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>> LQ 3.2 How do genes and chromosomes function?

To begin to understand the way that genes impact child development, we begin here with a basic description of genes and how they work within a cell, a field called molecular genetics. Our genetic inheritance begins at conception. A woman’s egg cells and a man’s sperm cells each contain half of our genetic material, organized into 23 chromosomes. When a man’s sperm penetrates a woman’s egg during fertilization, the fertilized egg that results is called a zygote. When fertilization occurs, the chromosome strands from the sperm join those from the egg to form 23 matched pairs in which genes with the same function pair up. As you can see in Figure 3.1, in 22 of these pairs of chromosomes (called autosomes) the two chromosomes look very similar. However, the chromosomes in the 23rd pair can be the same or different. These chromosomes have been named the X chromosome and the Y chromosome. It is the genetic information in this 23rd pair of chromosomes that determines the gender of the conception. Women have two X chromosomes in this pair, so the eggs they produce can only contain an X chromosome in the 23rd position. Because men have an X and a Y chromosome, the sperm they produce can contain either an X or a Y chromosome in this position. When the egg and the sperm unite, it is the man’s contribution of either an X or a Y chromosome that determines the sex of the child. A conception with an X chromosome from both parents is a female. One with an X chromosome from the mother and a Y chromosome from the father is a male.

Molecular genetics: Research focused on the identification of particular genes to describe how these genes work within the cell.

Chromosomes: The strands of genes that constitute the human genetic endowment.

Fertilization: The union of a man’s sperm and a woman’s egg to produce a zygote.

Zygote: A fertilized egg.

T/F #1

When a child is conceived, the mother’s genetic material determines the gender of the child. False

Figure 3.1 Human chromosomes. This image shows a full set of 23 pairs of chromosomes. What indicates that these are the chromosomes of a male? What would be different if these were the chromosomes of a female?

Source: Biophoto Associates/Science Source/Getty Images.

To understand the basic units of genetic inheritance, first look at Figure 3.2. You’ll see that chromosomes are found in the nuclei of each cell. Chromosomes are composed of chains of DNA (deoxyribonucleic acid) that twirl around each other in a double helix that looks much like a winding staircase with a banister. The basic building blocks of life are the four nitrogenous bases that make up DNA: guanine (G), adenine (A), thymine (T), and cytosine (C) (Klug, Cummings, Spencer, & Palladino, 2016). The complete sequence of these bases that make up the genetic instructions in an organism is called the genome. Within the sequence of bases are areas that work together to provide the codes needed to assemble proteins, which are the molecules within cells that control the structure and function of the body. These areas on the chromosomes are called genes.

Genome: The complete sequence of bases that makes up the genetic instructions of an organism.

Gene: A segment of DNA on a chromosome that creates proteins that are the basis for the body’s development and functioning.

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:

 Gotothegrocerystorepickupmilkcomehome

One way to divide this would be as follows:

 Got oth egro

 Ceryst orepick upm

 Ilk comeho me

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

 Go to the grocery store.

 Pick up milk.

 Come home.

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.

 Single nucleotide polymorphism (SNP):

 PICK UP MILK becomes PACK UP MILK

 Change in the number of copies:

 PICK UP MILK becomes PICK UPUPUPUPUPUP MILK

 Insertion:

 PICK UP MILK becomes PICK UP NO MILK

 Deletion:

 PICK UP MILK becomes PICK MILK

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.