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CHAPTER 14 MENDEL AND THE GENE IDEA
How are traits transmitted?
One theory-
The ____________ hypothesis- Genetic material
mixes much like yellow and blue paint makes green
Another theory- The _______________ hypothesis-
Parents pass on discrete heritable units - genes
- that retain their separate identities in
offspring.
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1. Mendel brought an experimental and
quantitative approach to genetics

• Gregory Mendel is the father of genetics
• entered Augustinian monastery in Czech republic
  in 1843
• University of Vienna from 1851 to 1853
• Influenced by a mathematician and a botanist to
  study variation in plants
• Around 1857, Mendel began breeding garden peas in
  a monastery to study inheritance

• Advantages of pea plants
• many varieties with distinct heritable features
  (characters)
• Mendel had strict control over which plants mated
  with which.

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1. Law of segregation
Mendel cross-pollinated peas
P1 x P2
_________________
The F2 generation revealed two principles of
heredity 1. _____________________
2._______________________
F1
_________________
F1 x F1
_________________
F2
1. __________________two alleles for a character
are packaged into separate gametes
Evidence
F1
F2
purple x white all purple
white purple
705
224
1 to 3 ratio
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1. Law of segregation

• Mendel reasoned that the purple flower is a
  _____________trait and the white flower is a
  _________________trait.
• The plant colors were not blended

Mendel also found the same 31 ratio in other
crosses (e.g wrinkled and smooth seeds)
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1. Law of segregation

• Mendel developed a hypothesis to explain these
  results that consisted of ________related ideas.
• 1. Alternative version of genes
  (______________) account for variations in
  inherited characters.
• Example The purple-flower allele and
  white-flower allele are two DNA variations at
  the flower-color locus.

Fig. 14.4
2. For each ___________, an organism inherits
twoalleles, one from each parent.

• E.g. a purple-flower allele from one parent and
  a white-flower allele from the other.

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1. Law of segregation

• 3. If two alleles differ, the_______________,
  is fully expressed in the organisms appearance.

• The ________________, has no noticeable effect on
  the organisms appearance.
• E.g. white-flower allele is recessive.

• 4. The two alleles for each character
  ___________ (separate) during gamete production.
• If different alleles are present, then 50 of the
  gametes will receive one allele and 50 will
  receive the other.
• The separation of alleles into separate gametes
  is summarized as Mendels ____________________

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1. Law of segregation

• A ________________ predicts the results of a
  genetic cross between individuals of known
  genotype.

Purple
White
Call the dominant allele P Call the recessive
allele p
PP x pp
All Pp
Purple
F1
Pp x Pp
F2
Predicts 75 purple 25 white Or 31 ratio
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1. Law of segregation

• Vocabulary
• ______________- An organism with two ________
  alleles for a character
• _______________- An organism with two ________
  alleles for a character
• __________- A description of an organisms _____
• ___________- A description of its genetic makeup

• Example- For flower color in peas, both PP and Pp
  plants have the same phenotype (purple) but
  different genotypes (homozygous and
  heterozygous).

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What about the law of independent
assortment??Law of _________ ___________each
pair of alleles segregates into gametes
independently
2. Law of independent assortment

• This law comes from Mendels observing
  inheritance of ______ different characters

Color and shape

• The allele for yellow seeds (Y) is dominant to
  the allele for green seeds (y).
• The allele for round seeds (R) is dominant to the
  allele for wrinkled seeds (r).

So what happens when we cross YYRR x yyrr??
Known as a ______________
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2. Law of independent assortment

• Predict- two characters are transmitted from
  parents to offspring as a__________.
• The Y and R alleles and y and r alleles stay
  together.

• Thus, ________ the F1 to be yellow, round
  seeds (YyRr) and F2 to be 31 (yellow/roundgreen/
  wrinkled
• But ________ four classes of gametes
  (________________) produced in equal amounts.

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• These combinations produce four distinct
  phenotypes in a ___________ ratio.

3
3
1
Fig. 14.8
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Mendelian inheritance reflects rule of __________.

• Each toss of a coin is an independent event, just
  like the distribution of alleles into gametes.

