Human Development- Chapter 2- From Conception to Birth
The Beginning of Life
• Everyone starts life as a single cell, called a zygote. Each zygote is distinct from any
other human cell ever created.
• Zygote: A single cell that is formed from the fusing of two gametes, a sperm and an
Genes and Chromosomes
• DNA (deoxyribonucleic acid): The molecule that contains the chemical instructions for
cells to manufacture various proteins.
• Chromosome: One of the 46 molecules of DNA (in 23 pairs) that each cell of the
human body contains and that, together, contain all the genes.
• Gene: a small section of a chromosome; the basic unit for transmission of heredity. A
gene consists of a string of chemicals that provide instructions for the cell to
manufacture certain proteins.
• Additional DNA and RNA surround each gene. In a process called methylation, the
material enhances, transcribes, connects, empowers, silence and alter genes. This
nongenetic material used to be called junk. Methylation continues throughout life, and it
can alter a gene’s expression even after a person is born. (EPIGENETICS)
• Gamete is the one exception to identical copies of chromosomes.
• Gamete: a reproductive cell; that is, a sperm or an ovum that can produce new
individual if it combines with a gamete from the other sex to form a zygote.
Variations among People
• Each man or women can produce 2²³ different gametes- more than 8 million versions of
their chromosomes (8,388,608).
• More variations occur because the DNA code contains 3 billion pairs of chemicals
organized in triplets (sets of three pairs), each of which specifies production of one of 20
possible amino acids. Those amino acids combine to produce proteins, and those
proteins combine to produce a person. Small variation or repetitions (copy number
variations) in the base pairs or triplets could make a notable difference in the proteins
and thus, eventually, in the person.
• Allele: Any of the possible forms in which a gene for a particular trait can occur.
• Genes that have various alleles are called polymorphic, and each variation is a single-
nucleotide polymorphism (SNP). • Most alleles cause minor differences, but alleles make one person unlike the other.
• Many gene pairs do not exactly match in every triplet due to the inherited alleles from the
sperm or ovum.
• Genotype: an organism’s entire genetic inheritance, or genetic potential.
• Phenotype: the observable characteristics of a person, including appearance,
personality, intelligence, and all other traits.
• Genotype is the beginning of diversity; the phenotype is the actual manifestation of it.
• Genetic diversity also allows adaptation.
More on Shared and Divergent Genes
• Genome: the full set of genes that are the instructions to make an individual member of
a certain species.
• The worldwide effort to map all the human genes led to the Human Genome Project
(completed in 2003).
• Any two men and women share 99.5% of their genetic code.
• Human and chimps share 98%.
• The human genome project found only about 20,000-23,000 genes.
• International project: HapMap aims to spot variations in the human genome. It has found
11 million differences among the 3 billion possible chemical pairs in humans. Can be
problematic since much remains to be understood about the connection between alleles
in the genotype and actual characteristics of the phenotype.
The 23 Pair
• Sex differences originate from one gene (SRY) on one chromosome.
• The closely matched pairs of chromosomes are referred to as autosomes,
and they can be inherited by a male or female.
• XX: a 23 chromosome pair that consists of two X-shaped chromosomes,
one of each from the mother and father. XX zygotes become females.
• XY: A 23 chromosome pair that consists of an X-shaped chromosome fro0m
the mother and a Y-shaped chromosome from the father. XY zygotes become
males. • Approx 120 males to every 100 females. However male embryos are more
vulnerable than female ones.
• About one in every 250 human conceptions, the zygote not only duplicates
but also splits apart completing, creating multiple separate zygotes, each
genetically identical as the first single cell.
• An uncompleted split creates conjoined twins.
• Monozygotic twins: twins who originate from one zygote that splits apart
very early in development (identical twins). Same genotype different
• Dizygotic twins: twins who are formed when two separate ova are fertilized
by two separate sperm at roughly the same time. (Fraternal twins).
