Psychological researchers study genetics in order to better understand the biological factors that contribute to certain behaviors While all humans share certain biological mechanisms we are each unique And while our bodies have many of the same parts brains and hormones and cells with genetic codes these are expressed in a wide variety of behaviors thoughts and reactions Why do two people infected by the same disease have different outcomes one surviving and one succumbing to the ailment How are genetic diseases passed through family lines Are there genetic components to psychological disorders such as depression or schizophrenia To what extent might there be a psychological basis to health conditions such as childhood obesity To explore these questions let s start by focusing on a specific genetic disorder sickle cell anemia and how it might manifest in two affected sisters Sickle cell anemia is a genetic condition in which red blood cells which are normally round take on a crescent like shape Figure 3 2 The changed shape of these cells affects how they function sickle shaped cells can clog blood vessels and block blood flow leading to high fever severe pain swelling and tissue damage Many people with sickle cell anemia and the particular genetic mutation that causes it die at an early age While the notion of survival of the fittest may suggest that people with this disorder have a low survival rate and therefore the disorder will become less common this is not the case Despite the negative evolutionary effects associated with this genetic mutation the sickle cell gene remains relatively common among people of African descent Why is this The explanation is illustrated with the following scenario Imagine two young women Luwi and Sena sisters in rural Zambia Africa Luwi carries the gene for sicklecell anemia Sena does not carry the gene Sickle cell carriers have one copy of the sickle cell gene but do not have full blown sickle cell anemia They experience symptoms only if they are severely dehydrated or are deprived of oxygen as in mountain climbing Carriers are thought to be immune from malaria an often deadly disease that is widespread in tropical climates because changes in their blood chemistry and immune functioning prevent the malaria parasite from having its effects Gong Parikh Rosenthal Greenhouse 2013 However full blown sickle cell anemia with two copies of the sickle cell gene does not provide immunity to malaria Malaria is rare in the United States so the sickle cell gene benefits nobody the gene manifests primarily in minor health problems for carriers with one copy or a severe full blown disease with no health benefits for carriers with two copies However the situation is quite different in other parts of the world In parts of Africa where malaria is prevalent having the sickle cell mutation does provide health benefits for carriers protection from malaria The story of malaria fits with Charles Darwin s theory of evolution by natural selection Figure 3 3 In simple terms the theory states that organisms that are better suited for their environment will survive and reproduce while those that are poorly suited for their environment will die off In our example we can see that as a carrier Luwi s mutation is highly adaptive in her African homeland however if she resided in the United States where malaria is rare her mutation could prove costly with a high probability of the disease in her descendants and minor health problems of her own Genetic Variation Genetic variation the genetic difference between individuals is what contributes to a species adaptation to its environment In humans genetic variation begins with an egg about 100 million sperm and fertilization Roughly once per month active ovaries release an egg from follicles During the egg s journey from the ovary through the fallopian tubes to the uterus a sperm may fertilize the egg The egg and the sperm each contain 23 chromosomes Chromosomes are long strings of genetic material known as deoxyribonucleic acid DNA DNA is a helix shaped molecule made up of nucleotide base pairs In each chromosome sequences of DNA make up genes that control or partially control a number of visible characteristics known as traits such as eye color hair color and so on A single gene may have multiple possible variations or alleles An allele is a specific version of a gene So a given gene may code for the trait of hair color and the different alleles of that gene affect which hair color an individual has When a sperm and egg fuse their 23 chromosomes combine to create a zygote with 46 chromosomes 23 pairs Therefore each parent contributes half the genetic information carried by the offspring the resulting physical characteristics of the offspring called the phenotype are determined by the interaction of genetic material supplied by the sperm and egg called the genotype A person s genotype is the genetic makeup of that individual Phenotype on the other hand refers to the individual s inherited physical characteristics which are a combination of genetic and environmental influences Figure 3 4 FIGURE 3 4 a Genotype refers to the genetic makeup of an individual based on the genetic material DNA inherited from one s genetic contributors b Phenotype describes an individual s observable characteristics such as hair color skin color height and build credit a modification of work by Caroline Davis credit b modification of work by Cory Zanker Note that in genetics and reproduction parent is often used to describe the individual organisms that 74 3 Biopsychology Access for free at openstax org contribute genetic material to offspring usually in the form of gamete cells sperm and egg The concept of a genetic parent is distinct from social and legal concepts of parenthood and may differ from those whom people consider their parents Most traits are controlled by multiple genes but some traits are controlled by one gene A characteristic like cleft chin for example is influenced by a single gene from each parent In this example we will call the gene for cleft chin B and the gene for smooth chin b Cleft chin is a dominant trait which means that having the dominant allele either from one parent Bb or both parents BB will always result in the phenotype associated with the dominant allele When someone has two copies of the same allele they are said to be homozygous for that allele When someone has a combination of alleles for a given gene they are said