Biology Chapter 6
Advanced Genetics
The Continuity of Life: Part II
6A - Chromosome and Gene Changes
mutation:
any sudden inherited change
two types of mutations:
(1) CHROMOSOME mutations:
(2)
GENE mutation:
the chromosome number is changed
or the sequence or number of
genes on a chromosome is changed
the gene itself is changed
Mutations Affecting Chromosome Numbers
genome:
a single
haploid
diploid
Read page 151
parthenogenesis:
I.
complete set of an organism's chromosomes
gamete - contains a complete genome
cell - contains two complete genomes
the development of an unfertilized egg
(example - drone bees)
Euploidy: the addition or loss of an entire genome
polyploids (extremely rare in animals): any cell that has
multiple genomes
A. Triploids: three genomes
they are sterile
Living triploids do not occur in animals and humans.
plants - grapes, oranges, other fruits (they lack seeds
must reproduce asexually)
B. Tetraploid: four genomes
common in plants; rare in animals
human liver cells (92 chromosomes)
plants - white or Irish potatoes
n=12; 2n=24; 4n=48
american upland cotton (52 chromosomes)
Although there are a few well-known natural tetraploids, most of
today's triploids and tetraploids results from human intervention and
are kept alive only by cultivation.
The Mule:
II.
Read pages 152-153
female horse X male donkey = mule
64 chromosomes 62 chromosomes 63 chromosomes
Aneuploids:
lack or have extra chromosomes (not a complete
genome)
common in plant kingdom (esp. cultivated crops like wheat)
nondisjunction:
trisomy:
the failure of a pair of homologous chromosomes
to separate during the first division of meiosis
an abnormal condition in which there are 3 chromosomes
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in a set instead of 2.
example in humans - Down's Syndrome (mongolism) is caused
by a trisomy of the 22nd (or 21st) chromosome. Characterized by - low
mentality, shortness, stubby hands and feet, and an extra fold of
skin on the eyelids and other defects. Trisomic conditions of #13 &
17 human chromosomes are occasionally born (individual is deformed;
life expectancy is only a few months)
monosomy: a condition in which there is only 1 of a chromosome
set
Various aneuploids of the sex chromosomes:
Turner's syndrome (XO) - this female lacks the second sex
chromosome; characteristics: underdevelopment of the
female sex glands; shortness; and other deformities
1 in approximately 6,000 female births
Klinefelter's syndrome (XXY) - male with an extra X sex
chromosome; characteristics: usually mentally
retarded; underdeveloped sex organs
1 in approximately 6,000 male births
Trisomy X (XXX) - female with 3 X sex chromosomes
characteristics: some resemble women with Turner's
syndrome; some are so called "super females" having a
tendency toward male characteristics; usually mentally
retarded; usually sterile
Mutations Within the Chromosome
crossing over:
the exchange of segments between homologous
chromosomes during meiosis
Crossing over results in gametes with the normal amount of genes, but
often in unexpected combinations.
translocation:
the transfer of a chromosome segment to a
nonhomologous chromosome
Some of the gametes formed after translocation have extra genes
and some formed are missing genes.
Approximately 4% of the people with Down's syndrome have a
translocation of genes from the 22 chromosome (to one of the others 14, 15, 21)
Gene Action & Gene Mutations
Genes may be turned off (inactive); turned on (active); or
altered (mutated).
Not all of the approximately 100,000 genes you have in each
nucleus are functioning.
Factors that control gene action:
(1) concentration of the proteins they produce
(2) chemicals
(3) environment (temperature)
(4) sex hormones
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sex-limited characteristic: a trait expressed in only one sex
even though the genes are present in the autosomes of
both sexes
example - brightly colored feathers of roosters
humans: body hair, beard, breast
development, and milk production
Gene Mutations
gene mutation:
(or point mutation) the alteration of an
individual gene
codons of mRNA can be affected in 3 basic ways:
1. addition (of a nucleotide)
2. deletion
3. substitution
Effects of mutations
spontaneous mutations: a mutation that occurs naturally
mutagen: a substance that induces mutation
somatic mutation: a mutation affecting the somatic cells (nongamete cell)
A mutation in a somatic cell will usually do 1 of 3 things:
(1) produce an odd protein
(2) have no effect
(3) kill the cell
germ mutation: a mutation in a cell that forms gametes
many germ mutations are eliminated by a genetic screen
genetic screen: a mechanism designed by God that prevents badly
deformed or genetically defective individuals
from living and reproducing
(natural abortions [miscarriage] & still births)
example of human gene mutations: sickle-cell anemia
albinism
achondroplastic dwarfism
6B – Modern Genetics
gene pool - all the genes in a population of a given organism
(all the genes that a type of organism can
conceivably possess)
a difference between individual organisms of the
variations
same kind
Techniques to manipulate the gene pool
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Mass Selection:
Hybridization:
Inbreeding:
Eugenics:
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the method for selecting breeding stock in which
only the desirable organisms are selected
Example: loblolly pine (killed by southern
pine beetles)
It attempts to cultivate an already existing
trait, not to develop a new trait
Breeders are seeking a pure line of some
characteristic already in the gene pool, not
expanding the gene pool.
the crossbreeding of two genetically unrelated
individuals
Examples: crossing two strains of corn
two kinds of apples
Egyptian married to an Englishman
cross between an orange and a
tangerine
cross between a peach and a plum
Heterosis (Hybrid vigor) - and increased capacity
for growth in a hybrid
Examples: resistance to disease
larger bodies
more milk production
more crop yield per acre
the mating of an organism with itself or with close
relatives
Produces pure strains (organisms which are homozygous for various traits)
In animals, inbreeding often results in inferior
offspring.
the science that deals with the improvement of the
human race by applying principles of genetics
means well-born "good genes"
Read pages 167-168
Eugenic Practices
1.
artificial insemination - the mechanical injection of sperm
into a female's body
Long time practice among the cattle breeders in U.S. responsible for approx. 95% or the cattle born in the U.S.
