Child DevelopmentWinter 2011Chapter 3

What is our basic genetic endowment, and how can human development go off track?

How do the environment and genetics work together to determine human characteristics?

Which human characteristics are significantly influenced by heredity?

What happens during the prenatal stages of development?

CHAPTER 3 KEY QUESTIONS

What are the threats to the fetal environment, and what can be done about them?

CHAPTER 3 KEY QUESTIONS

This first cell is created when a male reproductive cell, a sperm, pushes through the membrane of the ovum, the female reproductive

cell.

Gametes The sex cells from the mother and father that form a new cell at conception

Zygote The new cell formed by the process of fertilization

EARLIEST DEVELOPMENT

Genes The basic unit of genetic information

DNA (deoxyribonucleic acid) molecules The substance that genes are composed of that determines the nature of every cell in the body and

how it will function

Chromosomes Rod-shaped portions of DNA that are organized in 23 pairs

GENES AND CHROMOSOMES: THE CODE

OF LIFE

Through a process called mitosis, which accounts for the replication of most types of cells, nearly all the cells of the body will

contain the same 46 chromosomes as the zygote.

When the sperm and ova are formed in the adult human body in a process called meiosis, each gamete receives one of the two

chromosomes that make up each of the 23 pairs.

GENES AND CHROMOSOMES: THE CODE

OF LIFE

Monozygotic twins Twins who are genetically identical

Dizygotic twins Twins who are produced when two separate ova are fertilized by two separate sperm at roughly the same time

MULTIPLE BIRTHS: TWO—OR MORE—FOR THE GENETIC PRICE OF ONE

If the sperm contributes an X chromosome when it meets an ovum (which will always contribute an X chromosome), the child will have an

XX pairing on the 23rd chromosome and will be a female.

If the sperm contributes a Y chromosome, the result will be an XY pairing and will be a male

BOY OR GIRL? ESTABLISHING

THE SEX OF THE CHILD

We need to consider the basic mechanisms involved in the way that the genes we inherit from our parents transmit information.

Dominant trait The one trait that is expressed when two competing traits are present

Recessive trait A trait within an organism that is present, but is not expressed

THE BASICS OF GENETICS: THE MIXING AND MATCHING OF

TRAITS

Genotype The underlying combination of genetic material present (but not outwardly visible) in an organism

Phenotype An observable trait; the trait that actually is seen

Some of the genes form pairs called alleles, genes governing traits that may take alternate forms, such as hair or eye color.

THE BASICS OF GENETICS: THE MIXING AND MATCHING OF

TRAITS

Homozygous Inheriting from parents similar genes for a given trait

Heterozygous Inheriting from parents different forms of a gene for a given trait

THE BASICS OF GENETICS: THE MIXING AND MATCHING OF

TRAITS

We can see this process at work in humans by considering the transmission of phenylketonuria (PKU), an inherited disorder in which a child is unable to make use of phenylalanine, an essential amino acid

present in proteins found in milk and other foods.

TRANSMISSION OFGENETIC INFORMATION

Polygenic inheritance Inheritance in which a combination of multiple gene pairs is responsible for the production of a particular trait

Genes also vary in terms of their reaction range, the potential degree of variability in the actual expression of a trait due to environmental

conditions.

X-linked genes Genes that are considered recessive and located only on the X chromosome

POLYGENIC TRAITS

Mapping the specific sequence of genes on each chromosome stands as one of the most important moments in the history of genetics, and,

for that matter, all of biology.

Already, the mapping of the gene sequence has provided important advances in our understanding of genetics.

THE HUMAN GENOME AND BEHAVIORAL GENETICS: CRACKING

THE GENETIC CODE

Behavioral genetics The study of the effects of heredity on behavior

BEHAVIORAL GENETICS

GENETIC BASIS OF DISORDERS AND TRAITS

A recessive gene responsible for a disorder may be passed on unknowingly from one generation to the next, revealing itself only

when, by chance, it is paired with another recessive gene.

In some cases, genes become physically damaged.

INHERITED AND GENETIC DISORDERS: WHEN DEVELOPMENT GOES AWRY

For instance, genes may break down due to wear-and-tear or to chance events occurring during the cell division processes of meiosis

and mitosis. And sometimes, for no known reason, genes spontaneously change their form, a process called spontaneous

mutation.

