Chapter 2 - Genetics and Prenatal Development
Section 1 - Genetic Influences On Development
Genotype and Phenotype
-Nearly all cells in the human body contain 46 chromosomes in 23 pairs, with 1 chromo-
some in each pair inherited from the mother and the other inherited from the father
Chromosome: sausage-shaped structure in the nucleus of cells, containing
genes, which are paired, except in reproductive cells
-Chromosomes composed of DNA (deoxyribonucleic acid)
DNA: long strand of cell material that stores and transfers genetic information in
all life forms.
-DNA in chromosomes are organized into segments called genes
Genes: segment of DNA containing coded instructions for the growth and func-
tioning of the organism.
-The totality of an individual’s genes is the genotype, and the person’s actual character-
istics are called the phenotype.
Genotype: organism’s unique genetic inheritance
Phenotype: organism’s actual characteristics derived from its genotype.
-Dominant-recessive inheritance is another aspect of genetic functioning that influences
the relation between genotype and phenotype.
Dominant-Recessive Inheritance: pattern of inheritance in which a pair of
chromosomes contains one dominant and one recessive gene, but only the
dominant gene is expressed in the phenotype.
-Each pair of chromosomes has two forms of each gene (one from mother - other from
father). Each form is called an allele.
Allele: on a pair of chromosomes, each of two forms of a gene.
-On many alleles, dominant-recessive inheritance occurs. This means that the dominant
gene will influence the phenotype. Chapter 2 - Genetics and Prenatal Development
Sometimes there is incomplete dominance, where the phenotype is influence
primarily, but not exclusively, by the dominant gene.
Incomplete dominance: Form of dominant-recessive inheritance in which the
phenotype is influenced primarily by the dominant gene but also to some extent
by the recessive gene.
-More commonly the influence of genes is a consequence of polygenic inheritance,
which is the interaction of multiple genes rather than just one.
Plolygenic Inheritance: expression of phenotypic characteristics due to the
interaction of multiple genes.
Regulator Gene: Gene that directs the activities of other genes.
Gene Therapy: Method of treating genetic disorders that involves replacing the affected
genes with genes that do not have the disorder.
The Sex Chromosomes
Sex Chromosomes - determine whether the person will be male (XY) or female (XX).
Ovum contains an X chromosome but sperm may carry either an X or a Y chro-
mosome. Sperm determines the sex of the child.
X-linked inheritance: pattern of inheritance in which a recessive characteristic is ex-
pressed because it is carried on the male’s X chromosome.
Genes and Environment in Human Development
- Nature-Nurture debate: Debate among scholars as to whether human development
is influenced mainly by genes (nature) or environment (nurture)
-Consensus that both play key roles in development but the relative strength of
nature and nurture continues to be debated.
Principles of Behaviour Genetics Chapter 2 - Genetics and Prenatal Development
Behaviour Genetics: field in the study of human development that aims to identify the
extent to which genes influence behaviour, primarily by comparing persons who share
different amounts of their genes.
(studies done mainly with twins and adoption)
Monozygotic (MZ) twins: Twins who have exactly the same genotype; aka identical
Dizygotic (DZ) twins: Twins that result when two ova are released by a female instead
of one, and both are fertilized by sperm; aka fraternal twins. Have 40-60% of their genes
-Comparing twin studies with adoption studies, heritability is able to be calculated.
Heritability: Statistical estimate of the extent to which genes are responsible for the dif-
ferences among persons within a specific population, with values ranging from 0 to 1.00.
-Heritability estimates criticized for giving misleading impression of the influences of ge-
netics on development.
-Heritability estimates are a measure not just of genetic influences but of how
much the environment allows the genes to be expressed. They measure phenotype
rather than genotype.
-Concordance rate, another stat used in behaviour genetics.
Concordance rate: Degree of similarity in phenotype among pairs of family members,
expressed as a percentage. (0%-100%)
Gene-Environment Interactions: Epigenesis and Reaction Ranges
Epigenesis: in development, the continuous bidirectional interactions between genes
-Epigenetic theory, genetic activity responds constantly to environmental influences. Chapter 2 - Genetics and Prenatal Development
-Genes establish a reaction range of potential expression.
Reaction Range: range of possible developmental paths established by genes; envir-
onment determines where development takes place within that range.
The Theory of Genotype ---> Environment Effects
Theory of Genotype ---> Environment Effects: Theory proposing that genes influ-
ence the kind of environment we experience.
-Based on our genotypes, we create our own environments, to a considerable extent.
The three forms of Genotype --->Environment Effects
-Genotype ---> Environment effects take three forms: Passive, evocative, and active.
Passive Genotype ---> Environment Effects: In the theory of Genotype --->
Environment effects, the type that results from the fact that in a biological family, parents
provide both genes and environment to their children.
(ex: Father, graphic artist, Daughter, architect)
Evocative Genotype ---> Environment Effects: the type that results occur
when a person’s inherited characteristics evoke responses from others in the environ-
(Ex: Son, age 3, loves reading - buy him books. Daughter showing skill at basketball -
enrol her in basketball camp)
Active Genotype ---> Environment Effects: the type that results when people
seek out environments that correspond to their genotypic characteristics. Chapter 2 - Genetics and Prenatal Development
-People are drawn to environments that match their inherited abilities.
Genotype ---> Environment effects over time
The three “arrow” types mentioned operate throughout childhood, adolescence, and
adulthood, but their relative balance changes over time.
