Chapter 3 Forming a New Life PDF

Chapter 3 Forming a New Life  Conceiving New Life Most people think of development as beginning on the day of birth, when the new child—squalling and thrashing—is introduced to the world. However, development starts earlier. It starts at conception, as sper...

Chapter 3 Forming a New Life  Conceiving New Life Most people think of development as beginning on the day of birth, when the new child—squalling and thrashing—is introduced to the world. However, development starts earlier. It starts at conception, as sperm and egg meet and an entirely new individual is created from parental genomes. Development continues as the fertilized egg grows and differentiates and edges closer to independent life outside the womb. And it persists in the dance between nature and nurture that shapes the unique individual that is the product of these processes. HOW FERTILIZATION TAKES PLACE Fertilization Union of sperm and ovum to produce a zygote; also called conception. Zygote One-celled organism resulting from fertilization. HOW FERTILIZATION TAKES PLACE Dizygotic twins Twins conceived by the union of two different ova with two different sperm cells; also called fraternal twins; they are no more alike genetically than any other siblings. Monozygotic twins Twins resulting from the division of a single zygote after fertilization; also called identical twins ; they are genetically similar Mechanisms of Heredity The science of genetics is the study of heredity: the genetic transmission of heritable characteristics from parents to offspring. THE GENETIC CODE Deoxyribonucleic acid (DNA) Chemical that carries inherited instructions for the development of all cellular forms of life. Genetic code Sequence of bases within the DNA molecule; governs the formation of proteins that determine the structure and functions of living cells. Chromosomes Coils of DNA that consist of genes. Genes Small segments of DNA located in definite positions on particular chromosomes; functional units of heredity. THE GENETIC CODE Human genome Complete sequence of genes in the human body. Every cell in the normal human body except the sex cells (sperm and ova) has 23 pairs of chromosomes—46 in all. Through a type of cell division called meiosis, which the sex cells undergo when they are developing, each sex cell ends up with only 23 chromosomes—one from each pair. When sperm and ovum fuse at conception, they produce a zygote with 46 chromosomes, 23 from the father and 23 from the mother Through mitosis, a process by which the non–sex cells divide in half over and over again, the DNA replicates itself, so that each newly formed cell has the same DNA structure as all the others. Mutation Permanent alterations in genes or chromosomes that may produce harmful characteristics  WHAT DETERMINES SEX? Twenty-two pairs are autosomes, chromosomes that are not related to sexual expression. The twenty-third pair are sex chromosomes — one from the father and one from the mother—that govern the baby’s sex PATTERNS OF GENETIC TRANSMISSION Genes that can produce alternative expressions of a characteristic (such as the presence or absence of dimples) are called alleles. When both alleles are the same, the person is homozygous for the characteristic; when they are different, the person is heterozygous. Autosomes In humans, the 22 pairs of chromosomes not related to sexual expression. Sex chromosomes Pair of chromosomes that determines sex: XX in the normal human female, XY in the normal human male. Alleles Two or more alternative forms of a gene that occupy the same position on paired chromosomes and affect the same trait.  Homozygous Possessing two identical alleles for a trait. Heterozygous Possessing differing alleles for a trait. Dominant inheritance Pattern of inheritance in which, when a child receives different alleles, only the dominant one is expressed. Recessive inheritance Pattern of inheritance in which a child receives identical recessive alleles, resulting in expression of a nondominant trait. Polygenic inheritance Pattern of inheritance in which multiple genes at different sites on chromosomes affect a complex trait.  Genotypes and Phenotypes: Multifactorial Transmission Phenotype Observable characteristics of a person. Genotype Genetic makeup of a person, containing both expressed and unexpressed characteristics. Multifactorial transmission Combination of genetic and environmental factors to produce certain complex traits. Epigenesis: Environmental Influence on Gene Expression Epigenesis Mechanism that turns genes on or off and determines functions of body cells. Genes are turned off or on as they are needed by the developing body or when triggered by the environment. This phenomenon is called epigenesis, or epigenetics. Far from being fixed once and for all, epigenetic activity is affected by a continual bidirectional interplay with nongenetic influences. Epigenetic changes can occur throughout life in response to environmental factors such as nutrition, smoking, sleep habits, stress, and physical activity (Fraga et al., 2005). Cells are particularly susceptible to epigenetic modification during critical periods such as puberty and pregnancy. GENETIC AND CHROMOSOMAL ABNORMALITIES Most birth disorders are fairly rare, affecting only about 3 percent of live births (Waknine, 2006). Nevertheless, they are the leading cause of infant death in the United States, accounting for 19.5 percent of all deaths in the first year in 2007 (Xu et al., 2010). The most prevalent defects are cleft lip or cleft palate, followed by Down syndrome. Most of the time, normal genes are dominant over those carrying abnormal traits, but sometimes the gene for an abnormal trait is dominant. Incomplete dominance Pattern of inheritance in which a child receives two different alleles, resulting in partial expression of a trait. Chromosomal Abnormalities Chromosomal abnormalities typically occur because of errors in cell division, resulting in an extra or missing chromosome. Down syndrome The condition is also called trisomy-21 because it is characterized in more than 90 percent of cases by an extra 21st chromosome. The most obvious physical characteristic associated with the disorder is a downward sloping skin fold at the inner corners of the eyes. Children with Down syndrome, like other children with disabilities, tend to benefit cognitively, socially, and emotionally when placed in regular classrooms rather than in special schools GENETIC COUNSELING AND TESTING Genetic counseling Can help prospective parents like Alicia and Eduardo assess their risk of bearing children with genetic or chromosomal defects. Genetic Testing Genomics, the scientific study of the functions and interactions of the various genes, has untold implications for medical genetics, the application of genetic information to therapeutic purposes.  