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Chapter Two Beginnings of Life – Mitosis Life begins as a single cell or zygote that divides repeatedly Mitosis – genetic code carried into new cells in our bodies; the process by which each chromosome in a cell’s nucleus duplicates itself DNA breaks apart; double helix duplicates DNA forms two camps on either side of cell; cell divides. Each incomplete rung combines with its partner to form a new ladder; resulting identical copies of the DNA strand separate when cell divides; each is newly formed cell; genetic code is identical in new cells unless mutations occur through environmental influences such as radiation; mutations occur by chance Beginnings of Life - Meiosis Sperm and ova are produced through meiosis or reduction division 46 chromosomes within the cell nucleus first line up into 23 pairs DNA ladders unzip, leaving unpaired halves of chromosome; when cell divides each member of each pair goes to each newly formed cell Meiosis con’t Each new cell nucleus contains only 23 chromosomes, not 46 23 chromosomes come from the mother and 23 chromosomes come from the father; 22 pairs are autosomes and 23rd pair are sex chromosomes Twins Monozygotic (MZ)– zygote divides into two cells that separate so each develops into individual with same genetic makeup Dizygotic (DZ) – two ova are produced in the same month and they are each fertilized by a different sperm cell DZ runs in families; likelihood of twins increases as number of births increases Ovulation – as woman nears end of child-bearing years, ovulation becomes less regular, thus increasing likelihood of twins Dominant and Recessive Traits Traits determined by pairs of genes; each member of pairs of genes termed an allele Homozygous – both of the alleles for a trait are the same Heterozygous – alleles for a trait differ Incomplete dominance/codominance – effects of both alleles are shown Dominant & Recessive Traits con’t Dominant trait – trait whose influence will be shown each time the gene is present Recessive trait – trait whose influence will be shown only when it is paired with a second recessive gene Carriers – people who bear one dominant gene and one recessive trait Chromosomal Abnormalities Down’s Syndrome Caused by an extra chromosome of the 21st pair, resulting in 47 chromosomes Characteristic features of rounded face, protruding tongue, broad, flat nose, sloping fold of skin over the inner corners of the eyes Typically die from cardiovascular problems Most live to middle age Deficits in cognitive development Deficits in language and motor development Frequent disorders of ear, nose and throat XYY Chromosomal Abnormality Sex-linked chromosomal abnormalities – disorders stemming from abnormal number of sex chromosomes Most individuals with disorder are infertile Extra Y chromosome in males results in heightened male secondary sex characteristics Overrepresented in prisons (stealing is most common offense) XYY males have mildly delayed language development Most XYY males do not have records of aggressive criminal behavior Klinefelter Syndrome (XXY) 1 male in 500 has syndrome Caused by extra X sex chromosome Produces less testosterone than normal males Testes, deepening of voice, musculature, and male pattern of body hair does not develop properly Have enlarged breasts Typically mildly retarded, particularly language Turner Syndrome (X) 1 girl in 2,500 has syndrome Female has single X chromosome External genitals are normal, ovaries poorly developed, producing little estrogen Shorter than average, infertile Cognitive deficits with low estrogen: problems with visual-spatial skills, mathematics, and nonverbal memory Tend to be more interested in novel stimulation Triple X Syndrome (XXX) 1 girl in 1,000 has syndrome Normal in appearance Demonstrate lower-than-average language skills Poorer memory for recent events Development of external sex organs appear normal Increased incidence of infertility Genetic Abnormalities Phenylketonuria (PKU) Enzyme disorder transmitted by a recessive gene affecting 1 child in 8,000 If both parents carry the gene, PKU will be transmitted to one child in four Cannot metabolize an amino acid called phenylalanine; builds up in body and impairs functioning of the central nervous system (CNS) Results are mental retardation, psychological disorders, physical problems No cure, but PKU can be detected in new born children through blood or urine analysis; if identified placed on diets low in phenylalanine Huntington’s Disease Fatal, progressive degenerative disorder Dominant trait Physical symptoms include uncontrollable muscle movements Psychological symptoms include loss of intellectual functioning and personality change Onset during middle adulthood Half of their offspring will have disorder Affects 1 in 18,000 Americans No cure, but helpful medicines Sickle-Cell Anemia Most common among African Americans but also found in some people from Central and South America, the Caribbean, Mediterranean countries, and the Middle East Caused by a recessive gene Red blood cells take on the shape of a sickle and clump together, obstructing small blood vessels and decreasing the oxygen supply Lessened oxygen can impair cognitive skills Physical problems include painful and swollen joints, jaundice, and potentially fatal conditions such as pneumonia, stroke and heart and kidney failure Tay-Sachs Disease Caused by recessive gene Causes CNS to degenerate resulting in death Commonly found among children in Jewish families of Eastern European background 1 in 30 Jewish Americans carry recessive gene Children with disorder progressively lose control of muscles experiencing visual and auditory sensory losses, develop mental retardation, become paralyzed and die by end of early childhood (age 5) Cystic Fibrosis Caused by recessive gene Most common fatal hereditary disease among European Americans Approx. 