Mendel's Seed Shape Trials

Now we will see how well YOU do in following Mendel's footsteps.

Among Mendel's pea collection he noticed two varieties that differed only in the shape of the seeds in the pods. One variety had round seeds and the other had wrinkled seeds. The round seeds have higher starch content and so are used to make pea soup. The wrinkled seeds have higher sugar content and so are used as a fresh, frozen, or canned vegetable. Of course in Mendel's day there was no such technology as frozen vegetables...so at the abbey they had fresh peas of the wrinkled type in the summer for fresh peas and stored the starchy round types for soup in the winter.

Here is a diagram of the two varieties that Mendel started with. We call these two the P (parental) generation:

P    


Round

  x  


Wrinkled
These two appearances, round and wrinkled, are called the two parental:
Remember, Mendel's stock plants are true-breeding thanks to their long-term obligate self-crossing for many generations. This means that the two parental peas shown above are:

So you cross these two parentals, you pop open the pea pod that results, and you see the following F1 offspring:

F1:    


Round

Round

Round

Round

Round
What do all these round peas tell you about dominance?
The dominant allele is:
What do all these round peas tell you about the symbols we should use for our symbolic logic?
The symbols we should use are:

One of the strange situations in biology is that sometimes we use a Latin name to describe a particular feature...this was particularly true in Mendel's day. The Latin name for wrinkled is rugose. So in spite of the fact that you were directed to a correct choice in the question just above, in fact for pea geneticists the correct symbols for these seed shape alleles are R/r. Please just remember that, in this case, the r stands for rugose...not round!

With that in mind, the genotype of the round parent is:
The genotype of the wrinkled parent is:
The genotype of the five round F1 peas is:

On to the Next Generation

Now it is time to make the F2 generation. We do this by mating two F1 offspring from above. Remember of course, that all we have to do is plant those peas from that first pod, wait for the plants to reach flowering stage, and let them self-pollinate in the garden.

Here is how we would document the cross:

F1:    


Round
Rr

  x   F1:    


Round
Rr

Next we predict what kind of outcome we are going to expect from the cross above by filling in the Punnett square:

possible sperm of Rr parent
possible eggs
of Rr parent

Now, if you got all the entries correctly placed above, you should be predicting an F2 generation composed of 3/4 Round and 1/4 Wrinkled. Of course this is only a prediction. But you plant your F1 seeds and the first pod you open gives you these five actual progeny:

Actual F2:    


1.
Round

2.
Round

3.
Round

4.
Round

5.
Wrinkled

Now these results are not exactly in a 3:1 ratio of round:wrinkled. But we have a small family here. Small families almost never give expected ratios perfectly. The element of chance rears its ugly head! This important fact makes human genetics difficult to study; the families are almost always very small making firm conclusions difficult, if not impossible!

Testing the Hypothesis

To really test out what we have above, we will follow what Mendel did. He tested his hypothesis by determining the genotypes of each of his F2 offspring. In your case you need to identify the genotypes of the F2 peas numbered 1-5 above.

Which one of the F2 peas has a genotype you are sure of?
What is its genotype?

The other four F2 progeny (#1-#4) are all round. We cannot be sure of their genotypes yet. To determine their genotypes we will have to test-cross them. To do a test cross for each of these, what do we mate it with?

A plant with a genotype of:

So now we are going to do our four test crosses to determine the genotypes of F2 number 1 through F2 number 4 by mating them with F2 #5.


 

Here are the results of the test cross of Round F2 #1:


F2 #1
Round
R?

  x  


F2 #5
Wrinkled
rr

TestCross Progeny:    


Round

Round

Round

Wrinkled

Wrinkled

Wrinkled

Wow! You got exactly half-and-half. Maybe you should buy a lottery ticket today...are you feeling lucky?

What is the genotype of Round F2 #1?

 

Here are the results of the test cross of Round F2 #2:


F2 #2
Round
R?

  x  


F2 #5
Wrinkled
rr

TestCross Progeny:    


Round

Round

Round

Wrinkled

Round

Round

This time you did not get half round and half wrinkled among the progeny. In fact with just one wrinkled maybe the interpretation is more difficult...

What is the genotype of Round F2 #2?

Actually this decision was quite easy. It only took the one wrinkled pea (rr) among the offspring offspring to determine that the Round F2 was heterozygous. You cannot get a wrinkled offspring if the Round F2 was homozygous! Always remember, with such a small family (only six offspring), expected ratios rarely come out exactly!


 

Here are the results of the test cross of Round F2 #3:


F2 #3
Round
R?

  x  


F2 #5
Wrinkled
rr

TestCross Progeny:    


Round

Wrinkled

Wrinkled

Wrinkled

Wrinkled

Wrinkled

This time you did not get half round and half wrinkled among the progeny either. In fact with just one round maybe the interpretation is more difficult...

What is the genotype of Round F2 #3?

Actually this one was quite easy. It only took the one wrinkled pea offspring to determine that Round F2 #3 was heterozygous. Having five of them is just insurance that you are sure Round F2 #3 is heterozygous. The failure to get a perfect 50:50 ratio in the offspring is explained by the small family size. But does the fact that you got more wrinkled in this cross mean that...

wrinkled is now dominant to round?

Indeed double-recessives can be the most common on earth. This is true in many cases. Certainly among peas on earth, the vast majority have wrinkled seeds. They are double-recessive...but also very common. In human examples, the simplest example for hair color involves several alleles; but the most common hair color in humans world-wide, black, is produced by the double-recessive. Common does not necessarily mean dominant! As a graduate of my course, I want you not to be a victim of the misconception that dominant means common!


 

Here are the results of the test cross of Round F2 #4:


F2 #4
Round
R?

  x  


F2 #5
Wrinkled
rr

TestCross Progeny:    


Round

Round

Round

Round

Round

Round

This time you got all round offspring.

What is the genotype of Round F2 #4?

So through this process you have solved the problem assigned to you. You have determined the genotypes of the five F2 pea seeds. You determined that one was homozygous dominant, three were heterozygous, and one was homozygous recessive. As you may remember, you were expecting a 1:2:1 ratio. Your observed 1:3:1 ratio is a pretty good fit for a small family.

Congratulations! I think you are now ready to do the lab in class this week.

 

 

This page © Ross E. Koning 1994.

 

 

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Send comments and bug reports to Ross Koning at koningre∂gmail⋅com.