INHERITED AND GENETIC DISORDERS: WHEN DEVELOPMENT GOES AWRY

Other inherited and genetic disorders include:

Down syndrome A disorder produced by the presence of an extra chromosome on the 21st pair; once referred to as mongolism

Fragile X syndrome A disorder produced by injury to a gene on the X chromosome, producing mild to moderate mental retardation

INHERITED AND GENETIC DISORDERS: WHEN DEVELOPMENT GOES AWRY

Sickle-cell anemia A blood disorder that gets its name from the shape of the red blood cells in those who have it

Tay-Sachs disease A disorder that produces blindness and muscle degeneration prior to death; there is no treatment

Klinefelter’s syndrome A disorder resulting from the presence of an extra X chromosome that produces underdeveloped genitals,

extreme height, and enlarged breasts

INHERITED AND GENETIC DISORDERS: WHEN DEVELOPMENT GOES AWRY

Due to advances in behavioral genetics, genetic difficulties increasingly can be forecast, anticipated, and planned for before a child’s birth,

enabling parents to take steps before the child is born to reduce the severity of certain genetic conditions.

INHERITED AND GENETIC DISORDERS: WHEN DEVELOPMENT GOES AWRY

Genetic counseling The discipline that focuses on helping people deal with issues relating to inherited disorders

Possible genetic defects, such as the presence of an extra sex chromosome, can be identified by assembling a karyotype, a chart

containing enlarged photos of each of the chromosomes.

GENETIC COUNSELING: PREDICTING THE FUTURE FROM THE GENES OF

THE PRESENT

Ultrasound sonography A process in which high-frequency sound waves scan the mother’s womb to produce an image of the unborn

baby, whose size and shape can then be assessed

Chorionic villus sampling (CVS) A test used to find genetic defects that involves taking samples of hairlike material that surrounds the embryo

PRENATAL TESTING

Amniocentesis The process of identifying genetic defects by examining a small sample of fetal cells drawn by a needle inserted into the

amniotic fluid surrounding the unborn fetus

PRENATAL TESTING

FETAL DEVELOPMENTMONITORING TECHNIQUES

The newest role for genetic counselors involves testing people to identify whether they themselves, rather than their children, are

susceptible to future disorders because of genetic abnormalities.

As our understanding of genetics continues to grow, researchers and medical practitioners have moved beyond testing and counseling to

actively working to change flawed genes.

SCREENING FORFUTURE PROBLEMS

DNA-BASED GENETIC TESTS

DNA-BASED GENETIC TESTS, CONT’D

As developmental research accumulates, it is becoming increasingly clear that to view behavior as due to either genetic or environmental

factors is inappropriate.

A given behavior is not caused just by genetic factors; nor is it caused solely by environmental forces.

THE ROLE OF THE ENVIRONMENT IN DETERMINING THE EXPRESSION OF

GENES: FROM GENOTYPES TO PHENOTYPES

Temperament Patterns of arousal and emotionality that represent consistent and enduring characteristics in an individual

TEMPERAMENT

Such findings illustrate that many traits reflect multifactorial transmission, meaning that they are determined by a combination of

both genetic and environmental factors.

In multifactorial transmission, a genotype provides a particular range within which a phenotype may achieve expression.

INTERACTION OF FACTORS

Nonhuman Animal Studies: Controlling Both Genetics and Environment

It is relatively simple to develop breeds of animals that are genetically similar to one another in terms of specific traits.

By observing animals with similar genetic backgrounds in different environments, scientists can determine, with reasonable precision,

the effects of specific kinds of environmental stimulation.

STUDYING DEVELOPMENT: HOW MUCH

IS NATURE? HOW MUCH IS NURTURE?

Nature conveniently has provided the potential to carry out various kinds of “natural experiments”—in the form of twins.

Because the inherited backgrounds of identical, monozygotic twins are precisely the same, any variations in their behavior must be due

entirely to environmental factors.

CONTRASTING RELATEDNESS AND BEHAVIOR: ADOPTION, TWIN, AND

FAMILY STUDIES

Studies of nonidentical, dizygotic twins also present opportunities to learn about the relative contributions of nature and nurture.

The general conclusion resulting from decades of research: Virtually all traits, characteristics, and behaviors are the joint result of the

combination and interaction of nature and nurture.

CONTRASTING RELATEDNESS AND BEHAVIOR: ADOPTION, TWIN, AND

FAMILY STUDIES

Monozygotic twins are merely the most extreme example of the fact that the more genetically similar two people are, the more likely they

are to share physical characteristics.

Other, less obvious physical characteristics also show strong genetic influences. For instance, blood pressure, respiration rates, and even

the age at which life ends are more similar in closely related individuals than in those who are less genetically alike.

PHYSICAL TRAITS:FAMILY RESEMBLANCES

No other issue involving the relative influence of heredity and environment has generated more research than the topic of

intelligence.

Genetics plays a significant role in intelligence.

