Transcript Chapter 10
Chapter 2: Genetics Genetic Foundations Heredity & Environment Genetic Foundations Chromosomes: limits “Each of us carries a “genetic code” that we inherited from our parents. Because a fertilized egg carries this human code, a fertilized human egg cannot grow into an egret, eagle, or elephant.” Genetics and Behavior Nucleus Cell Chromosome Gene DNA Genes: Our Biological Blueprint Human Genome Project Initial phase completed about the year 2000 Humans have 20,000 – 25, 000 genes (21,667) There are far more proteins than genes – 10-20 million Genes (DNA) are dependent- collaborate with other sources of information Gene expression/activity is affected by context or environment Context is affected by hormones, light, nutrition, etc. The Sex Cells Sex cells are formed by meiosis rather than mitosis. Gametes (sperm and ova) have only 23 chromosomes total. At conception, these two unite resulting in a full complement of 46 chromosomes (23 pairs). A fertilized egg is called a zygote. Sources of Genetic Variation • Alleles are normal variations of a gene, found at the same location. • A child who inherits the same allele (type of gene) from both parents is homozygous for that trait. • A child who inherits different alleles from each parent is heterozygous for that trait. Sources of Variation Genetic Expression Influenced by the environment hormones light nutrition behavior stress (cortisol may cause a fivefold increase in DNA damage) Sources of Genetic Variation • • Patterns of Genetic Inheritance Dominant-recessive: the dominant gene (allele) will determine the characteristic Patterns of Genetic Inheritance Dominant-recessive inheritance • Examples of dominant genes Dark hair, curly hair, dimples, types A & B blood (vs. type O), traits for normality in vision, hearing, pigmentation, etc. Huntington’s Disease Patterns of Genetic Inheritance Dominant-recessive inheritance • Examples of recessive genes: Cystic fibrosis, PKU, Tay-sachs disease. Sickle-cell anemia Patterns of Genetic Inheritance Co-dominance and Additive • Co-dominance: both alleles contribute to the phenotype, although not to the same degree. • Additive: They contribute about equally (50%-50%). • Example of Co-dominance; Sickle-cell anemia X-linked (sex-linked) inheritance • Female children receive an X chromosome from the father which matches locations on the mother’s X. • Male children receive a Y from the father, which does not have all the gene locations of an X. • The defective gene on the mother’s X is offset by the gene on the normal X in females, but not in males. • So, males will show evidence of the defective gene (e.g., hemophilia, RG colorblindness). • Females will be normal, but carriers of the defective gene. Polygenic Inheritance Many genes interact to influence the characteristic Most psychological characteristics are polygenic (Where environmental factors are included, traits are said to be multi-factorial.) Chromosomal Abnormalities Usually happen during meiosis Involve breakage and failure to separate Usually result in miscarriage Those most commonly survived are: Down syndrome (trisomy 21) Sex-linked abnormalities Sex Chromosome Abnormalities XXY (Klinefelter) may have verbal difficulties. Tall, underdeveloped testes, possible breasts. 1/800 live male births. XO (Turner) have trouble with math and spatial skills. Short and have webbed neck; may be infertile. 1/2500 live female births XYY (Are they more aggressive, antisocial?) Gene-linked Abnormalities Over 7000 known (most rare), including: Cystic fibrosis Diabetes Hemophilia Huntington PKU (phenylketonuria) Sickle-cell anemia Spina bifida Tay-sachs disease Genetic Counseling – for whom? Family history of disease, mental retardation, physical defects History of miscarriages Mother over age 35 (rate of abnormality begins to rise sharply) Prenatal Diagnostic Methods May cause miscarriage (except ultrasound, maternal blood samples) Is the problem correctible? Genetic engineering is still in the future. Often the only decision is whether or not to abort the fetus. Prenatal Diagnostic Methods Chorionic villi sampling (6-8 weeks); detects genetic defects; risk of miscarriage, limb deformity Amniocentesis – (11 weeks, best after 15 weeks); detects genetic defects; smaller risk of miscarriage Infertility 1 in 6 couples in U.S. Waiting too late Sexually transmitted diseases Fertility technology (IVF, donors) Adoption Babies culturally unavailable Environmental Influence Environmental Influence Rats reared in an Impoverished environment Enriched environment environment enriched with playthings show increased development of the cerebral cortex Twins Identical twins Fraternal twins Identical Twins develop from a single fertilized egg that splits in two, creating two genetically identical organisms Fraternal Twins develop from separate eggs genetically no closer than brothers and sisters, but they share a fetal environment Same sex only Same or opposite sex Multiple Births – fraternal twins Dizygotic (two zygotes) Share approximately 50% of their genetic heritage like any two siblings. Major causes are maternal age and fertility drugs. Twinning dramatically on the increase since the 1970s. Multiple Births – identical twins Monozygotic – one zygote (same fertilized egg) Share 100% of genetic heritage Occurs about 3 per 1000 live births worldwide Factors may include temperature and oxygen levels and late fertilization Genetics Research Behavior Genetics study of the power and limits of genetic and environmental influences on behavior Molecular Genetics subfield of biology that studies the molecular structure and function of genes Nature-nurture Research • Molecular genetics • Human Genome Project Behavioral genetics Twin studies Equal environment assumptions Adoption studies Concordance rates Epigenesis – ongoing nature/nurture exchanges (bi-directional) Reaction range Canalization Genetic-environmental correlation • Passive • Evocative • Active (niche-picking)