30,000 Americans have disorder, 10 million more are carriers (1 in 31 people) Children suffer from excessive production of thick mucus that clogs the pancreas and lungs Most victims die of respiratory infections in their 20’s Sex-Linked Genetic Abnormalities Genetic defects only carried on the X sex chromosome Hemophilia and Duchenne muscular dystrophy are sex linked Involve recessive genes Females with two X sex chromosomes are less likely to show sex-linked disorder Sons of female carriers are more likely to be afflicted 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, chorionic villus sampling, ultrasound, and blood tests are examples of prenatal testing Amniocentesis Performed on mother 14–16 weeks gestation Syringe withdraws fluid from the amniotic sac; contains cells sloughed off by fetus Cells separated, grown in culture and examined for chromosomal abnormalities Routine for women over 35 to detect for Down’s Syndrome; Children of women with aging fathers at risk Identifies 100 chromosomal and genetic abnormalities Provides choice for aborting fetus or preparing to raise a special needs child Chorionic Villus Sampling Carried out between 9th and 12th week gestation Syringe inserted through vagina into uterus and sucks out threadlike projections (villi) from the outer membrane that covers the amniotic sac and fetus Results available in days CVS slightly higher risk than amniocentesis of spontaneous abortion; both increase the risk of miscarriage Some practitioners better at carrying out procedures than others Ultrasound Sound waves that are too high in frequency to be heard by human ear to obtain information about the fetus Ultrasound waves are reflected by the fetus and 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, used to locate fetal structures when intrauterine transfusions are needed for survival of fetus as with Rh disease 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 High 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; develop because of both genetic and environmental influences Kinship Studies More closely related genetically, more genes in common Parents and children 50% genetic overlap Siblings 50% genetic overlap Aunts, uncles 25% overlap with nieces and nephews; grandparents 25% overlap with grandchildren First cousins 12.5% overlap Twin Studies MZ twins share 100% of genes DZ twins share 50% of genes; same as other siblings MZ twins more alike than DZ twins in physical and psychological ways, such as height and cholesterol levels MZ twin preferences more similar than DZ MZ twins similar in IQ and social abilities MZ twins more likely to share autism, depression, schizophrenia, and vulnerability to alchoholism Adoption Studies Adopted children used in studies to observe differences in influences of nature and nurture on development When children reared apart from biological parents share the same trait as the biological parent a powerful argument is made for the genetic role of nature Conception Conception refers to the union of an ovum and a sperm cell Ova – women are born with all the ova they will ever have, about 400,000 Ova released from ovarian follicle and enters fallopian tube; 3-4 days later egg propelled by small, hair-like structures called cilia 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 of woman about 400 ova will ripen and be released Conception Continued Ova are larger than sperm, cannot be seen by human 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 spontaneous abortion than females, often times in the first month of pregnancy 150 million sperm ejaculated, only 1 in 1,000 can fertilize ovum Conception Continued Sperm have to fight vaginal acidity, gravity, and swim against current fluid coming from the cervix If sperm survive, reach fallopian tubes 60–90 minutes after ejaculation Approximately 2,000 enter the correct tube Sperm are “egged on” by a change in calcium ions that occurs when an ovum is released by a follicle Only one enters; have to thin the gelatinous layer of egg; secrete an enzyme that briefly thins layer Once sperm enters, layer thickens and locks out other sperm Conception Continued Chromosomes from the sperm cell line up across from corresponding chromosomes within the egg cell Form 23 new pairs with unique set of genetic instructions Couples not able to conceive within 6 months are recommended to receive infertility counseling Infertility Infertility problems with men: - Low sperm count (most common) - Deformed sperm - Low sperm motility - Infectious diseases (sexually transmitted infections) - Overheating of the testes - Chronic diseases such as diabetes - Injury of the testes - Autoimmune response - Pressure (eg. bike seats) Infertility Infertility problems with women: - Irregular ovulation, including failure to ovulate, stress, and malnutrition - Declining hormone levels related to aging - Inflammation of the tissue that is sloughed off during menstruation (“endometriosis”) - Obstructions or malfunctions of the reproductive tract, often caused by infections or diseases involving the reproductive tract - Infections such as Pelvic Inflammatory Disease Infertility Options Artificial Insemination – sperm is collected and quick-frozen; sperm is 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 removed surgically from the mother and placed in laboratory dish; father’s sperm also placed in dish; one or more ova are fertilized and injected into mother’s uterus to become implanted; used when fallopian tubes are blocked or father has low sperm motility; donor can be used Infertility Options Continued Donor IVF – female does not produce ova of her own but her uterus is capable of bringing a baby to term; ovum harvested in another woman, fertilized in vitro; fertilized ovum placed directly into the uterus of the recipient; embryo implanted for remainder of prenatal development Surrogate Mothers – mothers who bring baby to term for another woman who is infertile; can be artificially inseminated by partner of infertile woman Adoption – choosing to parent and form a family with children delivered by a different mother Preimplantation Genetic Diagnosis (PGD) Reliable method for selecting the sex of the child prior to implantation Ova are fertilized in vitro, leading to conception of perhaps six to eight embryos After a few days of cell division, cell is extracted from each Sex chromosomal structure of the cell is examined microscopically to determine whether embryo is male or female Embryos of desired sex are implanted into the female Prenatal Development Normal gestation period 280 days; from date of fertilization 266 days Divided into three periods: germinal stage (first two weeks), embryonic stage (3rd through 8th weeks), fetal stage (third month through birth) Germinal Stage Blastocyst – cells separate into groups that will become different structures, inner part of blastocyst has two distinct layers that form a thickened mass of cells called embryonic disk, these cells eventually become the fetus Germinal Stage con’t Trophoblast – four membranes that will develop and nourish the embryo; one membrane produces blood cells until the embryo’s liver develops and takes over this function; it then disappears; another membrane develops into umbilical cord and blood vessels of the placenta Implantation may consist of bleeding One third of miscarriages occur during first three months Embryonic Stage Development follows cephalocaudal (head to toe) and proximodistal (near to far) Growth of the head takes precedence over growth of the lower parts of the body Relatively early maturation of the brain and organ systems Embryonic Stage con’t Ectoderm – outer layer of cells; develops into nervous system, sensory organs, nails, teeth and outer layer of skin Endoderm – forms digestive and respiratory system, liver and pancreas Mesoderm – develops into excretory, reproductive, and circulatory systems, muscles, the skeleton and the inner layer of skin Embryonic Stage Continued Head and blood vessels form third week after conception Heart starts beating Major organ systems develop within first 2 months Arm buds and legs appear by end of 1st month Nervous system and brain begins to develop Facial features become distinct Embryo is 1 inch long and weighs 1/30th of an ounce During 2nd month nervous system begins to send messages Teeth buds are formed by end of stage Embryonic Stage Continued At 5–6 weeks internal and external genitals resemble primitive female structures By 7th week genetic code asserts itself causing sex organs to differentiate Genetic activity on Y sex chromosome causes the testes to begin to differentiate Ovaries begin to differentiate if the Y chromosome is absent Androgens begin to produce Embryo and fetus develop within protective amniotic sac; sac is surrounded by clear membrane and contains amniotic fluid Embryonic Stage Continued Amniotic fluid acts as natural air bag allowing embryo and fetus to move without injury Placenta is a mass of tissue that permits the embryo to exchange nutrients with mother Mother and embryo have separate circulatory systems Placenta permits oxygen and nutrients to pass from mother to embryo and permits carbon dioxide and waste products to pass to the mother from the embryo Embryonic Stage Continued Mother eliminates waste through her lungs and kidneys Teratogens such as aspirin, smoke, alcohol, tranquilizers can also pass through Placenta secretes hormones that preserve pregnancy, prepare breasts for nursing and stimulate contractions Placenta passes from the birth canal after the baby, hence the title afterbirth Fetal Stage Lasts from beginning of third month until birth Between 9th and 10th week fetus responds to external stimulation Major organ systems formed at end of first trimester Second trimester consists of further maturation and gain in size Weight gain reaches 2 pounds and grows 4 to 5 times in length (to 14 inches) Soft, downy hair grows on eyes and scalp Fetal Stage Continued Fatty layers give the skin a pinkish hue Fetus can open and shut eyes, suck thumb at end of second trimester Fetal hiccups occur Organ systems mature during 3rd trimester By 7th month fetus turns upside down toward delivery Doubles in weight by end of 7th month 90% survival if born at end of 7th month and given quality care Fetal Stage Continued By 13th week fetus responds to sound waves Experiment by DeCasper and Fifer (1980) demonstrated that a fetus is learning while in utero Fetal movements are noticeable during middle of 4th month At 29–30 weeks fetus moves limbs vigorously Fetus turns summersaults Slow squirming movements begin between 5–6 months Fetus gets cramped as it grows, becomes less active during 9th month Environmental Influences on Prenatal Development Maternal malnutrition linked to low birth weight, prematurity, retardation of brain development, cognitive deficiencies, behavioral problems, and cardiovascular disease Fetal malnutrition can sometimes be overcome by a supportive, care-giving environment Enriched day-care programs enhance intellectual and social skills by 5 years of age Supplementing diets of pregnant women shows positive effects on motor development of infants Maternal obesity linked with higher risk of still birth Environmental Influences on Prenatal Development Continued Women should gain between 25–35 pounds; heavier women