Campbell and Reece
Chapter 14
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Character: observable, heritable feature
that may vary among individuals in a
population
Trait: 1 of 2 or more detectable variants
in a genetic character
True-breeding: refers to organisms that
produce offspring of the same variety
over many generations of self-pollination
Hybridization: cross-breeding 2 truebreeding individuals
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2.
3.
4.
Advantages of using peas:
several characters with “either-or”
traits
short generation time
large #s of offspring
each flower contained both male &
female organs
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started with true breeders
cross-pollinated 2 contrasting, truebreeding pea varieties (hybridization)
true-breeding parents = P generation
their hybrid offspring = F1 generation
F1 self-pollinated = F2 generation
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1.
2.
did quantitative analysis of thousands of
genetic crosses
deduced 2 principles of heredity:
Law of Segregation (monohybrids)
Law of Independent Assortment
(dihybrids)
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alternative versions of a gene
1.
2.
3.
Alternative versions of genes (alleles)
account for variations in inherited
characters.
For each character, an organism inherits
2 copies of a gene, one from each parent.
If the 2 alleles @ a locus differ, then the
dominant allele determines the
organism’s appearance & the recessive
allele has no noticeable effect on the
organism’s appearance
4. Law of Segregation: the 2 alleles for a
heritable character separate during
gamete formation & end up in different
gametes. (correlates to 2 homologous
chromosomes separating in Meiosis I)
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diagramatical device for predicting the
allele composition of offspring from a
cros between individuals froma known
genetic makeup.
P signifies dominant
p signifies recessive
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Homozygous: having 2 identical alleles for a
given gene
Heterozygous: having 2 different alleles for a
given gene
Phenotype: the observable physical &
physiological traits of an organism, determined
by its genetic makeup
Genotype: the genetic makeup or set of alleles of
an organism
Testcross: breeding an organism of unknown
genotype with a homozygous recessive to
determine the unknown genotype
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states basically that in a dihybrid cross
each allele for the 2 characters being
crossed has equal opportunity
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Probabilities of all possible outcomes for
an event = 1
outcome of any particular toss of a coin is
unaffected by the results of any previous
tosses
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used to determine the probability that 2
or more independent events will occur
together in some specific combination
multiply the probability of 1 event by the
probability of the 2nd event
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the probability that any 1 of 2 or more
mutually exclusive events will occur is
calculated by adding their individual
probabilities (which we calculate using
the multiplication rule)
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Alleles can show different degrees of
dominance or recessiveness in relation
to each other
Mendel’s peas characters were examples
of complete dominance (all or none)
Incomplete Dominance: neither allele is
completely dominant or recessive
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Snapdragons
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2 alleles affect the phenotype in
separate, distinguishable ways
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When a dominant allele coexists with a
recessive allele in a heterozygote, they do
not actually interact.
It’s in the pathway from genotype to
phenotype that dominance &
recessiveness come into play
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Mendel’s peas Round/wrinkled
R allele codes for an enzyme that helps
convert an unbranched form of starch 
branched form in the seed
r codes for a defective form of same
enzyme leading to an accumulation of
unbranched starch which leads to excess
water entering seed by osmosis
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later, when seed dries it wrinkles
If R present, it makes enough enzyme to
make enough branched starch to prevent
wrinkling
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sometimes depends on how closely we
look
example: Tay Sachs disease
homozygous recessive
Those with it cannot metabolize certain
lipids in neurons  lipids accumulate 
child suffers neurological events
(seizures, blindness, degeneration of
motor & mental performance)
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when study heterozygotes vs. homozygous
dominant individuals: heterozygotes have an
intermediate level of the activity of enzyme
that metabolizes this lipid than do homozygous
dominant individuals
on biochemical level acts like incomplete
dominance since ½ the normal enzyme activity
is sufficient to prevent lipid accumulation,
heterozygotes have normal phenotype
on molecular level it is really an example of
codominance
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dominant allele not always more
frequent allele in a population
example:
polydactyly
extra fingers or toes
1/400 babies born in USA
some caused by presence of a dominant
allele
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most genes exist in >2 allelic forms
example: ABO blood groups
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most genes have multiple phenotypic
effects
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Greek: standing apart
phenotypic expression of a gene at one
locus alters that of a gene at 2nd locus
example: color of labs
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Quantitative Characters: phenotypes vary
in gradation along continuum in a
population (height, skin color)
Polygenic Inheritance: an additive effect
of 2 or more genes on a single
phenotypic character, several genes 
single phenotype (converse of
pleiotrophy: 1 gene  several
characters)
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for humans: very old ?
