PSYC 2450 Chapter 3: Chapter 3

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Developmental Psychology Textbook Notes
Chapter 3 Hereditary Influences on Development
Principles of Hereditary Transmission
The Genetic Material
Genotype: the genes that a person inherits
Phenotype: a person’s observable or measurable characteristics
Conception: the moment when an ovum released by a women’s ovary and on its
way to the uterus via the fallopian tube is fertilized by a man’s sperm
Conception steps:
o The first development that occurs after conception is protective: when a
sperm cell penetrates the lining of the ovum, a biochemical reaction repels
other sperm, thus preventing them from repeating the fertilization process
o Within a few hours, the sperm cell begins to disintegrate releasing its
genetic material
o The ovum also releases its genetic material, and a new cell nucleus forms
around the hereditary information provided by the father’s sperm and
mother’s ovum
o This new cell, called a zygote (1/20th the size of the head of a pin)
o Yet this tiny cell contains the biochemical material for the zygote’s
development from a single cell into a recognizable human being
o The new cell nucleus contains 46 elongated, threadlike bodies called
chromosomes, each of which consists of thousands of chemical
segments, or genes: the basic units of heredity that work to build a single
protein
o Chromosomes come in matching pairs
o One member of each chromosome pair comes from the mother’s ovum
and the other from the father’s sperm cell
Thus each parent contributes 23 chromosomes to each of their
children
Genes
o Genes are actually stretches of deoxyribonucleic acid or DNA, a complex
“double helix” molecule that resembles a twisted ladder and provides the
chemical basis for development
o DNA can duplicate itself:
The rungs of this ladder-like molecule split in the middle
o This special ability of DNA to replicate itself is what makes it possible for a
one-celled zygote to develop into a marvellously complex human being
Definitions:
o Conception: the moment of fertilization, when a sperm penetrates an
ovum, forming a zygote
o Zygote: a single cell formed at conception from the union of a sperm and
an ovum
o Chromosome: a threadlike structure made up of genes; in humans there
are 46 chromosomes in the nucleus of each body cell
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o Genes: hereditary blueprints for development that are transmitted
unchanged from generation to generation
o Deoxyribonucleic acid: long, double-stranded molecules that make up
chromosomes
Growth of the Zygote and Production of Body Cells
As the zygote moves through the fallopian tube toward its prenatal home in the
uterus, It begins to replicate itself through a process of mitosis:
o The process in which a cell duplicates its chromosomes and then divides
into two genetically identical daughter cells
Steps
o At first the zygote divides into two cells, but 2 soon becomes 4, 4 becomes
8…etc
o Just before each division, the cell duplicates its 46 chromosomes and
these duplicate sets move in opposite direction
o The division of the cell then proceeds, resulting in two new cells, each of
which has the identical 23 pairs of chromosomes (46 in all) and thus the
same genetically material as the original cell
o Mitosis continues throughout life; generating new cells that enable growth
and replacing old ones that are damaged
o With each division, the chromosomes are duplicated so that every new
cell contains an exact copy of the 46 chromosomes we inherited at
conception
The Germ (or Sex) Cells
Human beings have germ cells that serve one special hereditary function to
produce gametes: sperm in males and ova in females
This is different cell reproduction than the process of mitosis
The process shares some of the characteristics of mitosis, but it differs in ways
that make the resulting cells able to join with gametes to create a unique cell that
will become a unique individual
Production of Gametes through Meiosis
Meiosis: the process in which a germ cell divides, producing gametes (sperm or
ova) that each contain half of the parent cell’s original complement of
chromosomes; in humans, the products of meiosis contain 23 chromosomes
Male germ cells in the testes and female germ cells in the ovaries produce sperm
and ova through a process called meiosis
o the germ cell first duplicates its 46 chromosomes
An event called crossing-over: (a process in which genetic material is
exchanged between pairs of chromosomes during meiosis) often takes place
o Adjacent duplicated chromosomes cross and break at one or more points
along their length, exchanging segments of genetic material
o This transfer of genes during crossing over creates new and unique
hereditary combinations
Next, pairs of duplicated chromosomes segregate into two new cells, each of
which contains 46 chromosomes
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Finally, the new cells divide so that each of their resulting gametes contains 23
single, or unpaired, chromosomes
At conception, then, a sperm with 23 chromosomes unites with an ovum with 23
chromosomes, producing a zygote with a full complement of 46 chromosomes
Hereditary Uniqueness
When a pair of chromosomes segregates during meiosis, it is a matter of chance
which of the two chromosomes will end up in a particular new cell
Independent assortment: the principle stating that each pair of chromosomes
segregates independently of all other chromosome pairs during meiosis
Multiple Births
There is only one situation in which 2 people will share a genotype
o Occasionally a zygote will split into separate but identical cells, which then
become two individuals
o These are called monozygotic or identical twins because they have
developed from a single zygote and have identical genes
Dizygotic (or fraternal twins): pairs that result when a mother releases two ova
at the same time and each is fertilized by a different sperm
o even though fraternal twins are born together, they have no more genes in
common than any other pair of siblings
the incidence of multiple births involving 3 or more infants has increased a lot
o the increase is often attributed to the use of assisted reproductive
technologies
these include fertility drugs, artificial insemination, in vitro
fertilization
Male or Female?
Chromosomal portraits, or karyotypes reveal that 22 of the 23 pairs of human
chromosomes (called autosomes) are similar in males and females
Sex is determined by the 23rd pair (called the sex chromosome)
In males, the 23rd pair consists of one elongated body known as an X
chromosome and a short, stubby companion called a Y chromosome
In females, both of these sex chromosomes are Xs
When the sex chromosomes of a genetic (XY) male segregate into gametes
during meiosis, half of the sperm produced will contain an X chromosome and
half will contain a Y chromosome
By contrast, the ova produced by a genetic (XX) female all carry an X
chromosome
A child’s sex is determined by whether an X-bearing or a Y-bearing sperm
fertilizes the ovum
Autosomes: the 22 pairs of human chromosomes that are identical in males and
females
X chromosome: the longer of the two sex chromosomes; normal females have 2
X chromosomes, whereas normal males have one
Y Chromosome: the shorter of the two sex chromosomes; normal males have
one Y chromosome, whereas females have none
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