To better understand how Instructions relate to traits, let's do two things: first, we'll discuss the instructions used to make a bicycle and compare them to the Instructions living things use; and second, we'll start with simple examples and work toward complex ones.

A few traits of a bicycle are produced by a single instruction. Its color, for instance. If the bicycle assembly manual says "Paint the bicycle blue," the bicycle turns out blue. Similarly, if the manual says "Attach the kickstand to the bottom of the frame," then the bicycle will be able to stand by itself.

Living things have similar traits that are controlled by single Instructions. The color of sweet pea flowers is determined by one Instruction that makes a worker that changes a colorless molecule into a brightly colored pink dye. A few other traits are due to single Instructions. Some organisms that are resistant to antibiotics, pesticides, or herbicides owe these traits to a single worker (or perhaps only two or three) that allow it to deal with the poison. Many penicillin-resistant bacteria cut up penicillin by making a single worker with that job. Other antibiotic-resistant bacteria make transporters that simply pump the antibiotics out of their cells as fast as they get in. Some crop plants have been genetically engineered to resist weed-killers so that farmers can spray for weeds without killing their crop plants. This is also due to a single Instruction: in most plants, the herbicide gums up a worker needed to make other workers, but resistant plants make a worker that isn't affected by the herbicide.

In cases like these, where one Instruction produces a single worker that controls the trait, it's easy to see the connection between Instructions and traits. If the organism has that Instruction, it has that trait. More importantly, if the living thing displays that trait, it must have that Instruction. Going "backwards" from traits to the Instructions that produce them isn't always possible, as we'll soon see.

In many other cases, a small number of Instructions work together to produce a trait. A bicycle's height is set by a few instructions in its manual: the size of the wheels the instructions call for, how long the instructions say to make the frame's tubes, and how tall the instructions say to make the handlebars.

Living things also have traits that result from several Instructions. Some plants add a twist to producing flower color from one Instruction: instead, two Instructions control flower color. In these plants, the two color-determining factors are the presence or absence of a dye-maker Instruction, and the strength of the activator controlling the production of the dye-making worker. Such plants have bright pink flowers only if they both make the dye-maker and have a strong activator to cause the dispatchers to make a lot of dye-makers. If the activators are weaker, the flowers are pastel pink. If the activator is extremely weak, almost no dye-makers get made, and the flowers are white. But it's also possible for these plants to have white flowers if they just don't have a dye-maker Instruction. So, there are two ways for a plant to have white flowers, and it's not possible to tell by looking at the flowers which is the case. In general, while it's possible to tell what traits a living thing has if you know what Instructions are in its Book, it's not possible to tell what Instructions are in its Book by what traits it shows.

Finally, many common traits are controlled by the interaction of many instructions. These are often the traits that are easiest to see, or are the ones people are most interested in. For example, how fast a bicycle can be made to go depends on all the instructions for making its wheels (how big, how wide, the structure of the spokes), its frame, and its gears, as well as the instructions for streamlining. Many traits in living things are analogous. How much grain wheat will produce seems to depend on hundreds of Instructions. Likewise, how long a person will live seems to be due to many Instructions.

Where many Instructions produce a trait, it's all but impossible to determine what these Instructions are. For example, if we have a bicycle that goes 30 miles per hour, we would be hard pressed to say what instructions in its manual make it go that fast. Just by looking at it or riding it around, we can't say, "The bicycle's manual must say: 'To make the bicycle go 30 MPH, use 23-inch wheels and make the gears like so.'"

In fact, once the bicycle is put together, there's almost no way to figure out what the instructions that gave it its traits were. But one way to come close is to get another bicycle with different parts and try swapping the parts between them to see if a trait changes -- speed, in this example. For instance, if changing the handlebars changes the bike's speed, then we can tell that the instructions for making the bike's handlebars had something to do with it. Genetic engineers use this sort of technique to relate living things' traits to their Instructions. (Trying to build a better bicycle or a better living thing without understanding how its traits relate to its Instructions is hard, but people try it anyway using artificial selection.)

All living things display one very complex and very important trait: they make more living things. This trait has been studied closely, and we know many of the Instructions that produce it. We know that of the many, many things organisms do in order to reproduce, none are more important than making workers and copying their Instruction Books. Without workers, Life can't get anything done. Without copying the Instruction Book, living things can't pass their lessons on to their children. When we discussed how the dispatchers read the Instruction Book to carry out Life's work, we left out a lot of details about how it's done. The most important of those details is the nature of the Book itself. Without knowing what it's like, it's hard to understand how it's read and copied.