Child Development From Infancy to Adolescence. Laura E. Levine

      Figure 3.2 What are genes?

      Source: ttsz/Getty Images Plus.

      When the Human Genome Project completed mapping all the genes that make a human being, one of the biggest surprises was that the total came to only about 20,500 genes, not the 100,000 or more that researchers had expected to find (NHGRI, 2016a). If the number of genes alone governed how complex and sophisticated we are as a species, then it would appear we are only a little more complicated than mice or fruit flies. Even rice has about 38,000 genes (NHGRI, 2013a). Something other than simply the number of genes must account for the large differences between species.

      T/F #2

      Each human being has hundreds of thousands of genes that make him or her a unique individual. False

      We also now know that only 2% of the entire human genome is needed to make up those 20,500 genes (Plomin, 2013). The other 98% of the genome was long considered to be “junk” that had no effect, although this idea is changing rapidly. Using whole-genome sequencing, researchers have found that some of the areas formerly considered to have no function actually play an important role in regulating the way the 20,500 active genes are expressed (Plomin, 2013; Zhang et al., 2013).

      Although most of the genes in the human genome have been identified, we are still a long way from knowing exactly what each actually does (NHGRI, 2016a). Think of genes as a set of instructions, such as you might get when you purchase a new cell phone with many unfamiliar features. If you read and follow the instructions, you will be able to perform all the functions built into the cell phone. However, if you don’t, you may never use some of them. In the same way, genes may be either read or ignored, with consequences for the aspects of human structure and functioning to which they are related. You will learn more about the ways in which genes are either expressed or their instructions remain silent later in this chapter.

      As we have said, a gene is made up of the four bases (G, A, T, C) that come together in a way similar to the way that words come together to form a sentence. A sentence can contain all kinds of information and instructions depending on how the words in it are arranged. Likewise, a gene contains information and instructions for the body to make a protein (NHGRI, 2015a). The trick is to identify which sequences of Gs, As, Ts, and Cs constitute a gene. To better understand this process, look at the following sentence:

      One way to divide this would be as follows:

      We all know this is wrong and makes no sense. You would really divide this sequence of letters into three meaningful instructions:

      In a similar way, scientists have taken sequences of bases such as ATCATCTTTGGTGTT and figured out which sequences give clear instructions to produce proteins.

      All human beings share 99.5% of their genome; the remaining one-half of 1% is what contributes to our differences (NHGRI, 2012b). Changes that can occur in the structure of genes, called mutations, also differentiate among human beings. We can inherit mutations from our parents, but we each have approximately 175 new mutations that are unique to us as individuals (Plomin, 2013). Most are inconsequential because they make no difference to our growth and development, but evolution of the species depends on the occurrence of mutations that turn out to be adaptive and therefore help individuals survive in their environment. When individuals survive because of particular adaptive mutations, they hand down the mutations to their children (Wolters Kluwer Health, 2009). However, other mutations are harmful and have been linked to diseases such as cystic fibrosis and disorders such as autism and schizophrenia (Plomin, 2013).

      Mutations: Changes in the structure of genes that occur as cells divide.

      Mutations can occur in several different ways. A mutation can consist of variations in a single nucleotide, which is a combination of one of the four bases with a phosphate group and a sugar molecule. This type of mutation is referred to as a single nucleotide polymorphism, or SNP (Grigorenko & Dozier, 2013). Mutations can also consist of large-scale changes in the order of nucleotides in a gene. Groups of nucleotides can be inserted or deleted, or there can be variations in the number of copies of groups of nucleotides that appear in a gene. These various mutations can affect between one and thousands of nucleotides. Let’s return to our sentence analogy to illustrate the nature of these types of mutations. In each type of mutation described below, you can see that the initial instruction has been changed in some important way so the outcome will be quite different:

      Nucleotide: An organic molecule that contains one of the four chemical bases with a phosphate group, and a sugar molecule.

      With this very basic understanding of how genes operate within the cell, we now discuss how genes are translated into our physical appearance and behaviors.