At the present there are approx. 1 million people alive in america
(some over 20 years old) who are the result of artificial
insemination.
several places in the U.S. have sperm banks where frozen human sperm
are kept (by liquid nitrogen).
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First done with humans in 1785 in London by a doctor named John
Hunter.
Two types of artificial insemination:
1. AIH - using semen of the husband
2. AID - using semen of a donor
AIH:
AID:
Few Christians object to human in artificial insemination
between a husband and wife.
Some Christians consider artificial insemination with a
donor's sperm unacceptable, contending that it is
immoral for a woman to bear a child of a man who is not
her husband.
Those who condone AID argue that true purity is not
violated by this clinical technique.
Scientific Considerations:
1. possibility of accidental incest
sperm donors remain anonymous
avg. sperm-bank donor is used for up to 6 pregnancies
and some for as many as 50 pregnancies
potential for inbreeding (half brothers and half
sisters)
2. accidental transmission of unknown genetic traits
Most doctors only take oral histories from donors - only
29% performed a blood test
Theological Considerations:
1. Is AID an act of adultery?
One of the major factors in adultery is the attitude.
Jesus said that "anyone who looks at a women
lustfully has already committed adultery with her
in his heart" (Matt. 5:28). There is no such
attitude discernable in AID. Another factor is
action. Paul talks about becoming "one body" with
a prostitute (I Cor. 6:12-16); such a union is not
found in AID. Therefore it is not appropriate to
call AID a form of adultery.
2.
The biblical view of parenthood
While AID may be allowable in particular cases of last
resort to a married couple, this does not sanction
its use outside the bounds of marriage. Producing
children for single parents and lesbians or making
surrogate arrangements goes far beyond the bounds
of a biblical view of parenthood.
Somewhere in the process a child has ceased to be a
gift
of God. Ps. 127:3 "Lo, children are an
heritage of
the Lord: and the fruit of the womb
is His reward."
2.
in-vetro fertilization (ectogenesis) (test tube baby) removing an ovum from a female's body and then
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fertilize it with sperm in the lab (allows an ovum to
be fertilized and grown outside the womb for a short
period of time)
In order for the zygote to develop properly it must be
implanted again in a female's body.
world's first: Louise Brown born July 25, 1978
Scientific Considerations:
(1) hyperfertilization
more than 1 egg is harvested and fertilized; may be
associated with higher rates of chromosomal abnormality
(trisomy)
(2) May by-pass the natural screening of sperm
It appears that the female reproductive tract
selectively eliminates abnormal sperm so that very few
reach the site of fertilization.
(3) high rate of miscarriages or natural abortions
any technique that increases that risk also increases
the risk of problems for the mother
Ethical Considerations:
(1) The slippery slope argument
Moral choices often lead to unintended consequences in
the future. A pratice that may be moral can often very
naturally lead to immoral consequences.
(A) The possibility of establishing surrogate mothers
as an alternative form of reproduction.
Might lead to an entirely new profession prenatal baby care. Could easily develop into a
system in which women hire other women to carry
their babies while they themselves pursue their
careers.
(B) Artificial embryonation (AE)
A couple pays a fee for a woman to be artificially
inseminated by the husband. After a few days the
woman returns to the clinic so that the doctor can
flush out the embryo and implant it in the wife.
(C) Oocyte fusion product (OFP)
two ova are fused together; would create only
female offspring and requires only ova from
females; possible means of reproduction for
lesbian couples
(D) surrogate father
embryos could be fertilized in the lab, implanted
in men's abdomens, and delivered by Cesarean
section.
Such a procedure has already been performed by
Dr. Cecil Jacobsen at George Washington Univ. Med.
School with male chimps who delivered healthy
chimps via Cesarean section.
(E) Embryo adoption (EA)
Donor sperm is used, and a couple "adopts" the
embryo the same way they might adopt a child.
(2) The conflict between ends and means
Theological Considerations:
(1) The basis for the sanctity of human life
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(2)
3.
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We are seen as special in God's creation because we
are made in His image (Gen. 1:27).
Human beings are distinct from animals, because we
are created in the image of God. (Gen. 1:27; 5:1-2)
(a) loss of fetal life
Even with the more improved technigues, there is
still a 90% loss of fertilized ova.
(b) the practice of destroying fertilized ova if they
appear abnormal
In most clinics parents must agree to abort a
fetus that is found to be defective.
(c) the practice of hyperfertilization
Many ova are fertilized simultaneously, one
(sometimes 4) is selected for implantation, and
the others are thrown away.
The effect IVF will have on a biblical view of marriage
motherhood will be affected
lead to the breakdown of the family unit
Gentic Engineering - the manipulation of chromosomes or genes
by methods other than normal reproduction
examples: recombinate DNA research
cloning
The Problems of Eugenics
1.
2.
3.
Gov't domination
Genetic failures
Dehumanization
Genetic disorder (inherited disorder) - any undesirable function
or shape of an individual's body that is caused by a
gene or a group of genes.
Some are not apparent at birth.
Presently there is not cure for genetic disorders.
Some are apparently the wearing out or turning off of
a particular gene (diabetes).
When an individual has a genetic disorder, every cell
in his body has the genetic information for that
disorder.
Genetic Screening - the determining of an individual's genetic
makeup
Three basic screening methods widely used:
1.
2.
3.
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analysis of prospective parents
analysis of the unborn
analysis of the newborn