tend to gain less, thinner women tend to gain more; inadequate weight gain increases likelihood of premature or low-birth weight baby Teratogens – environmental agents that are toxic to the embryo or fetus; includes drugs taken by mother, lead, mercury Pathogens – disease causing organisms; bacteria and viruses Environmental Influences on Prenatal Development Continued Critical periods refers 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, HIV/AIDS can affect the development of the fetus Routine blood tests are given early in pregnancy to diagnose syphilis Environmental Influences on Prenatal Development Continued Women affected by rubella (German measles) during first 20 weeks of pregnancy stand 20% chance of bearing children with birth defects such as deafness, mental retardation, heart disease, eye problems (including blindness) Toxemia is life-threatening and characterized by high blood pressure that may afflict women late in 2nd trimester or early in 3rd trimester Rh incompatibility consists of antibodies produced by the mother and transmitted to a fetus or newborn infant that cause brain damage or death; happens with second born child Environmental Influences on Prenatal Development Continued Thalidomide used for treatment of insomnia and nausea; causes missing or stunted limbs during second month of pregnancy Antibiotics, especially tetracycline, can be harmful to the fetus; can cause yellowed teeth and bone abnormalities; other antibiotics are related to hearing loss Hormones are used to maintain high risk pregnancy Environmental Influences on Prenatal Development Continued Diethylstilbestrol (DES) used to prevent miscarriage during 40’s and 50’s Caused cervical and testicular cancer in some offspring 1 in 1,000 daughters of DES users will develop cancer in the reproductive tract; also more likely to have premature or low birth weight babies Daughters and sons have rates of infertility and immune disorders Environmental Influences on Prenatal Development Continued High doses of vitamins A and D associated with central nervous system damage, small head size, and heart defects Narcotics (heroine, methadone) easily pass placental membrane and cause fetuses to become addicted; after birth drug is substituted so serious withdrawal symptoms are minimized; addicted newborns may have behavioral effects, motoric delays, language delays Environmental Influences on Prenatal Development Continued Marijuana contributes to slower fetal growth, low birth weight Secondary marijuana smoke contributes as well; the greater the amount smoked or inhaled the greater the likelihood of fetal problems Study by Goldschmidt et. al. (2000) indicates prenatal exposure to marijuana can result in increased hyperactivity, impulsivity, problems paying attention, increased delinquency and aggressive behavior Environmental Influences on Prenatal Development Continued Cocaine use during pregnancy increases risk of stillbirth, low birth weight, and birth defects Infants are excitable, irritable, or lethargic; sleep is disturbed Suggestions of delays in cognitive development even at 12 months of age Prenatal exposure linked to lower receptive and expressive language abilities at older ages Environmental Influences on Prenatal Development Continued Heavy alcohol consumption during pregnancy can result in fetal alcohol syndrome (FAS) FAS babies are often physically smaller as well as have smaller brains; facial features include widely spaced eyes, underdeveloped upper jaw, flattened nose; malformation of the limbs, poor coordination, cardiovascular problems; psychological characteristics can include mental retardation, hyperactivity, distractibility, lessened verbal fluency and learning disabilities Facial deformities diminish; intellectual, academic, and behavioral deficits persist Environmental Influences on Prenatal Development Continued There is no safe amount of alcohol that can be consumed per day Some use of alcohol can result in fetal alcohol effects (FAE) Pregnant women who consume as little as one or two drinks per day increase the likelihood of miscarrying or growth-delayed babies Environmental Influences on Prenatal Development Continued Research regarding caffeine consumption is inconclusive 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 Environmental Influences on Prenatal Development Continued Carbon monoxide is toxic; decreases amount of oxygen available to the fetus; connected with cognitive and behavioral problems, including impaired motor development; cognitive difficulties can include academic delays, learning disabilities, mental retardation, and hyperactivity Smokers’ babies likely to be smaller than nonsmokers; babies of smokers more likely to be stillborn or 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 Environmental Influences on Prenatal Development Continued Environmental hazards consist of heavy metals such as lead, mercury, and zinc Exposure to lead related to delayed mental development at 1 and 2 years of age Consumption of PCB-contaminated fish from Lake Michigan may result in smaller and poorer motor functioning newborns Exposure to radiation has been linked to mental retardation and physical deformity; suggestion to avoid unnecessary x-rays Environmental Influences on Prenatal Development Continued Parents’ age affects development Older fathers more likely to produce older sperm 20’s ideal age for women to bear children Teenage pregnancy can result in higher incidence of infant mortality and low birth weight Teenage mothers less educated and less likely to obtain prenatal care than older mothers Stillborn or preterm babies increase as age of mother increases; adequate prenatal care decreases this likelihood even for first time older mothers