Nature and Nurture: Influences of Heredity And Environment STUDYING HEREDITY AND ENVIRONMENT it seeks to measure how much heredity and environment influence particular traits. This is the traditional goal of the science of behavioral genetics. Behavioral geneticists have developed a means of estimating how much of a trait is due to genetics and how much is the result of environmental influences by using a concept known as heritability. By looking at groups of people with known genetic relationships, and assessing whether or not they are concordant (Term describing tendency of twins to share the same trait or disorder), meaning the same, on a given trait, behavioral geneticists can estimate the relative influence of genes and environment. Heritability is expressed as a percentage ranging from 0.0 to 1.0: the higher the number, the greater the heritability of a trait. A heritability estimate of 1.0 indicates that genes are 100 percent responsible for variances in the trait within the population. A heritability estimate of 0.0 percent would indicate the environment shaped a trait exclusively. HOW HEREDITY AND ENVIRONMENT WORK TOGETHER Reaction range Refers to a range of potential expressions of a hereditary trait. Canalization Limitation on variance of expression of certain inherited characteristics. Many highly canalized traits tend to be those necessary for survival. Genotype-environment interaction Genotype-Environment Correlation 1. Passive correlations: Parents, who provide the genes that predispose a child toward a trait, also tend to provide an environment that encourages the development of that trait. 2. Reactive, or evocative, correlations: Children with differing genetic makeups evoke different reactions from others. 3. Active correlations: As children get older and have more freedom to choose their own activities and environments, they actively select or create experiences consistent with their genetic tendencies.  What Makes Siblings So Different? The Nonshared Environment Nonshared environmental effects The unique environment in which each child grows up, consisting of distinctive influences or influences that affect one child differently than another. SOME CHARACTERISTICS INFLUENCED BY HEREDITY AND ENVIRONMENT Physical and Physiological Traits Intelligence Personality and Psychopathology Temperament Characteristic disposition, or style of approaching and reacting to situations. There is evidence for a strong hereditary influence on such mental disorders as schizophrenia, autism, alcoholism, and depression. All tend to run in families and to show greater concordance between monozygotic twins than between dizygotic twins. However, heredity alone does not produce such disorders; an inherited tendency can be triggered by environmental factors. Prenatal Development Gestation Period of development between conception and birth. Gestational age Age of an unborn baby, usually dated from the first day of an expectant mother’s last menstrual cycle. STAGES OF PRENATAL DEVELOPMENT Germinal Stage (Fertilization to 2 Weeks) Implantation - The attachment of the blastocyst to the uterine wall, occurring at about day 6. The ectoderm, the upper layer, will become the outer layer of skin, the nails, hair, teeth, sensory organs, and the nervous system, including the brain and spinal cord. The endoderm, the inner layer, will become the digestive system, liver, pancreas, salivary glands, and respiratory system.  Germinal Stage (Fertilization to 2 Weeks) The mesoderm, the middle layer, will develop and differentiate into the inner layer of skin, muscles, skeleton, and excretory and circulatory systems. The amniotic sac is a fluid-filled membrane that encases the developing embryo, protecting it and giving it room to move and grow. The placenta allows oxygen, nourishment, and wastes to pass between mother and embryo. It is connected to the embryo by the umbilical cord. Embryonic Stage (2 to 8 Weeks) During the embryonic stage, from about 2 to 8 weeks, the organs and major body systems— respiratory, digestive, and nervous— develop rapidly. This process is known as organogenesis. Spontaneous abortion - Natural expulsion from the uterus of an embryo that cannot survive outside the womb; also called miscarriage.  Fetal Stage (8 Weeks to Birth) The appearance of the first bone cells at about 8 weeks signals the beginning of the fetal stage, the final stage of gestation. During this period, the fetus grows rapidly to about 20 times its previous length, and organs and body systems become more complex. Scientists can observe fetal movement through ultrasound, the use of high-frequency sound waves to detect the outline of the fetus. ENVIRONMENTAL INFLUENCES: MATERNAL FACTORS Teratogen Environmental agent, such as a virus, a drug, or radiation that can interfere with normal prenatal development and cause developmental abnormalities. Nutrition and Maternal Weight Malnutrition Physical Activity and Strenuous Work Drug Intake Alcohol Fetal alcohol syndrome (FAS) is characterized by a combination of retarded growth, face and body malformations, and disorders of the central nervous system. ENVIRONMENTAL INFLUENCES: MATERNAL FACTORS Nicotine Caffeine Marijuana, Cocaine, and Methamphetamine Maternal Illnesses An infection called toxoplasmosis, caused by a parasite harbored in the bodies of cattle, sheep, and pigs and in the intestinal tracts of cats, typically produces either no symptoms or symptoms like those of the common cold. Maternal Anxiety, Stress, and Depression Maternal Age Outside Environmental Hazards THE NEED FOR PRECONCEPTION CARE Physical examinations and the taking of medical and family histories Vaccinations for rubella and hepatitis B Risk screening for genetic disorders and infectious diseases such as STDs Counseling women to avoid smoking and alcohol, maintain a healthy body weight, and take folic acid supplements Interventions should be provided where risks are indicated and also between pregnancies for women who have had poor pregnancy outcomes in the past. Thank You!