Chapters 2–3: Biology, Prenatal Development & Birth

What is a gene?

A basic unit of genetic information made of DNA.

What does DNA determine?

The nature and functioning of every cell in the body.

How many genes and chromosome pairs does each human cell contain?

About 20,000–25,000 genes and 23 pairs of chromosomes.

What are chromosomes?

Rod-shaped portions of DNA organized into 23 pairs.

What was the Human Genome Project?

An ambitious project to map the complete set of human genes.

About how many genes do humans have according to the Human Genome Project?

About 21,500.

Do genes work independently?

No. Genes collaborate with each other and with nongenetic factors inside and outside the body.

How does the environment affect genes?

It can turn genes “on” or “off.”

What is mitosis?

Cell division in which the nucleus and chromosomes duplicate and divide into two genetically identical cells.

How many chromosomes do cells produced by mitosis have?

46 chromosomes, or 23 pairs.

Which cells undergo mitosis?

All cells except sperm and egg cells.

What is meiosis?

Specialized cell division that creates egg and sperm cells.

What happens during meiosis?

The cell divides twice, producing four cells with half the parent cell's genetic material.

How many chromosomes do cells produced by meiosis contain?

23 unpaired chromosomes.

What happens when sperm fertilizes an egg?

The 23 chromosomes from each parent link together.

What determines whether the typical chromosome pair is XX or XY?

Which chromosome the father contributes.

What is a genotype?

The underlying combination of genetic material in an organism.

What is a phenotype?

An observable trait.

What is a dominant gene?

A gene expressed when two competing traits are present.

What is a recessive gene?

A gene that is present but not expressed unless another recessive trait is also present.

What are sex-linked genes?

Genes involving a mutation carried on the X chromosome.

Why are males more likely to show X-linked diseases?

They do not have a second X chromosome to provide a backup gene.

What is polygenic inheritance?

When several genes interact to determine a characteristic.

What are examples of polygenic characteristics mentioned in the slides?

Asthma, cancer, arthritis, and immune-system functioning.

What causes Down syndrome?

The presence of an extra chromosome on the 21st pair.

What is Klinefelter's syndrome?

A disorder involving an extra X chromosome, producing XXY.

What characteristics are associated with Klinefelter's syndrome?

Underdeveloped genitals, extreme height, and enlarged breasts.

What is XYY syndrome?

A chromosomal condition for which the slides state no intervention/treatment is required.

What is Fragile X syndrome?

An abnormality of the X chromosome causing mild-to-moderate intellectual disability.

What is Turner syndrome?

A condition in which a female has only one X chromosome or a damaged second X chromosome.

What are characteristics of Turner syndrome?

Short/webbed neck, possible infertility, and difficulty with math.

What is PKU?

A condition in which a person cannot digest phenylalanine.

Why is untreated PKU dangerous?

Phenylalanine can build to toxic levels and cause brain damage.

What is sickle-cell anemia?

A blood disorder in which blood cells are misshaped.

What statistics do the slides give for sickle-cell anemia?

About 1/10 African Americans carry the disease; about 1/400 actually have it.

What are symptoms of sickle-cell anemia listed in the slides?

Poor appetite, stunted growth, swollen stomach, and yellowish eyes.

What is Tay-Sachs disease?

A disorder in which the central nervous system begins degenerating.

What happens to children with Tay-Sachs?

They usually die before reaching school age.

Which populations are Tay-Sachs said to occur mainly among?

Ashkenazi Jews and French Canadians.

What does behavior genetics study?

The influence of heredity and environment on individual differences in traits and development.

What do twin studies compare?

Dizygotic/fraternal twins with monozygotic/identical twins.

What would greater similarity among MZ twins suggest?

That a trait is influenced by genetics.

What would MZ twins being just as similar as DZ twins suggest?

A greater influence of nurture/environment.

What do adoption studies examine?

How traits vary among people raised apart from their biological relatives.

What question do adoption studies ask?

Whether people are more similar to their adopted or biological family members.

What is conception?

When sperm unites with an ovum through fertilization.

How long does prenatal development last?

Approximately 266 days, from fertilization to birth.

What are the three periods of prenatal development?

Germinal, embryonic, and fetal periods.

