Hockenbury Discovering Psychology 5th txtbk

Genetic Contributions to Your Life Story371Packed in the nucleus of the cellare the 23 pairs of chromosomesthat represent your uniquegenotype.A gene is a segment of the DNAthat contains the genetic instructionsfor making a particular protein.Genes are arranged in pairs.Each gene is a segment of DNA.The twisted strands that make upa DNA molecule resemble a spiralstaircase.ChromosomeGeneCellDNAreferred to as “junk DNA,” but most researchers today believe that it is involved inregulating gene functioning (Parker & others, 2009; Wray & Babbitt, 2008).What do genes do? In a nutshell, genes direct the manufacture of proteins. Proteinsare used in virtually all of your body’s functions—from building cells to manufacturinghormones to regulating brain activity. Your body requires hundredsof thousands of different proteins to function (Henzler-Wildman & Kern, 2007;Marcus, 2004). Each protein is formed by a specific combination of amino acids,and that combination is encoded in a particular gene.Figure 9.1 Chromosomes, Genes, andDNA Each chromosome contains thousandsof genes, and each gene is a unit ofDNA instructions. Incredibly fine, thestrands of DNA in a single human cellwould be more than three inches long ifunraveled. If the DNA present in one personwere unraveled, it would stretch fromEarth to Pluto and back—twice!Your Unique GenotypeAt fertilization, your biological mother’s egg cell and your biological father’s spermcell each contributed 23 chromosomes. This set of 23 chromosome pairs representsyour unique genotype or genetic makeup. With the exception of the reproductivecells (sperm or eggs), every cell in your body contains a complete, identical copy ofyour genotype.Scientists have recently mapped out the human genome—the complete set ofDNA in the human organism (Human Genome Program, 2008). One surprisingdiscovery was that the complete human genome contains only about 20,000 to25,000 protein-coding genes, far fewer than previous estimates that put the numberas high as 100,000 genes.Although all humans have the same basic set of genes, these genes can come indifferent versions, called alleles. As a general rule, your genotype contains two copiesof each gene—one inherited from each biological parent. These genes may be identicalor different. The range of potential alleles varies for individual genes (Baker,2004). Some genes have just a few different versions, while other genes have 50 ormore possible alleles. It is this unique combination of alleles that helps make yourgenotype—and you—unique.The best-known, although not the most common, pattern of allele variation isthe simple dominant–recessive gene pair. For example, the development of frecklesappears to be controlled by a single gene, which can be either dominant or recessive(Zhang & others, 2004). If you inherit a dominant version of the freckles genefrom either or both of your biological parents, you’ll have the potential to displayfreckles. But to be freckle-free, you would have to inherit two recessive “no freckles”genes, one from each biological parent.Unlike freckles, most characteristics involve the interaction of multiple genes(Wang & Zhao, 2007). For these characteristics, each gene contributes only a smallamount of influence to a particular characteristic.The 23 Pairs of Human Chromosomes Eachperson’s unique genotype is represented inthe 23 pairs of chromosomes found in thenucleus of almost all human body cells. Thisphotograph, taken through a microscope,depicts a karyotype, which shows one cell’scomplete set of chromosomes. By convention,the chromosomes are arranged in pairsfrom largest to smallest, numbered from 1to 22. The 23rd pair of chromosomes, calledthe sex chromosomes, determines a person’sbiological sex. The sex chromosomes in thiskaryotype are XX, indicating a female. Amale karyotype would have an XYcombination.