage of short plants) becomes constant (or almost so), and the ratio of one version of the trait to the other version also becomes almost constant!!


Pea Analyze and Interpret Your Data

For all of the genetic crosses you have carried out in this investigation, calculate the percentages of offspring that show one trait or the other, and also calculate the ratios of one trait to another.

What results did you get?



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Science at a Distance
© 1999 Professor John Blamire