What is the germinal period?

The first and shortest stage of prenatal development, lasting 0–2 weeks.

What happens to the zygote during the germinal period?

It divides and doubles, forming a blastocyst.

What is the blastocyst?

A ball of identical cells.

What is the trophoblast?

The outer layer of cells that forms around the blastocyst.

When does implantation occur?

About 10–14 days after conception.

Where does the blastocyst implant?

In the nutrient-rich wall of the uterus.

What begins once implantation occurs?

The embryonic period.

When does the embryonic period begin and end?

Around the 2nd week through the 8th week.

What three cell layers develop from the blastocyst?

Ectoderm, mesoderm, and endoderm.

What develops from the ectoderm?

Skin, hair, teeth, and central nervous system.

What develops from the mesoderm?

Muscles, bones, blood, and circulatory system.

What develops from the endoderm?

Digestive and respiratory systems.

What is organogenesis?

The process of organ formation.

What is the amnion?

A structure that develops from the trophoblast and contains amniotic fluid.

What does amniotic fluid do?

Provides temperature and humidity control and overall protection for the embryo.

What does the placenta do?

Provides nourishment and oxygen through the umbilical cord and removes waste.

What determines whether male or female genitalia develop?

The presence or absence of testosterone.

When is testosterone released by the testes?

Around the 7th–8th week.

What happens in genetic males?

A gene on the Y chromosome calls for testes to develop.

What do the testes produce?

Testosterone, the primary male hormone that stimulates the male internal reproductive system.

What happens in genetic females?

Ovaries form, and the female reproductive system develops because testosterone is absent.

What is the neural tube?

A hollow, pear-shaped tube on the embryo's back.

When do the top and bottom of the neural tube close?

Around the 24th day.

What happens if the neural tube does not close properly?

Neural-tube complications can occur.

What is anencephaly?

Failure of the portion of the neural tube that would form higher brain functions to close.

What happens with anencephaly?

Death occurs in the womb, during childbirth, or shortly afterward.

What is spina bifida?

Failure of part of the neural tube that would form the spinal cord to fully close.

What can spina bifida cause?

Varying degrees of paralysis.

How rapidly can new neurons proliferate at peak times after neural-tube closure?

About 200,000 per minute.

When does the fetal period begin?

When the major organs begin differentiating, around 8 weeks.

What happens during the fetal period?

Rapid increases in weight and length occur, and organs become more differentiated and begin functioning.

What can a fetus do by three months?

Swallow and urinate.

When can the mother usually feel fetal movement according to the slides?

By the fourth month.

What is the age of viability?

The earliest point at which a fetus can survive outside the womb.

When is the age of viability reached according to the text?

About 24–25 weeks.

What happens to neuronal proliferation during the fetal period?

It continues at high rates.

What is neuronal migration?

The movement of neurons to different areas of the brain.

What is neuronal differentiation?

Neurons increasingly take on specific functions based on where they migrated.

How many neurons do babies have at birth?

Approximately 100 billion.

How long does rapid brain development continue after birth?

It continues rapidly throughout the first two years.

How does the fetus's body proportion change?

At two months, the head is about half the fetus; at birth, it is about one-quarter of total body size.

What is infertility?

The inability to conceive after 12–18 months of trying.

What is artificial insemination?

A physician places a man's sperm directly into a woman's vagina.

What is in vitro fertilization (IVF)?

The woman's ova are removed, fertilized with sperm in a laboratory, and then implanted into the woman's uterus.

What is ultrasound sonography?

High-frequency sound waves scan the womb to produce an image of the unborn baby.

What can ultrasound assess?

The baby's size and shape and, with repeated use, developmental patterns.

When is ultrasound less accurate for identifying abnormalities?

Until later in pregnancy; the slide specifies about 7 weeks.

What is chorionic villus sampling (CVS)?

A needle is used to take a sample of hairlike material surrounding the embryo.

When is CVS performed?

Around 10–12 weeks of the first trimester.

What is a risk of CVS?

A 0.5%–1% risk of miscarriage, so it is used infrequently.

What is amniocentesis?

A needle is inserted into the amniotic fluid to obtain fetal cells for identifying genetic defects.

When is amniocentesis performed?

Around 15–18 weeks.