Invitation to the Life Span
by Kathleen Stassen Berger
Chapter 2 – Genes and Prenatal
Development
PowerPoint Slides developed by
Martin Wolfger and Michael James
Ivy Tech Community College-Bloomington
The Beginning of Life
• DNA (deoxyribonucleic acid):
– Molecule that contains the chemical
instructions for cells to manufacture various
proteins
– Promotes growth and sustains life
• Chromosomes:
– Molecules of DNA
– DNA consists of 46 chromosomes arranged in
23 pairs
The Beginning of Life
• Gametes:
– Reproductive cells (sperm and ova)
– Each gamete consists of 23 chromosomes.
• Zygote:
– Two gametes (sperm and ovum) combine and produce
a new individual with 23 chromosomes from each
parent.
• Genes:
– Specific sections of a chromosome that carry
instructions via four chemicals.
– Chemicals are organized in four pairs (AT, TA, CG,
GC).
– Each person has about 3 billion pairs.
The Beginning of Life
Genetic Variations and Similarities
Genes are passed down from
generation to generation
• Genotype:
– An organism’s genetic
inheritance, or genetic potential
– Unique for each organism
• Phenotype:
– The observable characteristics
of an organism, including
appearance, personality,
intelligence, and all other traits.
Genetic Variations and Similarities
• Many genes are identical for every human
being.
• Some genes vary slightly in their codes from
one person to another.
• Allele:
– Variation of a gene or any of the possible forms in
which a gene for a particular trait can occur.
– Effects of variations vary greatly from causing lifethreatening conditions to having no detectable effect
at all.
The Human Genome
• Genetic Diversity
– Distinguishes each person
– Allows the human species to adapt to pressures of
the environment (e.g. climate changes, illnesses).
• Genome:
– The full set of genes that are the instructions to make
an individual member of a certain species
• Similarity between two people: 99.5%
• Similarity between humans and chimpanzees: 98%
• Similarity between humans and every other
mammal: 90+%
Human Genome Project and
HapMap
• Human Genome Project:
– Found “only” about 20,000 genes in humans
– Exact number is unknown
• HapMap:
– International project trying to spot all
variations in the human genome
– Found 11 million variations among the
billion chemical pairs
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Siblings and Twins
•
Sibling Similarities
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Each sibling gets abut 10,000 genes from each
parent.
Millions of variations and billions of combinations
are possible.
Full siblings are not necessarily alike.
Monozygotic (identical) twins:
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Originate from one zygote that splits apart very
early in development.
Incomplete split results in conjoined twins.
Same genotype but slight variations in phenotype
are possible due to environmental influences.
Siblings and Twins
• Dizygotic (fraternal) twins
– Result from fertilization of two separate ova
by two separate sperm.
– Dizygotic twins have half their genes in
common and occur twice as often as
monozygotic twins.
– Incidence is genetic and varies by ethnicity
and age.
Male and Female
• Humans usually possess 46
chromosomes.
– 44 autosomes and 2 sex chromosomes
– Females: XX
– Males: XY
– Sex of offspring depends on whether the
father’s Y sperm or X sperm fertilizes the
ovum.
Male and Female
Genetic Interactions
• Almost every trait is:
– polygenic (affected by many genes)
– multifactorial (influenced by many factors)
• Regulator genes
– Direct the interactions of other genes,
controlling their genetic expression,
duplication, and transcription
– Responsible for differences between species
Additive Heredity
• Additive genes
– Genes that add something to some aspect of
the phenotype
– Effects of additive genes add up to make the
phenotype
– Example: Height is affected by the
contributions of about 100 genes
Dominant-Recessive Heredity
• Dominant gene is far more influential than the
recessive gene (non-additive).
• Dominant gene can completely control the
phenotype with no noticeable effect of recessive
gene.
– Genes for blood type B and Rh-positive blood are
dominant.
• Effect of recessive genes can sometimes be
noticed.
