Human Development 1
Chapter 2: Genetics and Prenatal Development
Chromosome: sausage-shaped structure in the nucleus of cells, containing
genes, which are paired, except in reproductive cells.
DNA: long strand of cell material that stores and transfers genetic
information in all life forms
Gene: segment of DNA containing coded instructions for the growth and
functioning of the organism.
Genome: entire store of an organism’s hereditary information
Genotype: organism’s unique genetic inheritance
Phenotype: organism’s actual characteristics, derived from its genotype.
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.
Allele: on a pair of chromosomes, each of two forms of a 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.
o Incomplete dominance in sickle cell inheritance: Two recessive
genes for the sickle cell trait results in sickle cell anemia, but having
one dominant and one recessive gene provides protection against
Polygenic 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: chromosomes that determine whether an organism is
male (XY) or female (XX).
X-linked inheritance: pattern of inheritance in which a recessive
characteristic is expressed because it is carries on the male’s X chromosome.
Genes and Environment in Human Development
Nature-nurture debate: debate among scholars as to whether human
development is influences mainly by genes(nature) or
Principles of Behaviour Genetics
Behaviour genetics: field in the study of human development that aims to
identify the extent to which gened influence behaviour, primarily by
comparing persons who share different amounts of their genes/
Monozygotic (MZ) twins: twins who have exactly the same genotype; also
called identical twins Dizygotic(DZ) twins: twins that result when two ova are released by a
female instead of one, and both are fertilized by sperm; also called fraternal
Heritability: statistical estimate of the extent to which genes are responsible
for the differences among persons within a specific population, with values
ranging from 0 to 1.00.
o The higher the heritability, the more the characteristic is believed to
o Heritability estimates have been criticized for giving misleading
impression of the influence of genetics on development.
o Heritability estimates are estimates based on comparison of persons
with different amounts of genetic material in common, not direct
measures of the activity of genes.
Concordance rate: percentage that indicated the degree of similarity in
phenotype among pairs of family members, expressed as a percentage.
Gene-environment Interactions: Epigenesis and Reaction Ranges.
Epigenesis: in development, the continuous bidirectional interactions
between genes and environment.
o According to epigenetic theory, genetic activity responds constantly to
environmental influences. Development is influences by genes but not
purely determined by them.
Reaction range: range of possible developmental aths established by genes;
environment determines where development takes place within that range.
The theory of genotype environmental effects: theory proposing that
genes influence the kind of environment we experience.
The 3 forms of genotype Environment effects
Passive genotype environment effects: the type that results from the
fact that in a biological family, parents provide both genes and environment
to their children.
Evocative genotype environment effects: the type that results occur
when a persons inherited characteristics evoke responses from others in the
Active genotype environment effects: the type that results when people
seek out environments that correspond to their genotypic characteristics.
Genotype Environment effects over time
The three types of genotype environment effects operate throughout
childhood, adolescence, and adulthood, but their relative balance changes
In childhood, passive genotype environment effects are especially
pronounced, and active genotype environment effects are relatively weak. Human Development 3
Chapter 2: Genetics and Prenatal Development
However, the balance changes as children move through adolescence and
Parental control diminishes, so passive genotype environment effects also
Autonomy increases, so active genotype environment effects also increase.
In adulthood, passive genotype environment effects fade entirely, and
active genotype environment effects move to the forefront.
Evocative genotype environment effects remain relatively stable from
childhood through adulthood.
The theory of genotype environment effects has been the source of
Genes and Individual Development
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)
Sperm and Egg Formation
Ovum: mature egg that develops in ovaries, about every 28 days in human
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.
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 original cell.
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 mother.
Crossing over: at the outset of meiosis, the exchange of genetic material
between paired chromosomes.
Follicle: during the female reproductive cycle, the ovum plus other cells that
surround the ovum and provide nutrients.
About 14 days into a womens cycle, the mature follicle cursts
and ovulation takes place as the ovum is releaded into the
The ovum is 2,000 times larger than a sperm because it
contains so much cytoplasm.
The cytoplasm will provide nutrients for the first 2 weeks of
growth if the ovum is fertilized, until it reaches the uterus and
begins drawing nutrients from the mother.
It is only during the first 24 hours after the ovum enters the
fallopian tube that fertilization can occur. It takes sperm from afew hours to a whole day to travel up the fallopian tubes, so
fertilization is most likely to take place if intercourse occurs on
the day of ovulation or the 2 previous days.
When the sperm reaches the ovum they begin to penetrate the
surface of the cell, aided by a chemical on the tip of the sperm
that dissolves that ovum’s membrane.
Once the sperm penetrates the ovum’s membrane, the head of
the sperm detaches from the tail and continues toward the
nucleus of the cell while the tail remains outside
When the sperm head eaches 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.
Zygote: following fertilization, the new cell formed from the union of sperm
Twins can also result when a zygote that has just begun the process of cell
division splits into 2 separate clusters of cells, creatins MZ twins
MZ twins are less common than DZ twins.
The Germinal Period (First 2 weeks)
Germinal period: the zygote travels down the fallopian tubes to the uterus
and implants in the uterine wall. As it travels, it begins cell division and
By week 1 following conception there is a ball of about 100 cells known as a
blastocyst which is divided into 2 layers.
o Trophoblast: The outer layers of cells which will go on to form
structures that provide protection and nourishment to the embryo.
o Embryonic disk: the inner layer of cells, which will go on to form the
During the second week after conception, implantation occurs as the
blastocyst becomes firmly embedded into the lining of the uterus.
The trophoblast begins to differentiate into several structures during this 2d
Part of it forms a membrane, the amnion, which surrounds the developing
organism and fill with fluid, helping to keep a steady temperature for the
organism and protect it against the friction of the mother’s movements.
In between the uterine wall and the embryonic disk a round structure, the
placenta, begins to develop. The placenta will allow nutrients to pass from
the mother to the developing organism and permit wastes to allow nutrients
to pass from the mother to the developing organism and permit wastes to be
removed. It also protects the developing organism from bacteria and wastes Human Development 5
Chapter 2: Genetics and Prenatal Development
in the mother’s blood, and it produces hormones that maintain the blood in
the uterine linning and cause the mothers breast’s to produce milk.
An umbilical cord also begins to develop, connecting the plac