Genes and Individual Development
Sperm and Egg Formation
Gametes: Cells, distinctive to each sex, that are involved in reproduction (egg cells in
the ovaries of the female and sperm in the testes of the male)
Ovum: Mature egg that develops in the ovaries, about every 28 days in human females
-Gametes form in the testes or ovaries through a process called meiosis
Meiosis: Process by which gametes are generated, through separation and duplication
of chromosome pairs, ending in four new gametes from the original cell, each with half
the number of chromosomes of the original cell.
-Meiosis is a variation of mitosis
Mitosis: process of cell replication in which the chromosomes duplicate themselves and
the cell divides into two cells each with the same number of chromosomes as the origin-
Cytoplasm: In an ovum, fluid that provides nutrients for the first 2 weeks of growth if the
ovum is fertilized, until it reaches the uterus and begins drawing nutrients from the moth-
Crossing Over: at the outset of meiosis, the exchange of genetic material between
paired chromosomes. Chapter 2 - Genetics and Prenatal Development
-explains sibling diversity
-Crossing over mixes the combinations of genes in the chromosomes, so that genetic
material that originated from the mother and father is rearranged in a virtually infinite
number of ways.
Follicle: during the female reproductive cycle, the ocum plus other cells that surround
the ovum and provide nutrients.
-when the sperm head reaches the nucleus of the ovum, the final phase of meiosis is
triggered in the ovum.
-Fertilization takes place as the 23 chromosomes from the ovum pair up with the 23
chromosomes from the sperm and a new cell is formed from the two gametes, the zy-
Zygote: following fertilization, the new cell formed from the union of sperm and ovum.
Section 2 - Prenatal Development and Prenatal Care
The Germinal Period (First 2 Weeks)
-First two weeks after fertilization = Germinal period
-This is when the zygote travels down the fallopian tubes to the uterus and implants in
the uterine wall. Chapter 2 - Genetics and Prenatal Development
-Cell division begins about 30 hours after conception
Blastocyst: ball of about 100 cells formed by about one week following conception
The blastocyst is divided into two layers:
Trophoblast: the outer layer of cells, which will go on to form structures that
provide protection and nourishment to the embryo.
Embryonic disk: the inner layer of cells, which will go on to form the embryo.
-Implantation begins during the second week after conception.
-Trophoblast begins to differentiate into several structures
Amnion: fluid-filled membrane that surrounds and protects the developing organism in
Placenta: in the womb, gatekeeper between mother and fetus, protecting the fetus from
bacteria and wastes in the mother’s blood, and producing hormones that maintain the
blood in the uterine lining and cause the mother’s breasts to produce milk.
Umbilical cord: structure connecting the placenta to the mother’s uterus.
The Embryonic Period (Weeks 3-8)
Three layers form: the ectoderm, mesoderm, and endoderm; neural tube develops and
produces neurons; heart begins beating; ribs, muscles, and digestive tract form
Gestation: in prenatal development, elapsed time since conception
Ectoderm: in the embryonic period, the outer layer of cells, which will eventually be-
come the skin, hair, nails, sensory organs, and nervous system (brain, and spinal cord). Chapter 2 - Genetics and Prenatal Development
Mesoderm: in the embryonic period, the middle of the 3 cell layers, which will become
the muscles, bones, reproductive system, and circulatory system.
Endoderm: in the embryonic period, the inner layer of cells, which will become the di-
gestive system and the respiratory system.
Neural tube: in the embryonic period, the part of the extoderm that will become the
spinal cord and the brain.
Neuron: Cell of the nervous system.
-during weeks 5-8; arms and legs develop, then fingers and toes; placenta and umbilical
cord function; digestive system develops; liver produces blood cells; embryo responds
The Fetal Period (Weeks 9-Birth)
-Genitals form and release sex hormones; fingernails, toenails, and taste buds develop;
heartbeat audible with stethoscope. (this occurs during weeks 9-12)
-During weeks 13-24, Mother feels movement; fetus kicks, turns, hiccups, sucks thumb,
breathes amniotic fluid; responds to sounds, especially music and familiar voices; vernix
and lanugo develop on skin.
-During weeks 25-38, lungs develop fully; over two-thirds of birth weight is gained; brain
development accelerates; sleep-wake cycles resemble newborn’s.
Prenatal Care in Traditional Cultures Chapter 2 - Genetics and Prenatal Development
-often includes massage as well as folk knowledge that may or may not have practical
consequences. (i.e. Many cultures advise pregnant women to avoid certain types of
Midwife: person who assists in pregnant women’s prenatal care and the birth process.
Scientifically Based Prenatal Care
-Essential elements include regular evaluations by a health care professional and
guidelines concerning diet, exercise, and avoiding teratogens.
Teratogen: behaviour, environment, or bodily condition that can have damaging influ-
ence on prenatal development.
-malnutrition and disease, alcohol, tobacco, and other drugs.
-pregnant women advised to gain 25-30 pounds in the course of pregnancy, and light to
moderate exercise is encouraged.
Section 3 - Pregnancy Problems
-Two major problems that can occur during pregnancy: Chromosomal Disorders and Te-
- Errors can take place during meiosis of the gametes; chromosomes fail to divide prop-
erly. Chapter 2 - Genetics and Prenatal Development
-1 in 200 live births involves a child that has a chromosomal disorder.
-two types of chromosomal disorders:
1) those that involve the sex chromosomes
2) Those that take place on the 21 pair of chromosomes, resulting in a condition known
as Down syndrome.
Sex Chromosome Disorders
-Sex chromosomes especially likely to be involved in chromosomal disorders.
(Either extra X or Y: XXY. XXX. XYY. Or