– Dominant brown eye gene and recessive blue eye
gene can result in hazel eyes.
Carrier
• A person whose genotype includes a gene
that is not expressed in the phenotype
– Unexpressed gene occurs in half of the
carrier’s gametes and is passed on to half of
the carrier’s offspring
– Offspring can be carrier or express the gene
in the phenotype (e.g. when unexpressed
gene is inherited by both parents)
Heritability
• Statistical term that indicates what portion
of the variation in a particular trait within a
particular population is inherited.
– Example: 90% of the height differences
among children of the same age is genetic.
• Environment can affect the expression of
inherited genes.
Genetic Problems
a) Chromosomal Abnormalities
Down Syndrome (Trisomy-21)
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Three copies of chromosome 21
Specific facial characteristics (thick
tongue, round face, slanted eyes)
Hearing losses, heart abnormalities,
muscle weakness, short stature
Slow to develop language
Accelerated aging (cataracts,
dementia, certain forms of cancer
common at age 40)
Genetic Problems
b) Abnormalities of the Sex Chromosomes
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Turner Syndrome (X only):
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Unusually short, underdeveloped female organs,
infertile
Slow to develop
Problems in spatial understanding
Klinefelter Syndrome (XXY):
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Symptoms include learning problems and infertility
Boy’s penis does not grow during puberty and fat
accumulates around the breast area
Sometimes not recognized until adulthood
Genetic Problems
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Gene Disorders
a)
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Dominant Disorders
Half of the offspring of parents with a dominant
disorder will have the disorder.
Most dominant disorders begin in adulthood (fatal
dominant childhood conditions cannot be passed on).
Many dominant disorders have relatively mild or
variable symptoms.
Tourette syndrome
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Some who inherit the dominant gene exhibit uncontrollable tics
and explosive outbursts
Most have milder, barely noticeable symptoms
Genetic Problems
b) Recessive Disorders
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Millions of different types, lethal conditions are
rare
Sickle-cell trait
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Offers some protection against malaria
African carriers are more likely than non-carriers to
survive
Cystic fibrosis
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More common among people with northern
European ancestors
Carriers may have been protected against cholera
Genetic Problems
Sex-Linked Conditions
All X-linked conditions are more common in boys;
girls are likely to be protected by their second X
• Fragile X Syndrome
– Most common form of inherited mental retardation
– Additional symptoms include muscle weakness, shyness, and
poor social skills
•
Hemophilia
– Blood clotting is inhibited
– Blood transfusions can save children’s lives
• Color blindness
– Most common sex-linked condition
– Often passed on because it is not debilitating
Sex-Linked Conditions
Advising Prospective Parents
•
Genetic Counseling
– Consultation and testing by trained professionals
– Enables prospective parents to learn about their
genetic heritage, including harmful conditions that
may be passed on to their offspring
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Ethical Guidelines
– Test results are kept confidential
– Decisions regarding sterilization, adoption, abortion,
or carrying a pregnancy to term are made by the
clients
Advising Prospective Parents
Presenting the Facts
Potential genetic counseling
issues
• Prospective parents must base
some decisions on odds
• Tests often reveal that more
tests are needed
• False positives and false
negatives
• True positives may cause
additional stress
PKU: A Success Story
Phenylketonuria (PKU)
• Recessive condition
• Results in inability to metabolize
phenylalanine (amino acid found in many
foods)
• Buildup of phenylalanine causes brain
damage, progressive mental retardation, and
other symptoms
• Early testing and a special diet usually results
in normal development
From Zygote to Newborn
Three main periods of prenatal development
1. Germinal Period (first two weeks after
conception): rapid cell division and beginning of
cell differentiation
2. Embryonic Period (3rd through 8th week): basic
forms of all body structures develop
3. Fetal Period (9th week until birth): fetus grows in
size and matures in functioning
The Germinal Period
• Zygote begins duplication and division within
hours of conception
• Development of the placenta
– Organ that surrounds the developing embryo and is
attached to the wall of the uterus
– Sustains life via the umbilical chord (nourishes
organism and carries waste products away)
• Implantation (about 10 days after conception)
– Developing organism burrows into the placenta that
lines the uterus
• Organism grows rapidly
– Pregnancy can be detected due to new chemicals in
the mother’s urine
The Germinal Period
The Germinal Period
The Embryonic Period
Embryo
• Developing human organism from about the 3rd
through the 8th week after conception
• Begins when the primitive streak appears down the
middle of the cell mass
• Primitive streak becomes the neural tube and later
forms the brain and spine of the CNS
• Head takes shape
• Eyes, ears, nose, and mouth form
• Heart begins to pulsate
• Extremities develop and webbed fingers and toes
separate
The Embryonic Period
The Fetal Period
Fetus
• Developing human organism from the
start of the 9th week after conception
until birth
• Genitals form and sex hormones
cause differences in fetal brain
organization
• Cephalocaudal and proximodistal
growth
• Heartbeat detectable via stethoscope
• All body parts and systems complete
prenatal development
• Cortex is not fully mature at birth (e.g.
