Читать книгу Introduction to Abnormal Child and Adolescent Psychology - Robert Weis - Страница 84

Our body contains approximately 50 trillion cells, each containing our complete genetic code. The code is written using deoxyribonucleic acid (DNA). DNA is shaped like a twisted ladder, or double helix. The “ropes” of the ladder are made up of sugars (deoxyribose) and phosphates. The “rungs” of the ladder consist of pairs of chemical bases held together by hydrogen bonds. Their structures allow them to combine only in certain ways, forming our unique genetic blueprint. DNA instructs each cell to build proteins, which form the structure and characteristics of the person (Frommlet, Bogdan, & Ramsey, 2016).

Segments of DNA are organized into genes. A single human cell contains approximately 20,000 genes. If the genes in each cell were connected together, end to end, they would be approximately 2 meters long. To save space in the cell, genes wrap around special proteins called histones. Histones are important because they can turn genes “on” and “off” by binding to them in certain ways (Rutter & Thapar, 2015).

www.genome.gov

Genes are organized into strands called chromosomes. In typically developing humans, each cell contains 23 pairs of chromosomes, for a total of 46. Twenty-two of these pairs, called autosomes, look the same in both males and females. The 23rd pair, the sex chromosomes, differs in males and females. Females have two X chromosomes, whereas males have one X and one Y chromosome (Image 2.3).

Most cells form in a process called mitosis. In this process, chromosome pairs split in two and duplicate themselves. Then, the cell divides, forming two cells with 23 pairs of chromosomes each. The resulting (daughter) cells are identical to the original (parent) cell. Each cell contains the entire genetic code, but certain segments of the code are switched on or off, telling the cell its function: to serve as lung tissue, heart tissue, or other parts of the body.

Sex cells (i.e., sperm and ova) form differently, in a process called meiosis. Just as in mitosis, chromosome pairs split and duplicate themselves. Unlike in mitosis, however, chromosome pairs line up and exchange genetic material with each other, a process called recombination. Finally, the recombined chromosomes split into two daughter cells that are genetically different from the parent cell and divide again into sex cells. The result is that the sex cells have slightly different genetic information than the parent cells and only one-half the number of chromosomes. When sex cells combine during fertilization, each parent contributes one set of chromosomes and his or her genetic diversity to the offspring. Many genetic disorders arise when problems occur during meiosis. For example, children may inherit too many or two few chromosomes from each parent. Down syndrome typically occurs when children inherit an extra 21st chromosome during fertilization (Frommlet et al., 2016).

Neurotypical individuals have the same genes; the differences in people’s appearance come from slight variations in these genes, called alleles. For example, all people have genes that determine their hair color. Different alleles influence whether someone will be a blonde, redhead, or brunette. These alleles are usually inherited from parents or develop spontaneously as a genetic mutation (Nussbaum, 2016).

Many people erroneously believe that genes determine behavior. For example, newscasters may incorrectly report that researchers have discovered a gene responsible for sexual orientation or a gene that makes people behave aggressively. Nothing could be further from the truth. Genes merely form a blueprint for the body’s creation of proteins. Some of these proteins partially determine our hair color, eye color, or skin pigmentation. Others influence our height, body shape, and (sadly) our cholesterol. No gene directs behavior. However, genes can lead to certain structural and functional changes in our bodies that predispose us to behave in certain ways (Jaffee, 2016).