Invitation to the Life Spanby Kathleen Stassen Berger

Chapter 2 – Genes and Prenatal Development

PowerPoint Slides developed by Martin Wolfger and Michael JamesIvy 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 life-

threatening 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 3

billion chemical pairs

Siblings and Twins

• Sibling Similarities– 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:– 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)– 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)

b) Abnormalities of the Sex Chromosomes• Turner Syndrome (X only):

– Unusually short, underdeveloped female organs, infertile

– Slow to develop– Problems in spatial understanding

• Klinefelter Syndrome (XXY):– 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

Genetic Problems• Gene Disordersa) 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

– Some who inherit the dominant gene exhibit uncontrollable tics and explosive outbursts

– Most have milder, barely noticeable symptoms

b) Recessive Disorders• Millions of different types, lethal conditions are

rare• Sickle-cell trait

– Offers some protection against malaria– African carriers are more likely than non-carriers to

survive• Cystic fibrosis

– More common among people with northern European ancestors

– Carriers may have been protected against cholera

Genetic Problems

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

• 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 development1. 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 PeriodFetus• 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 22-week 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, breast-

feeding, 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

• 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 low-

birthweight 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 conclusions1. Genes affect every aspect of human behavior2. Nongenetic influences begin at conception and

continue lifelong, sometimes altering genetic instructions

3. Most environmental influences on children raised in the same home are NOT shared, partly because parents treat each child differently

4. 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• Some zygotes carry genes that make them

vulnerable• Certain genes increase likelihood of cleft lip in

Japanese people

• Some women do not metabolize folic acid well• 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• 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

3. Amount of Exposure• Threshold effect: Certain teratogens are relatively

harmless until exposure reaches a certain level• Bisphenol A (BPA)

– 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

– High doses can be harmful

Significant Factors

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