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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