How long before genetic sequencing is able to tell us exactly how our children will look and act?
Category: Biology Published: April 5, 2013
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Assistant Professor of Physics at West Texas A&M University
Genetic sequencing will never be able to perfectly predict an organism's appearance and behavior for the simple reason that genes are not the only thing that determine biology. There are three basic factors that determine the nature of an organism: 1) genetics, 2) unconscious environmental factors, 3) learned and conscious behavior. Your genes only directly determine basic biological properties such as the number of eyes, the shape of bones, and gender. Your genes do not determine exactly where every mole will grow, how obese you will be, or which baseball team you will route for. The textbook Developmental Biology by Russ Hodge states, "Development is constantly influenced by subtle events in the environment, and many of an organism's features are not determined by genes. For example, every human embryo is programmed to develop fingerprints, but the precise patterns they form are determined by tiny, random events that vary from individual to individual."
Unconscious environmental factors affected who you are even before you were born. Many birth defects are caused by the exposure of the fetus to a harmful substance or by the pregnant mother providing insufficient vitamins. A damaging blow to a boy's gonads can, if untreated, reduce the levels of testosterone and thereby drastically influence the boy's physical appearance upon hitting puberty. Beyond the influence of nutrient levels and exposure to chemicals, the simple randomness of life affects your biological make-up. Identical twins have almost the exact same set of genes, but they don't have every hair in identically the same place. While genes dictate that there is (or is not) hair on your head, they do not dictate the exact location of every hair. Rather, the complex, random nature of chemistry plays a role. As a metaphor, consider the case where two identical cups of water are poured in exactly the same way onto two identical floors. While the average shape, depth, and size of the resulting puddles will match well, microscopic features of the puddles will be quite distinct. The individual molecules of both puddles will definitely not be in the exact same locations. Aside from physical environmental factors, unconscious psychological factors also play a role. A girl that grew up around pet snakes will most likely not shriek in terror as an adult at the sight of a snake.
Learned and conscious behavior also play a large role in who you are. Children grow up to speak very much like their parents. A boy who has a father that loves baseball and engages him in the sport will likely grow up to do the same with his child. While innate programming does tend to lead boys to plays with cars and girls to play with dolls, some of the child's gender behavior is learned from the prevailing cultural norms. Parents, teachers, friends, extracurricular activities, religious routines, books, and movies all play a role in shaping us. The act of learning not only affects your mind and emotions, but can also influence your body. For instance, a girl who does gymnastics has a more developed upper-body muscle structure than other girls her age. An avid guitar player develops finger calluses. Furthermore, humans are not robots that are programmed by genes and parents. Rather, as conscious, intelligent creatures, you also behave certain ways because you want to, despite the influence of your genes and your environment. For instance, alcoholism is often passed down continually through a family's generations because of its strong genetic and environmental links. But some children born into such a family line grow up and decide to break the generational cycle of alcoholism. They use their conscious choice to fight against their inherited and acquired alcoholic tendencies because they are aware of the harm it causes. Conscious choice opens the door for a person to learn new behavior patterns despite their genes and environment. Parents of identical twins discover that the twins develop different personalities, mannerisms, interests and hobbies after years of different conscious choices.
Even if your genes were the only thing that determined your appearance and personality, there would still be variability between people with identical genes. The reason for this is that your genes do not determine your biology in a strictly linear, forward fashion. Your body is not built as an exact replica of a blueprint contained in our DNA. Human biology is both a coded pattern and an emergent phenomenon. If you drop a number of identical balls through a grid of pegs known as a Galton box, a bell curve results. The bell curve is a purely emergent phenomenon, meaning that each ball does not contain a map of the final bell curve. The bell curve emerges after each ball interacts with the pegs. In a similar way, your genes only determine who you are in so far as their information is modified by the process known as gene expression. The gene expression process involves many steps, and there are feedback loops among these steps. This means that external factors can modulate how your genes get expressed. A journal article titled "RNA regulation of epigenetic processes" by J.S. Mattick and collaborators states, "It is also evident that RNA editing is a primary means by which hardwired genetic information in animals can be altered by environmental signals". In an extreme example of genetic expression, if a person's cells are insensitive to sex hormones, a person with male DNA (XY in every cell) can end up looking and feeling like a woman, a condition known as androgen insensitivity. Because of the effects of unconscious environmental factors, learning, free choice, and non-linear gene expression; genetic sequencing will never be able to completely predict a child's appearance and behavior.
One other item worth noting is that "genetic sequencing" is not the same as "gene decoding". Sequencing simply records the positions of the atoms along the length of the DNA but does not derive meaning from the sequence. The genes still have to be decoded in order to understand the effect of each gene. The complex decoding process is being carried out by a large field of biologists and is currently incomplete. The situation is like discovering a recipe for ancient Egyptian bread carved into the walls of a recently unearthed tomb, and the recipe is in an unknown script. Just because you have copied down the figures on the wall into your notebook does not mean that you have any idea how to read the recipe, let alone properly bake Egyptian bread according to the recipe. Complete sequencing of the human genome has been accomplished. Complete decoding has not.