Human Development Chapter 2

Gene

A specific sequence of nucleotides that serves as a recipe for making proteins.

Genes

Units of heredity located on chromosomes that influence development and traits.

Proteins

Molecules made by genes that influence the structure and function of cells.

Chromosomes

Structures in the nucleus of cells that contain genes.

Human cells

Normally contain 46 chromosomes (23 pairs).

Mitosis

Process where a cell copies its chromosomes and divides into two identical cells.

Meiosis

Process that produces gametes (sperm and eggs) with half the normal number of chromosomes.

Gametes

Reproductive cells (sperm and ova) containing 23 chromosomes.

Autosomes

The 22 pairs of chromosomes that are not sex chromosomes.

Sex chromosomes

The 23rd pair of chromosomes that determine biological sex.

XX

Genetic combination for a female.

XY

Genetic combination for a male.

Genotype

The complete set of genes an individual inherits.

Phenotype

The observable characteristics expressed from genes.

Alleles

Different versions of the same gene.

Homozygous

Having two identical alleles for a trait.

Heterozygous

Having two different alleles for a trait.

Dominant gene

A gene that is expressed even when paired with a different allele.

Recessive gene

A gene expressed only when paired with another recessive allele.

Polygenic traits

Traits controlled by multiple genes.

Incomplete dominance

When a dominant gene does not completely mask a recessive gene.

Carrier

A person with one recessive gene who usually does not show the disorder but can pass it to offspring.

Sickle Cell Disease

A recessive disorder causing sickle-shaped red blood cells that reduce oxygen transport.

Monozygotic (Identical) Twins

Twins formed when one fertilized egg splits into two embryos; they share the same genotype.

Dizygotic (Fraternal) Twins

Twins formed when two separate eggs are fertilized by two different sperm; they share about 50% of their genes.

Older maternal age

Increases the chance of having fraternal twins.

Fertility drugs

Increase the likelihood of fraternal twins.

Genetic disorder

A disease or condition caused by abnormal genes or chromosomes.

Dominant gene disorder

A disorder that occurs when one abnormal dominant gene is inherited.

Recessive gene disorder

A disorder that occurs only when two recessive genes are inherited.

Sex-linked disorder

A disorder caused by a defective gene on the X chromosome.

Tourette Syndrome

A dominant disorder characterized by uncontrollable motor and vocal tics.

Huntington's Disease

A fatal dominant disorder affecting the nervous system that usually appears in midlife.

Cystic Fibrosis

A recessive disorder causing thick mucus that affects breathing and digestion.

Phenylketonuria (PKU)

A metabolic disorder in which the body cannot break down phenylalanine.

Tay-Sachs Disease

A recessive disorder causing progressive brain damage due to an enzyme deficiency.

Albinism

A recessive condition with little or no melanin in the skin, hair, and eyes.

Fragile X Syndrome

An X-linked disorder causing learning and behavioral problems, usually more severe in males.

Hemophilia

An X-linked disorder in which blood does not clot normally.

Duchenne Muscular Dystrophy

An X-linked disorder causing progressive muscle weakness.

Chromosomal abnormality

A condition caused by having too many or too few chromosomes.

Trisomy

Three copies of a chromosome instead of the normal two.

Down Syndrome (Trisomy 21)

A chromosomal disorder caused by an extra chromosome 21, resulting in intellectual disability and characteristic physical features.

Trisomy 13

A chromosomal disorder caused by an extra chromosome 13 that usually results in severe birth defects and early death.

Trisomy 18

A chromosomal disorder caused by an extra chromosome 18 resulting in severe birth defects and early death.

Turner Syndrome (XO)

A disorder in females caused by the loss of one X chromosome, resulting in short stature, infertility, and incomplete sexual development.

Klinefelter Syndrome (XXY)

A disorder in males caused by an extra X chromosome resulting in low testosterone, infertility, and possible breast development.

Genetic counseling

A service that helps individuals understand inherited disorders, testing results, and risks for future children.

Behavioral genetics

The study of how genes and the environment interact to influence behavior.

