Hockenbury Discovering Psychology 5th txtbk

372 CHAPTER 9 Lifespan DevelopmentHuman Sex Chromosomes: X and YBiological sex is determined by the 23rd pairof chromosomes, the sex chromosomes.While every egg cell has one X chromosome,every sperm cell has either one X orone Y chromosome. Whether a zygote developsinto a male or a female depends onwhether the egg is fertilized by a sperm cellwith a Y chromosome (XY, resulting in amale) or by a sperm cell with an X chromosome(XX, resulting in a female). Notice thatthe X chromosome (left) is larger and hasmore genes than the Y chromosome (right).This confers some protection against certaingenetic disorders for females. Why? Becauseby having two X chromosomes, females aremore likely to have a normal allele than adisease-producing allele. If a male has a disease-producingallele on his X chromosome,he is less likely to have a normal allele onhis smaller Y chromosome to override it.This is why males are more likely to displayvarious genetic disorders, such as red–greencolor blindness and hemophilia, which theyinherit via the X chromosome contributedby their biological mother.Most of the genes in each person aredormant. Experience affects whichgenes are turned on (and off), andwhen. Thus, the environmentparticipates in sculpting expression ofthe genome.ADELE DIAMOND (2009)phenotype(FEEN-oh-type) The observable traits orcharacteristics of an organism as determinedby the interaction of genetics andenvironmental factors.sex chromosomesChromosomes, designated as X or Y, thatdetermine biological sex; the 23rd pair ofchromosomes in humans.From Genotype to PhenotypeWhile the term genotype refers to an organism’s unique genetic makeup, the termphenotype refers to the characteristics that are actually observed in an organism. Inthe past, a person’s unique genotype was often described as a “genetic blueprint.”The blueprint analogy implied that a genotype was a fixed, master plan, like an architecturalblueprint. Each person’s genetic blueprint was thought to direct andcontrol virtually all aspects of development as it unfolded over the lifespan. But wenow know that the genetic blueprint analogy is not accurate.The first problem with the genotype-as-blueprint analogy is that genes don’tactually control your physical development or behavior. Rather, your genes directthe synthesis and production of particular proteins. In turn, these proteins are thebuilding blocks of all your body’s tissues and functions, which ultimately do influenceyour development and behavior (Marcus, 2004).Second, environmental factors influence the phenotype you display. For example,even if your genotype contains a copy of the dominant “freckles” gene, you will notdevelop freckles unless the expression of that dominant gene is triggered by a specificenvironmental factor: sunlight. On the other hand, if you carry two recessive“no freckles” genes, you won’t develop freckles no matter how much time youspend in the sunlight.Here’s the important point: Different genotypes react differently to environmentalfactors (Baker, 2004; Rowe, 2003). Thus, psychologists and other scientists oftenspeak of genetic predispositions to develop in a particular way (Edwards & Myers,2007). In other words, people with a particular genetic configuration will be moreor less sensitive to particular environmental factors. For example, think of peopleyou know who sunburn easily, such as redheads or people with very fair skin. Theirgenotype is especially sensitive to the effects of ultraviolet light. One person’s frecklefactory is another person’s light tan—or searing sunburn.The New Science of EpigeneticsEach of us started life as a single-celled zygote that divided and multiplied. Eachnew cell contained the exact same set of genetic instructions. Yet some of those cellsdeveloped into bones, hair, eyes, joints, lungs, or other specialized tissues. Why,then, are cells so different? How does the single-celled zygote develop into a complex,differentiated organism with kidneys, eyelashes, navels, and kneecaps?The dramatic differences among the size, shape, and function of cells are due towhich genes are “expressed” or activated to participate in protein production. Putsimply, cells develop differently because different genes are activated at differenttimes. Some genes are active for just a few hours, others for a lifetime. Many genesare never expressed. For example, humans carry all of the genes to develop a tail,but we don’t develop a tail because those genes are never activated.What triggers a gene to activate? Gene expression can be triggered by the activity ofother genes, internal chemical changes, or by external environmental factors, such assunlight in our earlier freckles example. Thus, gene expression is flexible, responsive toboth internal and external factors (Panning & Taatjes, 2008; West & others, 2002).Scientists have only recently begun to understand the processes that guide anddetermine gene expression. This new field is called epigenetics—the study of themechanisms that control gene expression and its effects on behavior and health(Volkow, 2008). For any given cell, it’s the epigenetic “settings” that determinewhether it will become a skin cell, a nerve cell, or a heart muscle cell. Thus, epigeneticsinvestigates how gene activity is regulated within a cell, such as identifying thesignals that switch genes to “on” or “off.”To help illustrate epigenetic influences, consider identical twins, who develop froma single zygote. Each twin inherits exactly the same set of genes. Yet, as twins develop,differences in physical and psychological characteristics become evident. These differencesare due to epigenetic changes—differences in the expression of each twin’sgenes, not to their underlying DNA, which is still identical (Fraga & others, 2005).The study of epigenetic mechanisms is providing insights into how the environmentaffects gene expression and the phenotype. For example, consider the groundbreaking