• Almost every trait is polygenic (affected by many genes) and multifactorial (influenced by
• Additive gene: a gene that adds something to some aspect of the phenotype. Its
contribution depends on additions from the other genes, which may come from either
the same or the other (recessive gene).
• Less common are nonadditive genes, which do not contribute equal share.
• Dominant-recessive pattern: the interaction of a pair of alleles in such a way that the
phenotype reveals the influence of one allele (dominant gene) more than the other
• Carrier: a person whose genotype includes a gene that is not expressed in the
phenotype. Such an unexpressed gene occurs half the carrier’s gametes and thus is
passed on to half the carriers children, who will most likely be carriers, too. Generally,
the characteristic appears in the phenotype only when such a gene is inherited from
• X-linked: a gene carried on the X-chromosome. If a male inherits an X-linked recessive
trait from his mother, he expresses that trait because the Y from his father has no
counteracting gene. Females are more likely to be carriers of X-linked traits but are less
likely to express them.
From Zygote to Newborn • Germinal Period: the first two weeks of the prenatal development after conception,
characterized by rapid cell division and the beginning of cell differentiation.
• Embryonic Period: The stage of prenatal development from approximately the third
through the eight week after conception, during which the basic forms of all body
structures, including internal organs, develop.
• Fetal Period: The stage of prenatal development from the ninth week after conception
until birth, during which the fetus grows in size and matures in functioning.
Germinal: First 14 Days
• Within hours of conception, the zygote begins duplication and division.
• First cells are stem cells; cells from which any other specialized type of cell can form.
• After about the eight-cell stage, duplication and division continue and a third process,
differentiation begins. Soon cells specialize, taking different forms, and reproducing at
• About a week after conception the multiplying cells separate into two distinct masses.
The outer cells form a shell that will become the placenta, and the inner cells form a
nucleus that will become the embryo.
• First task of the outer cells is Implantation: the process, beginning about 10 days after
conception, in which the developing organism burrows into the tissue that lines the
uterus, where it can be nourished and protected as it continues to develop.
Embryo: From the Third through the Eighth Week
• The start of the third week after conception initiates the embryonic period, during which
the formless mass of cells becomes a distinct being.
• Embryo: the name for a developing human organism from bout the third through the
eighth week after conception.
• Approximately day 14, a thin line (primitive streak) appears down the middle of the
embryo, becoming the neural tube 22 days after conception, it eventually develops into
the central nervous system.
• The head appears in the fourth week, as eyes, ears, nose, and mouth start to form. A
minuscule blood vessel that will become the heart begins to pulsate.
• The fifth week, the buds will become limbs will emerge.
• 52-54 days after conception the fingers and toes separate. • Prenatally the head develops first, in cephalocaudal pattern, and the extremities form
last, in a proximodistal pattern.
• At the end of the eighth week, the embryo weighs just one-thirteenth of an ounce (less
than a gram) and is about 1 inch long. It has all its organs and body parts aside from the
sex organs. It moves frequently, about 150b times an hour.
Fetus: From the Ninth Week until Birth
• Fetus: the name for a developing organism from the start of the ninth week after
conception until birth.
• The fetal period encompasses dramatic change.
• In the ninth week sex organs develop, soon visible via ultrasound.
• Ultrasound: an image of a fetus (or an internal organ) produced by using high
frequency sound waves (sonogram).
• By three months, the fetus weighs approx 3 ounces and is about 3 inches.
• As prenatal growth continues, the cardiovascular, digestive and excretory systems
• The brain increases about six times from the forth to the sixth months. Enables
• Age of Viability: The age (about 22 weeks after conception) at which a fetus may
survive outside the mother’s uterus if specialized medical care is available.
Finally, a Baby
• About 38 weeks (266 days) after conception, the fetal brain signals the release of
hormones, specifically oxytocin, which prepares the fetus for delivery and starts labor.
• Average baby is born after 12 hours of