Nature vs. Nurture

The debate over whether genetics or environment has the greater influence on development.

Genotype-environment correlation

The process by which genes influence the environments people experience.

Passive genotype-environment correlation

Children inherit both genes and environments from their parents.

Evocative genotype-environment correlation

People's inherited traits influence how others respond to them.

Active genotype-environment correlation (Niche picking)

People seek environments that fit their genetic tendencies.

Genotype-environment interaction

Environmental effects differ depending on a person's genetic makeup.

Epigenetics

The study of changes in gene expression caused by environmental factors without altering DNA.

Prenatal development

Development from conception until birth.

Germinal period

The first two weeks after conception when the zygote divides and implants.

Zygote

The fertilized egg containing genetic material from both parents.

Blastocyst

A ball of about 100 cells formed after several days of cell division.

Embryonic disk

The inner cells of the blastocyst that become the embryo.

Trophoblast

The outer cells of the blastocyst that become the support system for the embryo.

Implantation

The attachment of the blastocyst to the uterine wall.

Embryonic period

Weeks 3–8 when major organs and body structures develop.

Placenta

The organ that provides oxygen and nutrients from the mother to the fetus.

Umbilical cord

The structure connecting the fetus to the placenta.

Cephalocaudal development

Growth from head to toe.

Proximodistal development

Growth from the center of the body outward.

Fetal period

Week 9 until birth when growth and maturation occur.

Age of viability

Around 24 weeks when a fetus may survive outside the womb with medical care.

Neural plate

The embryonic structure that develops into the nervous system.

Neural tube

The structure that develops into the brain and spinal cord.

Forebrain

The brain region responsible for thinking, memory, and emotion.

Midbrain

The brain region involved in vision, hearing, and movement.

Hindbrain

The brain region controlling breathing, heartbeat, and balance.

Neurogenesis

The formation of neurons.

Gray matter

Brain tissue containing neuron cell bodies.

White matter

Brain tissue containing myelinated axons.

Myelin

A fatty covering around axons that speeds nerve impulse transmission.

Teratology

The study of the causes of birth defects.

Teratogen

Any environmental factor that can cause birth defects.

Critical period

A stage of development when an organ or body system is especially vulnerable to damage.

Factors affecting teratogens

Timing, amount of exposure, number of teratogens, genetics, and fetal sex.

Alcohol

The leading preventable cause of intellectual disabilities during pregnancy.

Fetal Alcohol Spectrum Disorders (FASD)

A range of disorders caused by prenatal alcohol exposure.

Fetal Alcohol Syndrome (FAS)

The most severe form of FASD, causing facial abnormalities, learning disabilities, and behavioral problems.

Binge drinking

Consuming four or more alcoholic drinks within 2–3 hours during pregnancy, greatly increasing the risk of FASD.

Nicotine

A chemical in tobacco that reduces oxygen available to the fetus.

Effects of smoking during pregnancy

Low birth weight, premature birth, stillbirth, birth defects, SIDS, and learning disabilities.

Secondhand smoke

Smoke inhaled from another person's cigarette that can harm the fetus.

Thirdhand smoke

Toxic residue from tobacco smoke left on clothing, furniture, and surfaces.

Prescription drugs during pregnancy

Should only be taken under medical supervision because some can cause birth defects.

Ibuprofen

A pain reliever that may cause serious fetal blood flow problems during the last trimester.

Illicit drugs

Illegal or misused drugs that can harm fetal development.

Effects of cocaine

Low birth weight, miscarriage, and stillbirth.

Effects of heavy marijuana use

Problems with fetal brain development.

Neonatal Abstinence Syndrome (NAS)

Withdrawal symptoms in newborns exposed to addictive drugs before birth.

Effects of prenatal drug exposure

Premature birth, birth defects, heart defects, learning disabilities, behavioral problems, and increased risk of SIDS.

Lead

An environmental pollutant linked to fertility problems, miscarriage, premature birth, low birth weight, and slowed brain development.

Pesticides

Chemicals that can increase the risk of birth defects, miscarriage, premature birth, learning problems, and low birth weight.