INTELLIGENCE: MORE RESEARCH, MORE

CONTROVERSY

Studies have shown that the closer the genetic link between two individuals, the greater the correspondence of their overall IQ scores.

Although it is clear that heredity plays an important role in intelligence, investigators are much more divided on the question of the degree to

which it is inherited.

INTELLIGENCE: MORE RESEARCH, MORE

CONTROVERSY

Increasing research evidence suggests that some of our most basic personality traits have genetic roots.

Neuroticism, as used by personality researchers, is the degree of emotional stability an individual characteristically displays.

GENETIC AND ENVIRONMENTAL INFLUENCES ON PERSONALITY: BORN

TO BE OUTGOING?

Extroversion is the degree to which a person seeks to be with others, to behave in an outgoing manner, and generally to be sociable.

Clearly, genetic factors play a role in determining personality. At the same time, the environment in which a child is raised also affects

personality development.

GENETIC AND ENVIRONMENTAL INFLUENCES ON PERSONALITY: BORN

TO BE OUTGOING?

MEAN BEHAVIORAL SCORES

If genetics were the sole cause of psychological disorder, the risk for an identical twin would be 100%.

However, other factors account for disorders, ranging from structural abnormalities in the brain to a biochemical imbalance.

Several other psychological disorders have been shown to be related, at least in part, to genetic factors.

PSYCHOLOGICAL DISORDERS: THE ROLE OF GENETICS AND ENVIRONMENT

The role of genetics is often to produce tendency toward a future course of development. When and whether a certain behavioral

characteristic will actually be displayed depends on the nature of the environment.

PSYCHOLOGICAL DISORDERS: THE ROLE OF GENETICS AND ENVIRONMENT

According to Scarr, the genetic endowment provided to children by their parents not only determines their genetic characteristics, but also

actively influences their environment.

Children tend to actively focus on those aspects of their environment that are most connected with their genetically determined abilities.

CAN GENES INFLUENCE THE ENVIRONMENT?

Children also pay less attention to those aspects of the environment that are less compatible with their genetic endowment.

The genetically driven temperament of a child may evoke certain environmental influences.

CAN GENES INFLUENCE THE ENVIRONMENT?

Fertilization The process by which a sperm and an ovum—the male and female gametes, respectively—join to form a single new cell

When sperm enter the vagina, they begin a winding journey that takes them through the cervix, the opening into the uterus, and into the

fallopian tube, where fertilization may take place.

FERTILIZATION: THE MOMENT

OF CONCEPTION

Only a tiny fraction of the 300 million cells that are typically ejaculated during sexual intercourse ultimately survive the journey.

FERTILIZATION: THE MOMENT

OF CONCEPTION

The prenatal period consists of three phases:

germinal

embryonic

fetal

THE STAGES OF THE PRENATAL PERIOD: THE ONSET OF DEVELOPMENT

STAGES OF THE PRENATAL PERIOD

Germinal stage The first, and shortest, stage of the prenatal period, which takes place during the first 2 weeks following conception

Placenta A conduit between the mother and fetus, providing nourishment and oxygen via the umbilical cord

THE GERMINAL STAGE: FERTILIZATION TO 2 WEEKS

Embryonic stage The period from 2 to 8 weeks following fertilization during which significant growth occurs in the major organs and body

systems

The outer layer of the embryo, the ectoderm, will form skin, hair, teeth, sense organs, and the brain and spinal cord.

THE EMBRYONIC STAGE: 2 WEEKS TO 8 WEEKS

The endoderm, the inner layer, produces the digestive system, liver, pancreas, and respiratory system. Sandwiched between the ectoderm and endoderm is the mesoderm, from which the muscles, bones, blood

and circulatory system are forged.

THE EMBRYONIC STAGE: 2 WEEKS TO 8 WEEKS

Fetal stage The stage that begins at about 8 weeks after conception and continues until birth

Fetus A developing child, from 8 weeks after conception until birth

The brain becomes increasingly sophisticated during the fetal stage.

THE FETAL STAGE:8 WEEKS TO BIRTH

The two symmetrical left and right halves of the brain, known as hemispheres, grow rapidly, and the interconnections between neurons

become more complex.

The neurons become coated with an insulating material called myelin which helps speed the transmission of messages from the brain to the

rest of the body.

THE FETAL STAGE:8 WEEKS TO BIRTH

Infertility The inability to conceive after 12 to 18 months of trying to become pregnant

Artificial insemination A process of fertilization in which a man’s sperm is placed directly into a woman’s vagina by a physician

PREGNANCY PROBLEMS

In vitro fertilization (IVF) A procedure in which a woman’s ova are removed from her ovaries, and a man’s sperm are used to fertilize the

ova in a laboratory

Gamete intrafallopian transfer (GIFT) and zygote intrafallopian transfer (ZIFT) are procedures in which an egg and sperm or fertilized egg are

implanted in a woman’s fallopian tubes.

