Chromosomes and Genes Genetics - the field of biology that studies heredity –Fundamental in the transmission of physical traits –Also plays a role in psychological traits Chromosomes – Rod-shaped structures composed of genes that are found within the nuclei of cells Gene – The basic unit of heredity. Composed of DNA Polygenic – Resulting from many genes Deoxyribonucleic Acid (DNA) – Genetic material that takes the form of a double helix composed of phosphates, sugars and bases.
Identical and Fraternal Twins Monozygotic (MZ) - Zygote divides into two cells that separate so that each develops into an individual with the same genetic makeup. Dizygotic (DZ) - Two ova are produced in the same month, each fertilized by a different sperm cell. Ovulation - As women reach end of childbearing years, ovulation becomes less regular. -Results in months when more than one ovum is released
Chromosomal and Genetic Abnormalities Down syndrome –Caused by an extra chromosome of the 21 st pair, resulting in 47 chromosomes Characteristic features - rounded face -protruding tongue -broad, flat nose -sloping fold of skin over the inner corners of the eyes Show deficits in cognitive development and motor development
Genetic Counseling and Prenatal Testing Genetic counselors compile information about a couple’s genetic heritage to explore if their children will have a genetic abnormality. Couples with likelihood of passing on genetic abnormality tend to adopt or not have children of their own. Prenatal testing can indicate if the embryo or fetus is carrying genetic abnormalities.
Amniocentesis Performed on mother 14-16 weeks after conception Syringe withdraws fluid from the amniotic sac; contains cells sloughed off by fetus Cells separated, grown in culture, and examined for genetic and chromosomal abnormalities Routine for women over 35 to detect for Down syndrome; other chromosomal abnormalities increase dramatically as women approach 40 Amniocentesis carries some risk of miscarriage.
Chorionic Villus Sampling Carried out between 9 th and 12 th week of pregnancy Syringe inserted through vagina into uterus to suck out threadlike projections (villi) from the outer membrane that covers the amniotic sac and fetus Results available in days CVS slightly greater risk than amniocentesis of miscarriage; both increase the risk of miscarriage
Ultrasound Sound waves that are too high in frequency to be heard by human ear obtain information about the fetus Ultrasound waves are reflected by the fetus; computer uses the information to generate a picture of the fetus Picture is termed a sonogram Used to guide the syringe in amniocentesis and CVS by determining the position of the fetus Used to track growth of fetus, detect multiple pregnancies, detect structural abnormalities
Blood Tests Used to identify sickle-cell anemia, Tay-Sachs disease, and cystic fibrosis Alpha-fetoprotein (AFP) used to detect neural tube defects such as spina bifida and chromosomal abnormalities Neural tube defects cause elevation in the AFP level in the mother’s blood Elevated AFP levels related to increased risk of fetal death
Heredity and Environment Inheritance, nutrition, learning, exercise, accident, and illness contribute to development of traits Genotypes -Set of traits we inherit from our parents Phenotypes -Actual set of traits -Both genetic and environmental influences
Kinship Studies The more closely people are related, more genes they have in common Parents and children have 50% genetic overlap Siblings have 50% genetic overlap Aunts, uncles have 25% overlap with nieces and nephews; grandparents, 25% overlap with grandchildren First cousins have 12.5% overlap
Twin Studies Monozygotic (MZ) twins share 100% of genes -MZ twins resemble each other more closely than DZ twins on a number of physical and psychological traits. -MZ twins more likely to look alike and be similar in height -MZ twins more likely to share autism, depression, schizophrenia, and vulnerability to alcoholism Dizygotic (DZ) twins share 50% of genes; same as other siblings
Adoption Studies Children that are separated from their natural parents at an early age and reared by adoptive parents provide special opportunities for sorting out nature and nurture. When children who are reared by adoptive parents are nonetheless more similar to their natural parents in a trait, a powerful argument is made for a genetic role in the appearance of that trait. –Examples: Psychological disorders, addiction
Conception Conception refers to the union of an ovum and a sperm cell. Ova –Women born with all the ova they will ever have, about 400,000 –Released from ovarian follicle and enter fallopian tube; 3-4 days later, egg propelled by small, hairlike structures called cilia, and perhaps, by contractions in the wall of the tube If egg not fertilized, discharged through the uterus and vagina, along with endometrium that had formed to support an embryo, in the menstrual flow; during reproductive years, about 400 ova will ripen and be released
Conception (cont’d) Ova are larger than sperm, barely visible to the eye Sperm cells develop through stages; sperm with Y sex chromosomes swim faster than sperm with X sex chromosomes. Male fetuses suffer a higher rate of miscarriage than females, often during the first month of pregnancy. 150 million sperm ejaculated, only 1 in 1,000 can approach an ovum
Infertility Infertility problems with men - Low sperm count (most common) - Lack of sperm - Genetic factors - Environmental poisons - Diabetes - Sexually transmitted infections - Overheating of the testes - Pressure (e.g., bike seats) - Aging - Certain prescription and illicit drugs
Infertility (cont’d) Infertility problems with women - Irregular ovulation, lack of ovulation - Irregularities among the hormones that govern ovulation, stress, and malnutrition - Pelvic inflammatory disorder (PID) -infection that scars the fallopian tubes and other organs, impeding the passage of sperm -Endometriosis - can obstruct the fallopian tubes