• Like a coin toss, each ovum from a heterozygous
  parent has a _____ chance of carrying the
  dominant allele and a _____ chance of carrying
  the recessive allele.

• The probability that two coins tossed at the same
  time will land heads up is 1/2 x 1/2 1/4.
• Similarly, the probability that a heterozygous
  pea plant (Pp) will produce a white-flowered
  offspring (pp) is ______________.

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• Predicting phenotypes in dihybrid crosses- The
  multiplication rule.
• What is the probability that an F2 plant will
  have a YYRR genotype from a heterozygous parent
  (YyRr)
• 1/4 (YR ovum) x 1/4 (YR sperm) 1/16

Y
y
R
r
Yy
Rr
YY
RR
Y
R
yy
rr
Yy
Rr
y
r
¼ YY
X
¼ RR
1/16
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What are the chances of having a grandchild with
the muscular dystrophy (a recessive disease)?
Given Grandparents are RR and rr, where r is
the disease allele.
Their grandson has married a carrier.
Grandparents
Parents
R
R
R
r
Rr
Rr
Rr
RR
r
R
Rr
rr
Rr
Rr
r
r
F1 (Parents)- no disease
F2 - ¼ with disease
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2. The relationship between genotype and
phenotype is ___________________

• Mendels peas were relatively simple genetically.
• Each character is controlled by
  a____________________.
• Each gene has only_________________, one of which
  is completely dominant to the other.

Fig. 14.10
red
white
Many alleles show_____________________
heterozygotes show a distinct intermediate
phenotype, not seen in homozygotes.
pink
F1
Others show _______________ in which both
alleles affect the phenotype
red
F2
e.g. Two genes encode distinct proteins on blood
cells responsible for blood-typing
pink
white
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• Dominance/recessiveness relationships have three
  important points.
• 1. They range from_____________________, to
  incomplete dominance, to codominance.
• 2. They reflect the mechanisms by which
  specific alleles are expressed in the phenotype
  and _________ involve the ability of one allele
  to subdue another at the level of DNA.
• 3. They do not determine or correlate with the
  relative abundance of alleles in a population.

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• The genes covered so far affect _________
  phenotypic character.
• Most genes are_____________, affecting more than
  one phenotypic character.
• Example A single mutation can contribute to
  diabetes, eye disease, gangrene infections, and
  heart disease.

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• Phenotype depends on ______________________.
• A single tree has leaves that vary in size,
  shape, and greenness, depending on exposure to
  wind and sun.
• For humans, nutrition influences height, exercise
  alters build, sun-tanning darkens the skin, and
  experience improves performance
  on______________________.
• Even identical twins, genetic equals, accumulate
  phenotypic differences as a result of their
  unique experiences.
• The relative importance of genes and the
  environment in influencing human characteristics
  is a very old and hotly contested debate.

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• A ______________emphasis on single genes and
  single phenotypic characters presents an
  _____________ perspective on heredity and
  variation.
• A more comprehensive theory of Mendelian genetics
  must view organisms______________.

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3. Pedigree analysis reveals Mendelian patterns
in human inheritance

• In a_____________________, information about the
  presence/absence of a particular phenotypic trait
  is collected from as many individuals in a family
  as possible and______________________.
• The distribution of these characters is then
  mapped on the ____________________.

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A pedigree can help us understand the past and to
predict the future.

• Example- the occurrence of widows peak (W) is
  ______________ to a straight hairline (w).

Grandparents
Parents
Kids

• If the kids lacks a widows peak, but both
  parents have widows peaks, then her parents must
  be ____________ for that gene

Fig. 14.14
The chance of having a widows peak is (1/2 WW
1/4 Ww) _______
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4. Many human disorders follow Mendelian patterns
of inheritance

• Thousands of genetic disorders, including
  disabling or deadly hereditary diseases, are
  inherited as simple __________________________.
• From mild (albinism) to life-threatening (cystic
  fibrosis).

• __________________ may have no clear phenotypic
  effects, they are carriers who may transmit a
  recessive allele to their offspring.

