During the course of development, the genotype interacts with the environment in complex ways to produce the phenotype. It is the phenotypic expression of individual physical and behavioral characteristics that scientists study in an effort to understand how genes and the environment interact to produce each unique human being. THE PROCESS OF GENETIC TRANSMISSIONChromosomes and Genes Within each cell nucleus are threadlike structures called chromosomes, on which genes containing the genetic code are located. Genetic variability is the result of the huge number of chromosome combinations that are possible during the formation of sperm and egg cells, and to crossing over, which occurs during the kind of cell division called meiosis and involves the exchange of genes on homologous chromosomes. Sexual reproduction, or the union of ovum with sperm, also contributes to genetic variability, as 23 chromosomes from a woman unite with 23 chromosomes from a man to form the zygote. Through the process of mitosis, this new autosome, or cell, divides and continues to divide, eventually producing a new, multicellular organism.Genes, DNA, and Proteins Chromosomes are bound by molecules of deoxyribonucleic acid (DNA), which in turn are made up of nucleotides.Genes, portions of the DNA molecule, are located at particular sites on the chromosome where they code for the production of certain kinds of protein. When a gene is activated, a copy of it travels from the cell nucleus to the body of the cell where it serves as a template for building a protein molecule. Each of the many different kinds of proteins in the human body serves a different function. All of them, working together, make a living organism. GENETIC INFLUENCES ON DEVELOPMENTThe Transmission of Traits: A Basic Model At any given gene's position on two homologous chromosomes, there can be more than one form of that gene, called the gene's alleles. If the alleles are the same, the person is homozygous for that particular characteristic; if the alleles are different, the person is heterozygous. Heterozygous combinations may be expressed in three ways: (1) the person may have a trait intermediate between the traits that each of the two alleles codes for; (2) both alleles may express their traits simultaneously, an outcome call codominance; and (3) a dominant allele may overcome the other, recessive allele, resulting in the expression of the dominant allele's trait.Genes on the Sex Chromosomes: Exceptions to the Rule The 23rd pair of human chromosomes are the sex chromosomes, differing in males and females. Females have two large homologous sex chromosomes, forming an XX pattern; males have one X and a smaller Y chromosome, and XY pattern. Because an X chromosome is about five times longer than a Y chromosome, it carries more genes. This means that, in males, some genes on the X chromosome have no equivalent genes on the Y chromosome; the male inherits only one each of these X-linked genes. If the inherited gene happens to be a harmful recessive allele, the associated genetic disorder will automatically be expressed. Hemophilia, a disease in which the blood fails to clot, is an example of an X-linked recessive trait.Interactions Among Genes Many human characteristics are influenced by complex interactions among multiple genes acting together. This interaction of multiple genes may help explain why some traits that are influenced by genes do not tend to run in families. Their development depends on a configuration of many genes, and that whole configuration is not likely to be passed on from parent to child. Further adding to the complexity of genetic influences on development, a single pair of alleles may influence more than one trait, and, if they are modifier genes, they may do so indirectly through the effects they have on how other genes are expressed.Genetic Disorders Harmful alleles survive generally because, as in phenylketonuria (PKU), they are not harmful in the heterozygous state. They may also survive, as they do in people who carry the allele for sickle-cell anemia; when in addition to threatening people with that disease, they serve to protect people who carry them from getting malaria.Down syndrome is one example of the many identifiable human chromosome disorders. It is caused by inheriting three 21st chromosomes instead of the normal two and is characterized by both physical and mental retardation and a distinctive physical appearance. Abnormalities can also arise in the sex chromosomes. Examples are Turner syndrome (an XO pattern), the triple-X syndrome, Klinefelter's syndrome (an XXY pattern), the double-Y syndrome (an XYY pattern), and fragile X syndrome. The physical, psychological, and emotional characteristics of people with these chromosome aberrations vary widely depending upon the specific chromosome pattern and environmental factors. GENETIC COUNSELING AND GENETIC ENGINEERINGPrenatal Diagnostic Techniques Advances in biology and genetics have opened new opportunities for diagnosing genetic disorders before birth. The two methods most commonly used to collect samples of fetal cells for genetic analysis are amniocentesis and chorionic villi sampling. Other diagnostic methods include the alphafetoprotein assay and ultrasound.Gene Therapy Scientists hope eventually to locate the genes responsible for inherited disorders. A breakthrough occurred when researchers identified the genetic marker for Huntington disease, making it possible to develop a test for the Huntington gene. With the aim of treating or curing genetic disorders, scientists are exploring gene therapy, which involves inserting normal alleles into patient's cells to compensate for defective alleles. Theoretically, normal genes could even replace defective ones in sperm or egg cells or they could be inserted into a newly created zygote. HEREDITY-ENVIRONMENT INTERACTIONSHow the Environment Influences the Expression of Genes The concept of the range of reaction helps shed light on how environments influence genes. According to this concept, heredity does not rigidly fix behavior, but instead establishes a range of possible developmental outcomes that may occur in response to different environments. When a reaction range is extremely narrow, it is said to exhibit canalization. With a highly canalized trait, there are few pathways that development can take, and intense or more specific environmental pushes are required to deflect the course of development.How Genetic Makeup Helps to Shape the Environment Not only does environment influence genes, but genes also influence the environments to which people are exposed. One way this can happen is for parents with certain genetic predispositions to create a home environment that suits those predispositions, and which may also suit and encourage the inherited predispositions of their children. Another way is for people's inherited tendencies to evoke certain environmental influences from others. A third way is for genes to encourage people to engage in niche picking-seeking out experiences that are compatible with their inherited tendencies. HEREDITY, ENVIRONMENT, AND INDIVIDUAL DIFFERENCESMethods of Studying Individual Differences An important question researchers ask is why significant differences exist in the ways that people develop. Human behavior genetics seeks to answer this question by calculating heritability factors-percentage estimates of the contribution that genes make to some observed individual difference. Commonly, researchers study family members with known degrees of biological relatedness, such as monozygotic and dizygotic twins and adopted children, as well as the degree to which they inhabit shared or nonshared environments.Some Individual Differences and Their Contributors Family resemblance studies consistently show that individual differences in IQ scores are substantially influenced by genetic factors. In addition, individual differences in certain more specific cognitive abilities, including spatial skills and verbal proficiency, are also influenced by genes, as are differences in the timing and rate of mental development. Nevertheless, an enriched environment can boost a child's level of intellectual development considerably. Heredity contributes to many individual differences in temperament and personality, especially differences in emotionality, activity level, and sociability. However, the contribution of heredity to differences in these traits appears to decline with age, as people's personalities become increasingly influenced by their life experiences. In the area of psychopathology we find an influence of genes as well. Whether a person develops a certain psychological disorder or not often depends on a combination of genetic vulnerability and environmental stresses.