PREGNANCY PROBLEMS

In vitro fertilization is increasingly successful, with success rates of as high 33% for younger women (but with lower rates for older women).

Furthermore, reproductive technologies are becoming increasingly sophisticated, permitting parents to choose the sex of their baby.

PREGNANCY PROBLEMS

The use of surrogate mothers, in vitro fertilization, and sex selection techniques present a web of ethical and legal issues, as well as many

emotional concerns.

Even more troubling are concerns raised by sex selection techniques.

For the moment, many of these ethical issues remain unresolved.

ETHICAL ISSUES

A miscarriage—known as a spontaneous abortion—occurs when pregnancy ends before the developing child is able to survive outside the mother’s womb. The embryo detaches from the wall of the uterus

and is expelled.

MISCARRIAGE AND ABORTION

In abortion, a mother voluntarily chooses to terminate pregnancy.

Regarding risk of future psychological problems, the findings are mixed, and there are significant individual differences in how women

respond to the experience of abortion.

MISCARRIAGE AND ABORTION

There are certain aspects of a mothers’ and fathers’ behavior, both before and after conception, that can produce lifelong consequences

for the child.

Some consequences show up immediately, but half the possible problems aren’t apparent before birth.

Other problems, more insidious, may not appear until years after birth.

THE PRENATAL ENVIRONMENT: THREATS

TO DEVELOPMENT

Teratogen A factor that produces a birth defect

The timing and quantity of exposure to a teratogen are crucial.

At some phases of prenatal development, a certain teratogen may have only a minimal impact.

At other periods, however, the same teratogen may have profound consequences.

THE PRENATAL ENVIRONMENT: THREATS

TO DEVELOPMENT

Most of our knowledge of the environmental factors that affect the developing fetus comes from the study of the mother.

There are ways to counteract the types of maternal malnourishment that affect prenatal development.

MOTHER’S DIET

Dietary supplements given to mothers can reverse some of the problems produced by a poor diet.

Research shows that babies who were malnourished as fetuses can overcome some of the effects of their early malnourishment.

MOTHER’S DIET

More women are giving birth later in life than was true just 2 or 3 decades ago.

Women who give birth when over the age of 30 are at greater risk for a variety of pregnancy and birth complications than are younger mothers.

The risks involved in pregnancy are greater not only for older mothers but also for atypically young women.

MOTHER’S AGE

The higher mortality rate for babies of adolescent mothers reflects more than just physiological problems related to the mothers’ young

age.

Young mothers often face adverse social and economic factors which can affect infant health.

MOTHER’S PRENATAL SUPPORT

Mothers who eat the right foods, maintain an acceptable weight, and who exercise appropriately maximize the chances of having a healthy

baby.

A mother’s diseases may affect a developing fetus, depending on when the illness is contracted.

MOTHER’S HEALTH

Mothers’ use of many kinds of drugs—both legal and illegal—poses serious risks to the unborn child.

Even drugs prescribed by medical professionals have sometimes had disastrous consequences. (Thalidomide)

Illicit drugs may pose equally great, and sometimes even greater, risks for the environments of prenatal children.

MOTHER’S DRUG USE

Increasing evidence suggests that even small amounts of alcohol and nicotine can disrupt the development of the fetus.

Fetal alcohol syndrome (FAS) A disorder caused by the pregnant mother consuming substantial quantities of alcohol during pregnancy, potentially resulting in mental retardation and delayed growth in the

child

MOTHERS’ USE OF ALCOHOLAND TOBACCO

Fetal alcohol effects (FAE) A condition in which children display some, although not all, of the problems of fetal alcohol syndrome

due to the mother’s consumption of alcohol during pregnancy

Smokers are two times as likely as nonsmokers to have babies with an abnormally low birthweight, and smokers’ babies are

shorter, on average, than those of nonsmokers.

MOTHERS’ USE OF ALCOHOLAND TOBACCO

Furthermore, women who smoke during pregnancy are 50% more likely to have mentally retarded children.

MOTHERS’ USE OF ALCOHOLAND TOBACCO

It is becoming increasingly clear that fathers’ behavior may indeed influence the prenatal environment.

Fathers-to-be should avoid smoking.

A father’s use of alcohol and illegal drugs can have significant effects on the fetus.

Fathers who are physically or emotionally abusive to their pregnant wives can damage their unborn children.

DO FATHERS AFFECT THE PRENATAL ENVIRONMENT?