Infertility Options Artificial insemination -Sperm is collected and quick-frozen -Sperm then injected into woman’s uterus at time of ovulation; addresses low sperm count as well as low sperm motility -Can be used for a woman to get pregnant who does not have a partner In vitro fertilization -Ripened ova are surgically removed from the mother and placed in laboratory dish; father’s sperm also placed in dish -One or more ova fertilized and injected into mother’s uterus to become implanted -Used when fallopian tubes are blocked or father has low sperm motility -A donor can be used
Infertility Options (cont’d) Donor IVF - Mother does not produce ova -Ovum from another woman is fertilized and injected into the uterus of the mother-to-be Surrogate mothers -Mothers who bring baby to term for other women who are infertile -Can be artificially inseminated by partner of infertile woman Adoption -Another way for people to obtain children that results in the formation of loving new families
Preimplantation Genetic Diagnosis (PGD) Reliable method for selecting the sex of the child prior to implantation Ova are fertilized in vitro After a few days of cell division, cell is extracted from each Sex chromosomal structure of the cell is examined microscopically to determine its sex Embryos of desired sex are implanted in the woman’s uterus
Normal gestation period 280 days; from date of fertilization 266 days Divided into three periods 1. Germinal stage (Weeks 0 - 2) 2. Embryonic stage (Weeks 3 - 8) 3. Fetal stage (Weeks 9 - Birth)
The Germinal Period First two weeks after conception Creation of the fertilized egg (the zygote) Cell division Implantation -- the attachment of the zygote to the uterine wall – takes place about 10 to 14 days after conception
The Embryonic Period Occurs from two to eight weeks after conception Rate of cell differentiation intensifies Every body part eventually develops –Neural tube –Organogenesis -- the process of organ formation during the first two months of prenatal development organs are especially vulnerable to environmental influences
The Fetal Period Fetal period begins two months after conception (normal gestation is 38-40 weeks) Three months after conception -- fetus is about 3 inches long; weighs about 3 ounces By the end of the fifth month, the fetus is about 12 inches long and weighs close to a pound –Vernix –Lanugo At birth, the average American baby weighs 7½ pounds and is about 20 inches long
Three Trimesters The germinal and embryonic periods occur in the first trimester The fetal period begins toward the end of the first trimester and continues through the second and third trimesters Age of Viability (the chances of surviving outside the womb) occurs at the beginning of the third trimester –About 24-25 weeks after conception
Environmental Influences on Prenatal Development Maternal malnutrition effects -low birth weight -prematurity -retardation of brain development -cognitive deficiencies -behavioral problems -cardiovascular disease Fetal malnutrition can sometimes be overcome by a supportive, care-giving environment. Supplementing diets of pregnant women shows positive effects on motor development of infants. Maternal obesity linked with higher risk of stillbirth
Environmental Influences on Prenatal Development (cont’d) Women should gain between 25-35 pounds -Overweight women may gain less -Slender women may gain more Teratogens -Environmental agents can harm the embryo or fetus -Includes drugs taken by mother, lead, mercury Pathogens -Disease-causing organisms -Bacteria and viruses
Environmental Influences on Prenatal Development (cont’d) Critical periods refer to the times when organs are developing. Particular teratogens at a particular time can be harmful to the fetus. Sexually transmitted infections such as syphilis and HIV/AIDS can affect the development of the fetus. -Routine blood tests are given early in pregnancy to diagnose syphilis
Drugs Taken by the Parents (cont’d) Alcohol Heavy alcohol consumption during pregnancy can result in fetal alcohol syndrome (FAS). FAS babies -often smaller, with smaller brains -facial features include widely spaced eyes, underdeveloped upper jaw, flattened nose Psychological characteristics appear to reflect dysfunction of the brain. Maladaptive behaviors such as poor judgment, distractibility, and difficulty perceiving social cues are common.
Drugs Taken by the Parents (cont’d) Caffeine Research regarding caffeine consumption is inconsistent - Several studies have found that pregnant women who take in a good deal of caffeine are more likely than nonusers to have a miscarriage or a low-birth-weight baby.
Drugs Taken by the Parents (cont’d) Cigarettes –Consist of nicotine, carbon monoxide, and hydrocarbons (tars) - Nicotine and carbon monoxide pass through placenta and reach the fetus -Nicotine stimulates the fetus; long-term effects unknown -Carbon monoxide decreases amount of oxygen available to the fetus -Connected with impaired motor development academic delays, learning disabilities, mental retardation, and hyperactivity Smokers’ babies likely to be smaller than those of nonsmokers -Babies of smokers more likely to be stillborn or to die soon after birth Men who smoke are more likely to produce abnormal sperm. -Babies of fathers who smoke have higher rates of birth defects, infant mortality, lower birth weights, and cardiovascular problems.
Parents’ Age Older fathers more likely to produce abnormal sperm 20’s ideal age for women to bear children Teenage pregnancy can result in higher incidence of infant mortality and low birth weight. Stillborn or preterm babies increase as age of mother increases; adequate prenatal care decreases this likelihood even for first-time older mothers.