prefrontal lobes)
• Brain at birth is far bigger than any
other part of the baby
The Fetal Period
Age of viability
• Age at which a preterm
newborn may survive outside
the mother’s uterus if medical
care is available
• About 22 weeks after
conception
• Brain is able to regulate basic
body functions (e.g.
breathing)
• Chances of survival increase
with each day after the 22week mark
The Fetal Period
Birth
• Fetal brain signals the release of hormones to
trigger the female’s uterine muscles
• Labor begins
– Average duration for firstborn babies: 12 hours
– Quicker labor for later-born babies
• Apgar scale
– Quick assessment of newborn’s heart rate, breathing,
muscle tone, color, and reflexes
– Completed twice (1 minute and 5 minutes after birth)
– Score of 0, 1, or 2 in each category
– Desired score: 7 or above
Birth
Traditional and Modern Birthing
Practices
• Home births
• Hospital births
• Doula
– Woman who helps with labor, delivery, breastfeeding, and newborn care
– May be related to lower rate of cesarean
sections
Cesarean Section (C-Section)
• Surgical birth
• Fetus can be removed quickly
• Rates and reasons for c-sections vary greatly
– Lower rates in poorer countries (emergencies)
– Higher rates in richer countries (planned for the
convenience of mother and/or obstetrician)
– 1/3 of births in the United States
• Less trauma for the newborn but slower
recovery for the mother
• Subsequent cesarean deliveries may be
necessary
Medical Intervention
• Infant mortality has decreased due to better
medical care
– 1900: 5%
– Today: <1 in 200
• Childbirth has become safer for mothers
– Death rate in poorest nations: 1 in 20 women
• Excessive medical care also has disadvantages
– Increase in unnecessary c-sections is associated with
higher rate of low-birth weight babies
Low Birthweight
• Low birthweight (LBW)
– Less than 2,500 grams (5½ pounds) at birth
– United States
• Steady increase in LBW over the past 25 years
• 8% of newborns are seriously underweight
• More susceptible to teratogens, higher birth risks, lower
survival rate
• Very low birthweight (VLBW)
– Under 1,500 grams (3 pounds, 5 ounces) at birth
• Extremely low birthweight
– Under 1,000 grams (2 pounds, 3 ounces) at birth
Preterm or Slow Growing?
