PSYC 2500H Chapter Notes - Chapter 3: Intellectual Disability, Sickle-Cell Disease, Gregor Mendel
26 Jun 2018
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Chapter 3 – Hereditary Influences on Development
-Principles of Hereditary Transmission
How one’s genotype is expressed as a phenotype
oGenotype: genetic endowment that an individual inherits
oPhenotype: ways in which a person’s genotype is expressed in observable or measurable
characteristics
oBegins with conception: when a sperm penetrates an ovum and forms a zygote
The Genetic Material
oWhen a sperm penetrates the lining of the ovum, a biochemical reaction repels other sperm,
preventing them from repeating the fertilization process
Within hours sperm begins to disintegrate, releasing genetic material
Ovum also releases its genetic material
A new cell nucleus forms around both
oZygote: a single cell formed at conception from the union of a sperm and an ovum (46
chromosomes, 23 from dad and 23 from mom)
oChromosome: a threadlike structure made up of genes’ in humans there are 46 chromosomes in
the nucleus of each body cell
oGenes: hereditary blueprints for development that are transmitted unchanged from generation to
generation
oDeoxyribonucleic acid (DNA): long, double-stranded molecules that make up chromosomes
Ladder-like molecule that splits in the middle, opening like a zipper
Each remaining half guides the replication of its missing parts
This makes it possible for a one-celled zygote to develop into a human
Growth of the Zygote and Production of Body Cells
oZygote moves through the fallopian tubes towards its prenatal home in the uterus, replicating itself
through mitosis
oMitosis: process in which a cell duplicates
its chromosomes and then divides into two
genetically identical daughter cells
Continues throughout life,
generating new cells that enable
growth and replacing ones that are
damages
Every new cell contains an exact
copy of the 46 chromosomes we
inherited at conception
The Germ (or sex) Cells
oGerm cells serve the hereditary function of
producing gametes (Sperm in males and
ova in females)
oProduction of Gametes Through Meiosis
Meiosis: process by 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, products of meiosis contain 23
chromosomes
Crossing-over: process in which genetic material is exchanged between pairs of
chromosomes during meiosis
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Creates genetically unique combinations
1.Germ cells first duplicates its 46 chromosomes, crosses over
2.Pairs of duplicated chromosomes (some of which have been altered by crossing over)
segregate into two new cells, each of which contains 46 chromosomes
3.New cells divide so that each of the resulting gametes contains 23 single (unpaired)
chromosomes
4.At conception a sperm with 23 chromosomes unites with an ovum with 23
chromosomes, producing a zygote with a full set of 46 chromosomes
oHereditary Uniqueness
Independent assortment: the principle that each pair of chromosomes segregates
independently of all other chromosome pairs during meiosis
Multiple Births
oMonozygotic (identical) twins: twins who develop from a single zygote that later divides to form
two genetically identical individuals
two people share a genotype
Zygote will split into separate but identical cells, which then become two individuals
oDizygotic (fraternal) twins: twins that result when a mother releases two ova at roughly the same
time and each is fertilized by a different sperm, producing two zygotes that are genetically
different
A mother releases two ova at the same time and each is fertilized by a different sperm
Male or Female
oChromosomal Portraits (Karyotypes) reveal that 22 of the 23 pairs of human chromosomes are
autosomes
Autosomes: chromosomes not involved in determining the sex of an organism
The 22 pairs of human chromosomes that are identical in males and females
Sex is determined by the 23rd pair (called the sex chromosome)
Sex chromosome: determines szex of an organism
X chromosome: longer of the two sex chromosomes
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Y chromosome: shorter of the two sex chromosomes
Males: 1 X chromosome, 1 Y chromosome
Females: 2 X chromosomes
Child’s sex is determined by whether an X-bearing or a Y-bearing sperm fertilizes the
ovum
What do Genes do?
oCall for the production of amino acids, which form enzymes and other proteins that are necessary
for the formation and functioning of new cells
Regulate production of melanin in the iris of an eye
oGuide differentiation of cells, making some part of the brain and central nervous system, others
part of the circulatory system, bones, skin, etc.
Genes influence and are influenced by the biochemical environment surrounding them
during development
i.e. a particular cell might become part of an eyeball or part of an elbow depending on
what cells surround it during early embryonic development
oSome are responsible for regulating the pace and timing of development
Specific genes are ‘turned on’ or ‘turned off’ by other regulatory gees at different points
in the lifespan
i.e. genes are ‘turned on’ and produce growth spurt during adolescence
oEnvironmental factors influence the function of genes, combining with genetic influences to
determine how a genotype is translated into a certain phenotype
Experience-expectant interactions: effect of the external environment experienced by all
humans
Experience-dependent interactions: experienced only by some individuals
oGenes do not simply ‘code’ for human characteristics, but they interact with the environment at
many levels to produce proteins that eventually influence human characteristics
How are Genes expressed?
-There are four main patterns of genetic expression: simple dominant-recessive inheritance, codominance,
sex-linked, and polygenic (or multiple-gene) inheritance
Single-Gene Inheritance Patterns
-Sometimes human characteristics are determined by the actions of a single gene
-Sometimes characteristics are determined by the actions of many genes working together; this is called
polygenic inheritance
Simple Dominant-Recessive Inheritance
-Many characteristics are influences by only one pair of genes (called alleles): one from the mother, one
from the father
-Gregor Mendel major discovery was a predictable pattern to the way in which two alternative
characteristics appeared in the offspring of cross-breeding
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Document Summary
The genetic material: when a sperm penetrates the lining of the ovum, a biochemical reaction repels other sperm, preventing them from repeating the fertilization process. Within hours sperm begins to disintegrate, releasing genetic material. Ladder-like molecule that splits in the middle, opening like a zipper. Each remaining half guides the replication of its missing parts. This makes it possible for a one-celled zygote to develop into a human. Continues throughout life, generating new cells that enable growth and replacing ones that are damages. Every new cell contains an exact copy of the 46 chromosomes we inherited at conception. The germ (or sex) cells: germ cells serve the hereditary function of producing gametes (sperm in males and ova in females, production of gametes through meiosis. Meiosis: process by 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, products of meiosis contain 23 chromosomes.