generally, genotype is NOT associated
with a rigidly defined phenotype
see range of phenotypic possibilities due
to environmental influences
phenotypic range is called: norm of
reaction for a genotype
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generally, broadest for polygenic characters
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The environment contributes to the
quantitative nature of polygenic
characters which are referred to as
multifactorial
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influenced by genetics & environment
(nutritional status, exposure to infectious
disease, general well-being)
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in place of looking at organisms as single
gene  single phenotype
view organism as whole: emergent
properties of all genes  all aspects of its
phenotype
In most cases, a gene’s impact on
phenotype is affected by genes & by the
environment
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In light of all the possibilities of gene
interaction it was extremely lucky that
Mendel chose to study inheritance in the
garden pea he chose.
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a diagram of a family tree with
conventional symbols, showing the
occurrence of heritable characters in
parents & offspring over multiple
generations
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generally, the recessive homozygous
either has a malfunctioning protein or no
protein at all
heterozygous individuals produce
enough of the normal protein to have
“normal” phenotype & are called carriers
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generally, genetic disorders are NOT
evenly distributed among all groups of
people
uneven distribution results from
different genetic histories of world’s
people when populations were more
geographically isolated
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when a disease-causing recessive allele is
rare it is relatively unlikely that 2
carriers will meet & mate
if the 2 carriers are closely related (1st
cousins) the probablity of passing on
recessive traits increases
(consanguinous matings)
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little or no pigment in skin, hair, eyes
affects: vision, skin
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most common lethal genetic disease in
USA
1/2500 people of European descent have
CF
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4% are carriers
normal allele codes for membrane
protein that functions in transport of Claffected individuals have defective or no
Cl- membrane channel
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result of abnl Cl- channel:
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abnl high Cl- in extracellular fluid  mucus
that coats certain cells to become thicker,
stickier than normal
mucus more tenacious, builds up in
pancreas, lungs, digestive tract, testes
has pleiotropic effects: poor digestion &
absorption of nutrients (fats), chronic
bronchitis, frequent bacterial infections,
infertility (males), diabetes
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autosomal recessive
growths grow on
nerves
skin changes (3-5%
growths cancerous)
hearing loss
bone damage
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most common inherited disorder among
people of African descent
1/400 African-Americans
single a.a. substituted in hgb
homozygous recessive individuals: all
RBCs sickle shaped
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when O2 content of affected individual is
low (hi altitudes, physical exertion) the
sickle cell hgb molecules aggregate into
rods  sickle shape to RBC
sickled RBCs will clog small vessels 
weakness, pain, organ damage, paralysis
transfusion help prevent brain damage
no cure
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1/10 African-Americans
unusually high frequency of
heterozygosity considering homozygous
recessive phenotype has such
detrimental effects
Malaria parasite spends part of its life
cycle in RBCs & even with only some
sickeled cells present it lowers the
density of the parasite  reduced
malarial symptoms
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those that are lethal less common than
recessive disorders
most cause death of afflicted individual
all lethal alleles arise by mutations in
gametes
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form of dwarfism found in heterozygotes
1/2500 people have achondroplasia
(0.01% of US)
If you do not have this form of dwarfism
you are homozygous recessive for it
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example of a lethal dominant allele that
is passed on to offspring (50%) because
it does not cause death until individual
in mid-forties (phenotype normal til
then)
degenerative, irreversible, untreatable
disease of nervous system
can test DNA (tip of chromosome 4)
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genetic (usually polygenic) + environmental
components
examples:
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heart disease
alcoholism
schizophrenia
bipolar disease
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use multiplication rule to determine if
potential parents are carriers
each child represents an individual event
it is incorrect to think: “If we have 1 child
with a recessive disease then our next 3
children will have the normal
phenotype”.
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available for several of the recessive
disorders
law passed in 2008 forbids
discrimination by insurance carriers (or
employers) from dropping coverage for
known carriers
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Amniocentesis:
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amniotic fluid sample taken in 2nd trimester
karyotype done on fetal cells
biochemical marker assayed
Chorionic Villus Sampling:
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1st trimester test by sampling placental
tissue (1 layer formed by fetus)
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newest technology: test mom’s blood 
find fetal cells  culture & test them
Ultrasound (US) can identify many
anatomical abnl in fetus
Fetoscopy: scope in amniotic cavity for
diagnosis, possible treatment
Intra-amniotic surgery: repair neurotube
defects, heart defects…..
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most hospitals screen using heel prick
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PKU (phenylketonuria)
recessive
 1/10,000 to 1/15,000
 cannot metabolize phenylalanine
 causes severe drop in mental capacity
 TX: diet free of phenylalanine
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