Chapter 3 Outline I. Becoming Parents A. Conception 1. Changing Theories of Conception Two-seed theory: (Hippocrates) Joining of male and female seeds. There are no significant anatomical differences between male and female genitalia except that men s are located outside the body and women s are located inside the body. Theory remained influential until 17 th century in England. Female as Defective Theory: (Aristotle) Sperm is responsible for conception. Females resulted when development was disturbed. Animalculists: Little people are contained in the heads of sperm which grow when deposited in the womb. Ovists: Female ovaries contain tiny, already formed humans whose growth is activated by male sperm. 2. How Fertilization Takes Place Fertilization: Union of sperm and ovum fuse to produce a single new cell called a zygote; also called conception. Gametes: Sex cells. Zygote: One celled organism resulting from fertilization. Follicle: Small sac which holds ovum. Ovulation: Rupture of a mature follicle in ovary and expulsion of its ovum. Occurs about once every 28 days until menopause. Cilia: Small hair cells that sweep the ovum along the fallopian tube. Cervix: Opening of the uterus. B. Infertility Infertility: Inability to conceive after 12 months of trying. 1. Causes of Infertility Most common cause of infertility in men is low sperm count. Most common cause of infertility in women is blockage of the fallopian tubes. 2. Alternative Ways to Parenthood Assisted reproductive technology (ART) o In vitro fertilization (IVF): Fertilization done outside mother s body. o In vitro maturation (IVM): Harvesting of follicles before ovulation is complete. o Intracytoplasmic sperm injection (ISI):A single sperm is injected into the ovum. o Artificial insemination: Injection of sperm into a woman s cervix. o Artificial insemination by a donor: If man is infertile, a donor may be matched with prospective father for physical characteristics.
o Ovum transfer: A donor egg is fertilized in lab and transferred to mother s uterus. o Donor egg: When a woman is producing poor quality ova or has had her ovaries removed. o Blastocyst transfer: Fertilized ovum is kept in culture until the develops into a blastocyst and then implanted into mother s uterus. o Gamete intrafallopian transfer(gift) and Zygote intrafollopian transfer (ZIFT): Egg and sperm or fertilized egg is inserted into the fallopian tube. o Surrogate motherhood: A fertile woman is impregnated by the prospective father and agrees to give the baby to the couple. II. Mechanisms of Heredity Heredity: Inborn factors which are inherited from the biological parents that affect development. A. The Genetic Code Deoxyribonucleic acid (DNA): Chemical that carries inherited instructions for the formation and function of body cells. Bases: Chemical units which make up DNA (A, T, C, G) and form pairs. o adenine + thymie o cytosine + guanine The Genetic Code: Sequence of base parts within DNA that determine inherited characteristics. Chromosomes: Coils of DNA that carry genes. Genes: Small segments of DNA located in definite positions on particular chromosomes. Meiosis: Complex process of cell division in which each gamete (sperm or ovum) ends up with only 23 chromosomes-one from each pair. Monomorphic genes: The three quarters of the genes every child receives that are identical to the genes that every other child receives. Polymorphic genes: The other quarter of the genes which define each child as an individual. Mitosis: Process by which cells divide in half over and over again duplicating themselves. B. What Determines Sex? Autosomes: 22 pairs of chromosomes not related to sexual expression. Sex chromosomes: Pair of chromosomes that determine sex: XX in the normal female, XY in the normal male. SRY gene: Gene for maleness. Wnt-4 molecule: Controls femaleness. C. Patterns of Genetic Transmission 1. Dominant and Recessive Inheritance
