Step 1: Supplies
a #160 balloon (#160 balloons are nominally 1" diameter and 60" long when inflated)
a small-diameter pump suitable for inflating #160 balloons
Sources: Qualatex makes reliable #160 twisting balloons and a suitable pump (the #47512 pocket pump.) You can find them sold online.
Step 2: The Word Game
You can insert ud or np anywhere.
On the first line of your empty page, write either ud or np. It's your choice. (We'll always use lowercase letters because the shape of the letters will become important.)
OK, let's apply the same rule again. There is now a choice of three sites at which to make an insertion: at the beginning, between the two letters, or at the end. Mark with an editor's caret the site where you choose to make an insertion. Above the caret, in small letters, write your choice of what to insert, ud or np. Rewrite a clean copy, including the insertion, on the next line.
We now have even more sites for insertion, but let's hold off applying rule one again and see if you have a chance to apply the second rule.
Definition: according to the direction of their strokes, there are two kinds of letters: the up letters, u and d; and the down letters, n and p.
An up letter can shuffle past a down letter.
Putting it another way: if an up letter is right next to a down letter (in either order) they can trade places. See if your word has an up letter right next to a down letter. If it does, you may opt to invoke the second rule. As an editor's mark, make a tie connecting the two letters you will be switching. Rewrite a clean copy, including the transposition of the two letters, on the line below.
(Both of these rules can be made reversible. The first rule is reversible if we also allow ud or np to be deleted anywhere we find them. The second rule reverses itself whenever it is applied a second time to the same letters.)
Play the word game as long as you like. Make as long a word as you like.
Now, make a clean copy of your word at the stage in its evolution when it is just four letters long. That's an ample length for your first attempt at balloon twisting.
Step 3: Reading Your Genotype
Scrutinize the shapes of your letters. Try reading them out loud, from bottom up, using the terms throw/catch and left/right. For example undp reads, "Throw left, throw right, catch left, catch right."
Now you're ready to twist.
Step 4: Twisting Your Phenotype
Squeeze the nozzle of the balloon to prevent deflation; remove it from the pump; stretch it; tie it in an overhand knot; and pull it tight.
By "twisting" a balloon, we really mean, "pinch—then twist." The pinch accurately locates where the balloon will twist. We are going to aim at twisting balloons in consistent three-inch (8 cm) segments that we'll call links. Discover some part of your hand that is three inches wide and refer back to it frequently as a reference when making your links. Consistently twisting three-inch links allows you to twist the phenotype of a 6-letter word from a single #160 balloon.
Pinch at a place 3 inches from the knot of your balloon, then twist the balloon about a full turn. (It is natural to fear the balloon will pop while twisting, but this is quite rare.)
Try letting go of the balloon right now and you will discover a problem. Until the twisting reaches a certain state (coinciding with the completion of a letter) it is prone to untwist if not continuously held. To grasp multiple links it is convenient to fan-fold them in your hand as they are completed. If there is too much balloon for your hand to hold, use your chest to help. (Each time you complete a letter, you can let go.)
Your first letter is always an open one. To make an open letter, twist off three links, then twist the last two links together around their common base. This creates a doubled link. It is these doubled links that we imagine the open letters "throw" and the closed letters "catch."
Between letters, take time to organize your balloon so that the "left" and "right" throw of each open letter is correctly oriented. You will find that the doubled links rather obligingly move to the correct side when pushed.
The "catch" indicated by a closed letter is made by twisting a single link and threading it through the nearest doubled link on the specified side. After threading through, force the neck of the link up against the outer neck of the doubled link—it will snap into place right there almost as securely as if it was knotted. I find it necessary to hold the doubled link open with two hands and "walk" the threaded length through with my fingers. If some other doubled link pops apart while you are threading, don't worry: it can be very easily re-twisted after you finish threading.
At the last letter there is a special rule: stretch the nozzle of the balloon (the place where you started) and wrap it around the twist where you finished. Ta da!
Step 5: Summary
Can these particular mutations ever produce a word with an odd number of letters? or a word ending with an open letter?
Can more than one genotype specify (i.e., map to) the same phenotype?
Why allow only one mutation per generation? (Is it ever very likely that two closely interacting mutations would originate in a single individual?)
If we insert one of these genotypes into another genotype, will the resultant be a word we can twist? (In other words, could the same resultant be obtained by sequential applications of rules one and two?)
For longer words, you will need to stop after completing the last letter the current balloon is able to accommodate. Then stretch and wrap the nozzle of the next balloon around the last twist. Then keep going!