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Chapter 3
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Chapter 3: Hereditary influences on development
Principles of hereditary transmission
- Conception: the moment when an ovum released by a woman’s ovary and on its way to the
uterus via the fallopian tube is fertilized by a man’s sperm to form a zygote
The genetic material
First development that occurs after conception is protective: when a sperm cell penetrates the
lining of the ovum, a biochemical reaction repels other sperm prevent them form repeating
fertilization
Phenotype: the ways in which a person’s genotype is expressed in observable or measurable
characteristics
Genotype: the genes that a person inherits
Zygote: a single cell formed at conception from the union of sperm and ovum
Chromosome: a threadlike structure made up of genes; in humans there are 46 chromosomes
in the nucleus of each body cells
Genes: hereditary blueprints for development that are transmitted unchanged from
generation to generation (units that builds single protein)
- Genes produce enzymes and other proteins that are necessary for creation and functioning
of new cells, and regulate the timing of development
- Internal and external environmental influence how genes function
Deoxyribonucleic acid (DNA): long, double –stranded molecules that make up
chromosomes. It can duplicate itself= makes it possible for one-celled zygote to develop into
a marvelously complex human being
Growth of the zygote and production of body cells
Mitosis: the process in which a cell duplicates its chromosomes and then divides into two
genetically identical daughter cells.
Step 1: original parent cells
Step 2: each chromosome splits lengthwise, producing a duplicate
Step 3: the duplicate sets of chromosomes move to opposite ends of the parent cell, which
then begins to divide
Step 4: the cell completes its division, producing two daughter cells that have identical sets of
chromosomes
The germ (or sex) cells
Function: to produce gametes (sperm or ova)
Production of gametes through meiosis
Meiosis: the process in which a germ ell divides production gametes that each contain half of
the parent cell’s original complement of chromosomes; in humans the products of meiosis
contain 23 chromosomes.
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Step 1: Each of the germ cell’s original chromosomes duplicates itself and the duplicate
remains attached
Step 2: Crossing-over takes place among adjacent chromosomes, thus creating new
hereditary combinations (crossing-over: process in which genetic material is exchanged
between pairs of chromosomes)
Step 3: The original cell now divides to form 2 parent cells each of which has 23 duplicated
chromosomes
Step 4: Finally, each chromosome and its duplicate now split and segregate into separate
gametes. Thus, each gamete has only half the chromosome of its parent cell.
Hereditary uniqueness
Independent assortment: the principle stating that each pair of chromosomes segregates
indecently of all other chromosome pairs during meiosis
Multiple births
Monozygotic (identical) twins: twins who develop from a single zygote that later divides to
form two genetically identical individuals
(Occurs in about 2 of every 250 births around the world)
Dizygotic (or fraternal) twins: twins that result when a mother release two ova at roughly
the same time and each is fertilized by a different sperm, producing two zygote that are
genetically different
(Occurs in about 1 of every 125 births)
- Sex is determined by the 23rd pair (sex chromosome)
- Female 23rd chromosome= XX
- Male 23rd chromosome= XY
- Father determines the sex of their children, depending on whether the sperm that fertilizes the
ova contains X or a Y chromosome
Male or female?
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 two X
chromosomes, whereas normal males have but one
Y chromosome: the shorter of the two sex chromosomes; normal males have one Y
chromosome, whereas females have none
What do genes do?
Production of amino acids; forms enzymes and other proteins that are necessary for the
formation and function of new cells
Genes regulate the production of pigment melanin in iris of the eye
Guide cell differentiation
Influence and are influenced by biochemical environment surrounding them during
development
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Some are responsible for regulating the pace and timing of development
Environmental factors clearly influence how genes function (combine with genetic influences
to determine how a phenotype is translated into a particular phenotype (feel, look, thinks,
behaves)
Environment affects actions of genes at several levels:
1. Within the nucleus may affect the expression of genetic expression
(Molecular)
2. The internal environment that surrounds the cell may affect the genes
Expression (cellular)
3. External environment affects the expression of the genetic material (organism-
environment, experience-expectant, experience-dependent)
How are genes expressed?
Single-gene inheritance patterns
1. Simple dominant recessive inheritance:
Alleles: alternative forms of a gene that can appear at a particular site on a chromosome
(influence human characteristics by one pair of genes: one from mother one from father)
Simple dominant-recessive inheritance: a pattern of inheritance in which one allele
dominates another so that only its phenotype is expressed
Dominant allele: a relatively powerful gene that is not expressed phenotypically when
paired with a dominant allele
Recessive allele: a less powerful gene that is not expressed phenotypically when paired with
a dominant allele
Homozygous: having inherited two allele for an attribute that are identical in their effects
Heterozygous: having inherited two alleles for an attribute that have different effects
Carrier: a heterozygous individual who displays no sign of a recessive allele in his or her
own phenotype but can pass this gene to offspring
(See figure 3.6 for an example, p.82)
(Also see box 3.1 for a list of common dominant and recessive traits p.83)
2. Codominance:
Some genes are codominant: the phenotype they produce is a compromise between the two
genes
(E.g. human blood alleles A and B are equally expressive and neither dominates the
other)
Codominance: condition in which two heterozygous but equally powerful alleles produce a
phenotype in which both genes are fully and equally expressed
Other type of codominance happens when one of 2 heterozygous alleles is stronger than the
other but fails to mask all its effects (e.g. sickle cell trait; problem because they cluster
together, distribute less oxygen)
- Consequence more severe for people who inherit two recessive sickle cell genes they develop:
sickle cell anemia: a genetic blood disease that causes red blood cells to assume an unusual
sickled shape and to become inefficient at distributing oxygen
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