Alleles: Paired genes (alike or different) that produce alternative expressions of a characteristic. Homozygous: Possessing two identical alleles for a trait. Heterozygous: Possessing different alleles for a trait. Dominant inheritance: Pattern of inheritance in which, when a child recieves contradictory alleles, only the dominant one is expressed. Recessive inheritance: Pattern of inheritance in which a child recieves identical recessive alleles resulting in expression of a nondominant trait. Polygenic Inheritance : Interaction of several genes. Mutations : Permanant alterations in genetic materials. Epigenesis: Mechanism that turns genes on or off and determines functions in the body. 2. Genotypes and Phenotypes: Multifactorial Transmission Phenotype: Observable characteristics of a person. Genotype: Genetic makeup of a person, containing both expressed and unexpressed characteristics. D. Genetic and Chromosomal Abnormalities 1. Dominant or Recessive Inheritance of Defects Incomplete Dominance: A trait that is not fully expressed. 2. Sex-Linked Inheritance of Defects Sex-linked inheritance: Pattern of inheritance in which certain characteristics carried on the X chromosome inherited from the mother are transmitted differently to her male and female offspring. Carrier: Parent does not have the disorder but can pass it on to children. 3. Genome Imprinting Genome imprinting: Differential expression of genetic traits depending on whether the trait has been inherited from the mother or the father. 4. Chromosomal Abnormalities Down syndrome: Chromosomal disorder characterized by moderate to severe mental retardation and a downward sloping skin fold at the inner corners of the eyes. Also called trisomy-21. E. Genetic Counseling and Testing Genetic counseling: Clinical service helps prospective parents assess their probable risk of having children with genetic or chromosomal defects. Karyotype: A chart of chromosomes sorted according to size and structure.
III. Nature and Nurture: Influences of Heredity and Environment A. Studying the Relative Influences of Heredity and Environment Behavioral genetics: Quantitative study of how much heredity and environment influence particular traits. 1. Measuring Heritability Heritability: Statistical estimate of how great a contribution heredity makes to individual differences in a specific trait at a certain time within a given population. Family studies: Measure the degree to which biological relatives share certain traits and whether the closeness of the family relationship is associated with the degree of similarity. Adoption studies: Look at similarities between adopted children and their adoptive families and also between adopted children and their biological families. Studies of twins: Compare pairs of monozygotic twins and same-sex dizygotic twins. Concordant: Term describing a statistical tendency to show the same trait. B. How Heredity and Environment Work Together Developmental System: A combination of constitutional, social, economic and cultural factors that shape development. 1. Reaction Range and Canalization Reaction range: Range of potential expressions of an inherited trait. Canalization: Limitation on variance of expression of certain inherited characteristics. 2. Genotype-Environment Interaction Genotype-environment interaction: Effects of similar environmental conditions on genetically different individuals. 3. Genotype-Environment Correlation Genotype-environmental correlation: Tendency of certain genetic and environmental influences to act in the same direction; may be passive, reactive (evocative) or active. o Passive correlations: Parents, who are genetically related to the child, provide the rearing environment for the child. Child does not control environment. o Reactive, or evocative, correlations: Child s genotype elicits certain types of physical and social environments. o Active correlations: Children seek out environments that they find companionable and stimulating. Niche-picking: Tendency of a person, especially after early childhood, to seek out environments compatible with his or her genotype. 4. 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.
C. Some Characteristics Influenced by Heredity and Environment. 1. Physical and Physiological Traits Obesity: Extremely overweight in relation to age, sex, height, and body type; sometimes defined as having a body mass index (weight-to-height) at or above the 85 th or 95 th percentile of growth curves for children of the same age and sex. 2. Intelligence Heritability estimates: 50 to 60 percent for verbal abilities and 50 percent for spatial abilities. 3. Personality Temperament: A person s characteristic style of approaching and reacting to situations. Heritability estimates: On 5 major aspects of traits, 40 percent: on scales without measurement errors, 66 percent. 4. Psychopathology Schizophrenia: Mental disorder marked by loss of contact with reality; symptoms include hallucinations and delusions. May originate in a lack of a chemical called reelin. Autism: Pervasive developmental disorder of the brain, characterized by the lack of normal social interactions, impaired communication and imagination, and repetitive, obsessive behaviors.