chapter 2 heredity and prenatal development. the influence of heredity on development

CHAPTER 2CHAPTER 2

Heredity and Prenatal DevelopmentHeredity and Prenatal Development

The Influence of Heredity The Influence of Heredity on Developmenton Development

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.

Mitosis

• Life begins as a single cell or zygote that divides repeatedly.

• Mitosis – Genetic code carried into new cells in our bodies

• DNA breaks apart; double helix duplicates

• DNA forms two camps on either side of the cell, and then the cell divides

Fig. 2-1, p. 24

Fig. 2-2, p. 25

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, one member of each pair goes to each newly formed cell.

• Each new cell nucleus contains only 23 chromosomes, not 46.

Fig. 2-3, p. 25

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

Dominant and Recessive Traits

• Traits determined by pairs of genes

– each member of pairs of genes termed an allele

• Homozygous– having two identical alleles

• Heterozygous– having two different alleles

• Incomplete dominance/codominances

– when both alleles are shown

• Dominant trait – trait that is expressed

• Recessive trait – trait that is not expressed

when genes have been paired with dominant genes

• Carriers – people who bear one

dominant gene and one recessive trait

Fig. 2-4, p. 26

Table 2-1, p. 27

Chromosomal and Genetic Abnormalities

• Down syndrome– Caused by an extra chromosome of the 21st 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

Fig. 2-5, p. 28

Sex-Linked Chromosomal Abnormalities

MALES

• 1 male in 700-1,000 has XYY– leads to heightened male

secondary sex characteristics– most individuals with disorder are

infertile

• 1 male in 500 has Klinefelter syndrome (XXY)

– leads to inadequate development of male primary and secondary sex characteristics

FEMALES

• 1 female in 2,500 has a single X resulting in Turner Syndrome

– poorly developed ovaries, short, and infertile.

• 1 female in 1,000 has XXX sex chromosomal structure called Triple X syndrome

– normal in appearance but tend to show lower-than-average language skills and poorer memory for recent events

Klinefelter Syndrome (XXY)

• 1 male in 500

• Caused by extra X on sex chromosome

• Produces less testosterone than normal males

• Characteristic features of testes, deepening of voice, musculature, and male pattern of body hair do not develop properly

• Typically treated with testosterone replacement therapy

Turner Syndrome (X)

• 1 female in 2,500

• Single X chromosome

• External genitals are normal, ovaries poorly developed, producing little estrogen

• Shorter than average and infertile

• Cognitive deficits with low estrogen: problems with visual-spatial skills, mathematics, and nonverbal memory

Triple X Syndrome (XXX)

• 1 female in 1,000

• Appears normal, but demonstrates lower-than-average language skills

• Poorer memory for recent events

• Development of external sex organs appears normal

• Increased incidence of infertility

Phenylketonuria (PKU)

• Enzyme disorder transmitted by a recessive gene affecting 1 child in 8,000

• 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 children can be placed on diets low in phenylalanine and develop normally

Huntington’s Disease (HD)

• Affects 1 in 18,000 Americans• Fatal, progressive degenerative disorder• Dominant trait• Physical symptoms include uncontrollable muscle

movements• Psychological symptoms include loss of intellectual

functioning and personality change• Onset of HD is delayed until middle adulthood• Possibly half their offspring will develop it• Medicines can deal with some symptoms

Sickle-Cell Anemia• Carriers

-1 in 10 African Americans-1 in 20 Latino or Latina Americans

• 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

-painful and swollen joints-jaundice-potentially fatal conditions such as pneumonia, stroke, and heart and kidney failure

Tay-Sachs Disease

• 1 in 30 Jewish Americans are carriers

• Caused by recessive gene

• Causes the central nervous system to degenerate, resulting in death

• Commonly found among children in Jewish families of Eastern European background

Cystic Fibrosis

• Approx. 30,000 Americans have disorder, 10 million more are carriers (1 in 31 people)

• Caused by recessive gene• Most common fatal hereditary disease among

Caucasians• Children suffer from excessive production of thick

mucus that clogs the pancreas and lungs• Most victims die of respiratory infections in their 20s

Sex-Linked Genetic Abnormalities

• Genetic defects only carried on the X sex chromosome • Hemophilia – genetic disorder in which blood does not

clot properly• Duchenne – muscular dystrophy; sex linked• Involve recessive genes• Females with two X sex chromosomes are less likely

than males 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

• 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.

