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Two Factor Crosses
- round Two

### What do all these numbers mean?

"Now what?"

"Now it gets complicated," Brother Gregory replied. "We have these three sets of F1 seeds, and by carrying out a second set of crosses to produce the F2 offspring, we should be able to calculate what kind of "transmission elements" were contained in the original parents."

"You are going to have to explain that one," Brother Joseph said with a rueful laugh, "I'm not sure I understand how you can deduce the nature and types of transmission elements in the parents from what happens in the F2 offspring."

"Very well, let's see if I can make it more clear," Mendel said, scratching his thinning hair. He picked up a pen and pulled a sheet of Abbot Napp's best notepaper towards him. Money troubles constantly plagued the monastery, and the Abbot was not thrilled with his monks taking and using his best linen paper for their experiments.

A hypothesis is a guess or speculation that tries to propose a solution to a problem "First, we must state our hypothesis unambiguously - how about 'All transmission elements are linked to one another' - or it's converse - 'All transmission elements are separate and distinct'."

"Which do you favor?"

Any hypothesis is as good as another until empirical evidence eliminates it from consideration "The second. I've been studying the behavior of these controlling elements for some years now, and they certainly behave as if they are separate. At least the ones I've been following seem not to be linked to one another."

"In that case, let's use the hypothesis that states each transmission element is separate. What do we do next?"

Brother Gregory began to write on the paper, and Brother Joseph watched him as he slowly and pedantically printed out a series of letters.
"These," said Mendel, pointing with his pen, "represent the elements themselves as found in the parent plants."

using letters to represent his "transmission elements" was another of Mendel's good ideas

"I have used a capital letter such as 'A' to stand for two things; the transmission element, and the version of the trait it controls. For example, the letter 'A' could represent the transmission element that controls the height to which the plant grows, and the fact that it is a capital letter indicated that it is the version of the trait which produces 'tall' plants. "Here I have indicated the second element. I've called it 'B' to represent a transmission element that controls the pod color trait, and the fact that it produces the 'green pod' version of that trait."

"I understand that part," Brother Joseph told him. "In your hypothesis, you see transmission elements as small particles, therefore it is permissible to represent them as letters of the alphabet. On the other hand, if the elements had been indistinct fluids, for example, you would have had to use a different way of representing them. Possibly using blocks of color, which would have allowed you to blend them together, or as numbers so that you could add them together."

using letters defined the 'elements' as particles, not 'liquids'

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"Exactly. The fact that we can write out these 'elements' as letters defines, in a way, their particulate character, and the fact that the can be found in one of two distinct states, with no intermediate forms."

"What ever these 'transmission elements' turn out to be," said Brother Joseph, "they will be found in only one of two definite states; on or off, red or white, tall or short, green or yellow. They will never be found to be some mixture or some vague intermediate between these states."

"I think you are right, but that will be for future scientists to determine," said Brother Gregory, working with his pen once more. "Now to our hypothesis of unlinked transmission. Look at what I have just written on the paper."

transmission of linked elements "In case Number One, I've connected the 'A' and the 'B' elements as if they were linked. In this form they would travel together into the body of the F1 offspring and subsequently into the body of the F2 offspring. Thus, in this generation, we would expect that any plant showing the 'A trait' would also show the 'B trait'."

"All the F2 plants that were 'tall' would also have 'green pods' in that case," Brother Joseph concluded.

"But only if the elements were linked!" Brother Gregory insisted. "If the elements are unlinked, then we get a different result."

transmission of unlinked elements "Here I have drawn the elements unconnected. As you can see, when they are not joined, they can separate and thus be transmitted into different offspring. An F2 plant that receives the 'A trait' may not also receive the 'B trait'."

"It might or it might not, the results would be a mixture of plants with both traits, or either one of the traits," said Brother Joseph. "That is testable, is it not?"

"It is indeed," exclaimed Brother Gregory, grabbing a bag from the table. "Here are some F1 seeds we collected from a series of two-factor crosses last year. Let us plant them and see what kinds of inheritance patterns we see in the F2 generation."

the start of the investigation

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Investigation - continued
- what happens when two traits are transmitted?

B rother Gregory wants you to investigate what happens next when two traits are transmitted from the F1 plants into the F2 offspring. This is the second phase in your analysis, and the second round of genetic crosses needed for this investigation.

The second part of this investigation requires:

• performing a genetic cross using the F1 plants produced in the first genetic crosses to produce the F2 hybrids,

• recording the form(s) of the trait seen in the F2 generation of plants,

• 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.

• determining if the results are what should be expected from the original hypothesis.

the question still remains What patterns of inheritance are seen during genetic crosses, that involve more than one trait? How can these results be interpreted?

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Two Factor Cross; round Two
- to produce F2 hybrids

Go to the Special Peas and select "an F1 seed from a tall/short : purple/white cross". Click on it once, and this will become "TRAIT ONE" and "TRAIT TWO" of Parent One, or possibly Parent Two, it doesn't matter which.

Do exactly the same thing (click on this special pea), and the second parent will receive the same traits.

You should now have two parent plants, both showing the same versions of two different traits.

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 record the results.

This is now the data for the 'pattern of inheritance' seen as two different transmission elements are passed from the F1 plants into the second generation of hybrids, the F2 hybrids.

answer these questions What did you find?

What was the pattern of inheritance seen the 'F2 hybrids'?

What ratios did you find (a) for each trait separately, and (b) for each different phenotype of hybrid seen? What can you deduce from these results?

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Testing the F1 hybrids

Brother Gregory also wants you to carry out some 'test crosses' or 'back crosses' on the original F1 hyprids. These special crosses will help you determine which 'transmission elements' are being carried by these plants.

As before, click on the "Special Pea" that will give you the F1 hybid for one of the parents. This plant should have the two traits of "tall" height and "purple" flowers.

For the other parent choose the other version of one of these traits (e.g. "short plants") from the regular "Traits" menu.

You now have two parent plants, carry out the cross, collect and plant the seeds. What results did you get?

Repeat this type of genetic cross with the other version of the other trait (e.g. "white flowers") from the regular "Traits" menu.

The results of these types of crosses should tell you which combination of "transmission elements" were being carried by the F1 hybids.

Why?

For consistency, you should also repeat the above types of genetic crosses with the other kinds of F1 hyprid "special peas". Do you get exactly the same results?

answer this question Were your results consistent with Brother Gregory's hypothesis that the 'transmission elements' were unlinked? Why or why not?

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Science at a Distance