• Preterm
– Birth that occurs at 35 or fewer weeks after
conception
– Usually associated with low birthweight
• Small for gestational age (SGA)
– Birthweight is significantly lower than
expected, given the time since conception
– Suggests impairment throughout prenatal
development and serious problems
Causes of Low Birthweight
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Genetic factors
Maternal illness
Exhaustion
Infection
Malnutrition
Drug use
Multiple births
Mothers, Fathers, and a Good Start
The Father’s Role
• Supportive father helps mother stay healthy
• Father can decrease or increase mother’s
stress (affects fetus)
• Most fathers are helpful to their pregnant
wives
• Two way street: Pregnant mothers should
support, involve, and encourage fathers
The Importance of Close Contact
• Birth complications can have lingering impact on
later life
• Mothers and fathers should help with early
caregiving if newborn must stay in the hospital
• Kangaroo care
– Child-care technique in which the mother of a lowbirthweight infant holds the baby between her breasts
– Allows baby to hear mother’s heartbeat and feel her
body heat
– Research confirms beneficial effects
Postpartum Depression
• Sadness and inadequacy felt by 8-15% of new
mothers in the days and weeks after giving birth
• Symptoms range from baby blues to postpartum
psychosis
• Depressed mothers find baby care burdensome
and may think about mistreating the infant
• Paternal involvement can have beneficial effect
– Some fathers are depressed themselves
• Causes for Postpartum Depression vary
Bonding
Parent-Infant Bond
• The strong, loving connection that forms as
parents hold, examine, and feed the newborn
• Early skin-to-skin contact is not essential
• Cross-fostering in monkeys
– Newborns are removed from their mothers and
raised by another female or male
– Strong and beneficial relationship sometimes
develops
Nature, Nurture, and the
Phenotype
Four general research conclusions
1.
2.
3.
4.
Genes affect every aspect of human behavior
Nongenetic influences begin at conception and
continue lifelong, sometimes altering genetic
instructions
Most environmental influences on children raised in
the same home are NOT shared, partly because
parents treat each child differently
Children , adolescents, and adults “niche-pick,”
choosing environments that are compatible with their
genetic inheritance
Nearsightedness or Myopia
• “High” (more severe) and “low”
(less severe) nearsightedness
• Low nearsightedness runs in
families and is associated with
minor variations in the Pax6 gene
• Environment also plays a role
– Increase in nearsightedness among
East Asian schoolchildren
– Increased schoolwork may have
caused nearsightedness in children
with a Pax6 allelle
Alcoholism
• Alcoholism probably has a genetic basis
– Genes can cause an overpowering addictive pull in
some people
– Environmental conditions can modify the genetic
effects
– Nature and nurture must combine to create an
alcoholic
• Fetal Alcohol Syndrome (FAS)
– A cluster of birth defects that may occur in the child of
a woman who drinks alcohol while pregnant
– Includes abnormal facial characteristics, slow physical
growth, and retarded mental development
Prenatal Teratogens
• Substances and conditions that can impair
prenatal development and result in birth defects
or even death
• Not all teratogens can be avoided
• Structural abnormalities are obvious at birth
• Behavioral teratogens
– Affect the child’s developing brain
– Can cause developmental retardation, hyperactivity,
and learning disabilities
– Effects do not become evident for months or years
Significant Factors
1. Genetic Vulnerability
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Some zygotes carry genes that make them
vulnerable
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Certain genes increase likelihood of cleft lip in
Japanese people
Some women do not metabolize folic acid well
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Increases rates of neural-tube defects (e.g. spina
bifida)
Folic acid was added to many foods in the United
States and Canada and rates have decreased
Significant Factors
2. Timing of Exposure
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Critical period: time when a body part
develops
– First two months for structural abnormalities
– Often before woman knows she is pregnant
– Time when most spontaneous abortions
occur
• Sensitive period: time when teratogens can
interfere with recent growth
Significant Factors
3. Amount of Exposure
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Threshold effect: Certain teratogens are relatively
harmless until exposure reaches a certain level
Bisphenol A (BPA)
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Chemical compound used to make clear plastic
Small doses are harmless but threshold for humans is
unknown
Pregnant mothers should avoid plastic containers and
dishware
Prescription drugs
Vitamin A
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High doses can be harmful
Resolving Uncertainties
• Pregnancy does not have to be an anxious
time
• Good prenatal care can
– teach women what they can do to have
healthy babies
– save lives
– reassure parents that all is well