Fig. 2-6, p. 30

Chorionic Villus Sampling

• Carried out between 9th and 12th 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 the EnvironmentHeredity and the Environment

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.

Conception: Against All OddsConception: Against All Odds

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


• 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.

• 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


• Chromosomes from the sperm cell line up across from corresponding chromosomes in the egg cell.

• Form 23 new pairs with unique set of genetic instructions

• The direction that sperm travel is guided by a change in calcium ions that occurs when an ovum is released.

Fig. 2-8, p. 33

Fig. 2-9, p. 34

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

Prenatal DevelopmentPrenatal Development

Prenatal Development

• 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 Stage

• The period from conception to implantation

• 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

• Trophoblast – Four membranes that will protect and nourish the embryo

1. One produces blood cells, then it disappears2. One develops into umbilical cord and blood vessels of

the placenta3. One develops into the amniotic sac4. One becomes the chorion, which will line the placenta

The Embryonic Stage

• Development follows cephalocaudal (head to tail) 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 organs that lie near the spine

• Ectoderm-Outer layer of cells-Develops into nervous system, sensory organs, nails, hair,

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

The Embryonic Stage (cont’d)

• 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-Eyes, ears, nose, and mouth take shape-Nervous system and brain begins to develop

• During 2nd month, nervous system begins to send messages

• Embryo is 1 inch long and weighs 1/30th of an ounce• Teeth buds are formed by end of stage


• 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


• 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 and waste with the mother.

• Mother and embryo have separate circulatory systems.

• Placenta permits oxygen and nutrients to reach the embryo from the mother, and permits carbon dioxide and waste products to pass to the mother from the embryo.


• Mother eliminates waste through her lungs and kidneys.

• Drugs such as aspirin, narcotics, alcohol, tranquilizers, and others cross the placenta and affect the fetus.

• Placenta secretes hormones that preserve pregnancy, prepare breasts for nursing, and stimulate contractions.

• Placenta passes from the birth canal after the baby, hence the term afterbirth.

The 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 advances from 1 ounce to 2 pounds and grows from about 3 to 14 inches

The Fetal Stage (cont’d)

• Fetus can open and shut eyes, suck thumb at end of second trimester

• Gains about 5 1/2 pounds and doubles in length

• 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

The Fetal Stage (cont’d)

• By 13th week, fetus responds to sound waves

• Experiment by DeCasper and Fifer (1980) demonstrated that a fetus is learning while in uterus

• Fetal movements are noticeable during middle of 4th month

• At 29-30 weeks, fetus moves limbs vigorously and turns summersaults

• Fetus gets cramped as it grows, becomes less active during 9th month

Fig. 2-12, p. 41

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


• 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


• Rubella-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)

• Preeclampsia (or Toxemia)-Life-threatening; 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

Drugs Taken by the Parents

• Thalidomide-A sedative used for treatment of insomnia and nausea-Causes missing or stunted limbs during second month of pregnancy

Drugs Taken by the Parents (cont’d)

• Hormones-Women at risk for miscarriages have been prescribed hormones to help maintain their pregnancies.

-Progestin

-Synthetic version of naturally occurring progesterone

-Can masculinize the external sex organs of female embryos

-Diethylstilbestrol (DES) -Used to prevent miscarriage during 1940s and 1950s -Caused cervical and testicular cancer in some offspring -1 in 1,000 daughters of DES users will develop cancer in the

reproductive tract


• 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


Marijuana• Contributes to slower fetal growth, low birth weight

• Babies of women who regularly used marijuana show increased tremors and startling, suggesting immature development of the nervous system.

• 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


Cocaine• 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


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.

Fig. 2-13, p. 44


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.


• 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.

Environmental Hazards

• 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 resulted in babies that were smaller and showed poorer motor functioning and memory defects.

• Exposure to radiation has been linked to mental retardation and physical deformity; suggestion to avoid unnecessary x-rays

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.

chapter 2 heredity and prenatal development. the influence of heredity on development

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