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Two Factor Crosses
- round One

### Are the Tansmission Elements Linked?

"Now you have a problem," Brother Joseph said, with just a hint of laughter in his voice. It was a hot summer day and the two friends had been taking their ease behind the greenhouse. Mendel leaned against the bricks, wiped his perspiring forehead with a damp piece of cloth and put down the rake.

"Another one!" he laughed, "what problem do I have now?"

"I've been thinking of your experiments with plant hybrids and some of the results you have been getting. If you are interpreting the numbers properly, your ratios of traits appearing in the F1 and F2 offspring seem to suggest that tiny "transmission elements" are being passed from the parent plants, into the seeds of the offspring, and onto the F1 and F2 offspring."

Mendel was right, no one understood his mathematical reasoning "Yes," Brother Gregory agreed, "I'm sure I am right about that, but the evidence - the ratios of traits being inherited in the offspring - needs a lot of explaining, and I don't know how many people will believe me. You know what people are like when you start using mathematics to interpret biology."

"Don't worry about that now," Brother Joseph told him, picking up two pebbles from the dirt around them. "Let's think about your "transmission elements" for a moment. Obviously, we have no idea about how the elements work or how they control factors such as flower color or pod shape, but assume for one moment that these pebbles represent the elements themselves."

Mendel was so far ahead of his time, that it is probable even he didn't fully understand it implications "You are thinking that the 'elements' are like 'atoms' - small, indivisible particles of information - but we have no evidence for that hypothesis," Mendel interrupted.

"No," said Brother Joseph, "but we can use it as a working hypothesis until something better comes along. Anyway, that is not important, you have a different kind of problem."

"Oh, yes, what?"

linked or un-linked genetic elements "Are your transmission elements all small separate atoms or particles of information, like these pebbles?" he paused for a moment and threw the pebbles of dirt into the air to show that they were separated from each other, then he went on, "Or are they more like beads on a chain; all linked together in a string?" Here he touched his rosary, which he had at his waist.

"Hummm, I see you point," Brother Gregory said, thoughtfully, "but what difference does it make, won't the inheritance data be all the same?"

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"I don't think so," Brother Joseph replied. He took the two pebbles in his hand and placed them on a flat stone on the ground. "Here, these pebbles are two of your elements controlling two traits - flower color and height. In the first hypothesis they are separate 'atoms' of information, independent of one another. Would you agree so far?"

"Of course," Mendel agreed, "here they are in the parent plant." He touched the pebbles on the rock.

"Now, at the time of inheritance, if the elements are not connected in any way, they can be separated, and this one," he picked up the larger pebble, "passed onto one offspring," he put the pebble down near a small weed, "while the second element could be passed to a different offspring." He moved the smaller pebble under a different weed.

linked transmission of linked elements Mendel looked at what Brother Joseph had done, and moved the pebbles with his toe. "Yes," he said at last, "I see the implications. If the controlling transmission elements are not connected to each other in any way, there would be nothing to stop them being inherited by different offspring. Hummm, a very interesting idea."

"But," Brother Joseph hurried on, "if the elements are connected to each other in the parent," here he picked up the two pebbles and kept them touching one another, "then it is more than likely that they will be inherited together, as a pair, when they are passed to the offspring." He put both pebbles under the same weed.

"I agree," said Mendel after some thought. "What you say makes a lot of sense, and I think I know how we can test this hypothesis."

"How?"

"We will follow what happens when we carry out a genetic cross in which two factors, or traits, are inherited at the same time."

"And how will that give us the answer?"

"Wait and see."

the start of the investigation

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Investigation
- what happens when two traits are transmitted?

B rother Gregory wants you to investigate what happens when two traits are transmitted from parent plants into the F1 offspring.

The first part of this investigation requires:

• two parent plants that show versions of two different traits,

• 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,

• counting the number of times a version of a trait occurs in the F1 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 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 One
- to produce F1 hybrids

Select "tall plants" from the Traits menu. This will become "TRAIT ONE" of Parent One.

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

Select "green pods" from the Traits menu. This will become "TRAIT TWO" of Parent One.

Select "yellow pods" from the Traits menu. Click on "TRAIT TWO" of Parent Two.

You should now have two parent plants, both showing different 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 then use the other combinations of traits, and record the results.

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

answer these questions What did you find? Was the pattern of inheritance seen the 'F1 hybrids' consistent with the results see in previous experiments in which only one trait was followed? Where there any patterns in the "F1 hybrids' inconsistent? How do you explain your results?

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Science at a Distance