PSYC 2314Lifespan Development

Chapter 3

Heredity and Environment

Development Begins

• Conception occurs when the male gamete (sperm) penetrates the membrane of the female gamete (the ovum); the gametes then fuse, and their genetic material combines, to form the one-celled zygote.

• Within hours, the zygote initiates the human development through the processes of duplication and division. Soon after, differentiation begins. Each body cell created from these processes carries an exact copy of the zygote’s genetic instructions.

Development Begins

• Instructions on each gene that specify the form, location, and function of each body cell are “written” in a chemical code made up of four bases:– Adenine– Guanine– Cytosine– Thymine

Development Begins

• The 23rd pair of chromosomes, which determines sex, is designated XY in the male and XX in the female. The critical factor in the determination of the zygote’s sex is which sperm reaches the ovum first, a Y sperm, creating a boy, or an X sperm, creating a girl.

Development Begins

• Genes accomplish two goals that are essential to our survival:– Ensure genetic continuity across the species– Ensure genetic diversity within it

Development Begins

• Age-related changes in the sex ratio– Worldwide, slightly more males than females

are born each year.– Because males have a slightly higher rate of

childhood death, a balance occurs at about 25.– After age 50, the sex ratio favors women.

Development Begins

• Monozygotic Twins– A single zygote splits into two separate

identical cells that develop into genetically identical, who have the potential for developing the same physical appearance and psychological characteristics and the same vulnerability to specific diseases.

Development Begins

• Dizygotic Twins– Begin life as two separate zygotes and share no

more genes than any other siblings.

Genotype to Phenotype

• Polygenetic: affected by many genes

• Multifactorial: influenced by many factors, including environmental

• Genotype: the sum total of all the genes a person inherits

• Phenotype: the sum total of all the genes that are actually expressed

Genotype to Phenotype

• Additive gene: when genes interact additively, the phenotype reflects the contributions of all the genes that are involved.

• Dominant gene: the stronger of an interacting pair of genes.

• Recessive gene: the weaker of an interacting pair of genes.

Genotype to Phenotype

• X-linked Genes: genes that are on the X chromosome.– If an X-linked gene is recessive—as are the

genes for most forms of color-blindness, many allergies, several diseases, and some learning disabilities—the fact that it is on the X chromosome is critical.

Genotype to Phenotype

• Genetic Imprinting– Certain genes tend to be expressed differently

when they are inherited from the mother than when they are inherited from the father.

– Some of the genes that influence height, insulin production, and several forms of mental retardation affect a child in different ways—even in opposite ways—depending on which parent they came from.

Genotype to Phenotype

• Effects of genes and environment on development– Research has revealed that most behavioral traits are

affected by the interaction of large number of genes with environmental factors.

– Genes are part of a person’s basic foundation, affecting many aspects of life while determining none.

– Even traits that show a strong genetic influence are also affected by environment.

Environmental Influences on Emotional Impairment

• The following psychological traits have strong genetic influences, but they are also affected by the social atmosphere:– Shyness (inhibition)– Intelligence– Emotionality– Activity level– Aggression

Environmental Influences of Mental Disorders

• Both genes and environment are influential in determining psychopathologies:– Depression– Phobias– Antisocial behavior– Schizophrenia– Alcoholism

Inherited Abnormalities

• Major Methods of Prenatal Diagnosis:– Pre-implantation Testing– Alphafetoprotein Assay– Ultrasound

Inherited Abnormalities

• Prenatal Testing:– Fetoscopy– Amniocentesis– Chorionic Villi Sampling

Chromosomal Miscount

• Down Syndrome (Trisomy-21)– Individual has 3 chromosomes at site 21– Specific facial characteristics: round face, thick

tongue, unusual eyes.– Also, heart abnormalities, hearing problems,

muscle weakness, mental retardation, and short stature

Chromosomal Miscount

• Abnormalities at the 23rd location– At the 23rd pair, either missing a X chromosome

or has two or more other sex chromosomes in addition to the first X.

– Impair cognitive and psychosocial development and sexual maturation

Chromosomal Discount

• The Fragile X– A disorder in which part of the X chromosome

is attached to the rest of it by a very slim string of molecules.

– Mental deficiency– Inadequate social skills and extreme shyness

Harmful Genes

• Huntington’s Chorea– A central nervous system disease caused by a gene that

remains inactive until adulthood, by which time a person could have had many children.

• Tourette Syndrome– Exhibit recurrent uncontrollable tics and explosive

outbursts of verbal obscenities (30%). An occasional twitch and a postponal impulse to speak inappropriately (70%).

Genetic Counseling

• Situations in which couples should seek genetic testing and counseling:– Individuals who have a parent, sibling, or child

with serious genetic condition.– Couples who have a history of early

spontaneous abortions, stillbirths, or infertility.

Genetic Counseling

• Situations in which couples should seek genetic testing and counseling:– Couples who are from the same ethnic group or

subgroup—especially if the group is a small one or most particularly if the couple are close relatives.

– Women over age 34.