• Most people with ____________________are born to
• ______________with normal phenotypes.
• Two carriers have a 1/4 chance of having a child
  with the
• disorder, 1/2 chance of a carrier, and 1/4 free.

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Examples of recessive disorders

• 1. _________________-one of every 2,500 whites of
  European descent.
• One in 25 whites is a _____________.
• The normal allele codes for a membrane protein
  that transports Cl- between cells and the
  environment.
• If these channels are defective or absent, there
  are abnormally high extracellular levels of
  chloride that causes the mucus coats of certain
  cells to become thicker and stickier than normal.
• This mucus build-up in the pancreas, lungs,
  digestive tract, and elsewhere favors bacterial
  infections.
• Without treatment, affected children die before
  five, but with treatment can live past their late
  20s.

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Examples of recessive disorders

• 2. ___________________________
• Caused by a dysfunctional enzyme that fails to
  break down specific brain lipids.
• Symptoms- seizures, blindness, and degeneration
  of motor and mental performance a few months
  after birth.
• Child dies after a few years.
• Among ____________________(those from central
  Europe) this disease occurs in one of 3,600
  births, about 100 times greater than the
  incidence among non-Jews or Mediterranean
  (Sephardic) Jews.

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Examples of recessive disorders
Figure not in text

• 3. ___________________
• The most common inherited disease among blacks
  (1400 African Americans)
• Cause- substitution of a single amino acid in
  hemoglobin.
• Effects- sickle-cell hemoglobin crystallizes into
  long rods.
• This deforms red blood cells into a sickle shape.

A pleiotropic disease

• Multiple problems

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Examples of recessive disorders

• Why does sickle cell disease remain in the
  population?
• Individuals with one sickle-cell allele have
  increased resistance to malaria (whereas those
  with normal alleles die)
• Its relatively high frequency in African
  Americans is a vestige of their African roots.

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Examples of recessive disorders
Why is incest genetically dangerous?

• Relatively unlikely that two carriers of the same
  rare harmful allele will meet and mate.
• However, ___________________matings, those
  between close relatives, increase the risk.
• These individuals who share a recent common
  ancestor are more likely to carry the same
  recessive alleles.
• Example- _______________in royal British lineages
• Most societies and cultures have laws or taboos
  forbidding marriages between close relatives.

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Examples of ____________disorders

• 1. __________________, a form of dwarfism, has an
  incidence of one case in 10,000 people.
• Heterozygous individuals have the dwarf
  phenotype.
• Those who are not achodroplastic dwarfs, 99.99
  of the population are homozygous recessive for
  this trait.
• Lethal dominant alleles are much less common than
  lethal recessives
• Why? because if a lethal dominant kills an
  offspring

• One example -__________________________, a
  degenerative disease of the nervous system that
  is fatal.
• The ________________________________has no
  obvious phenotypic effect until an individuals is
  about 35 to 45 years old.

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• Many disorders normally involve multiple genes-
  these are called _________________
• Multifactorial disorders include heart disease,
  diabetes, cancer, alcoholism, and certain mental
  illnesses, such a schizophrenia and
  manic-depressive disorder.
• The genetic component is typically polygenic.

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5. Technology is providing new tools for genetic
testing and counseling

• Most children with recessive disorders are born
  to parents with a normal phenotype.
• A ________ to assessing risk is identifying if
  prospective parents are carriers of the recessive
  trait.
• However, issues of confidentiality,
  discrimination, and adequate information and
  counseling arise.

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A hypothetical case

• Consider a hypothetical couple, John and Carol,
  who are planning to have their first child.
• In both of their families histories a recessive
  lethal disorder is present and both John and
  Carol had brothers who died of the disease.
• While neither John and Carol nor their parents
  have the disease, their parents must have been
  carriers (Aa x Aa).
• John and Carol each have a 2/3 chance of being
  carriers and a 1/3 chance of being homozygous
  dominant.
• The probability that their first child will have
  the disease 2/3 (chance that John is a carrier)
  x 2/3 (chance that Carol is a carrier) x 1/4
  (chance that the offspring of two carriers is
  homozygous recessive) 1/9.

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• If their first child is born with the disease, we
  know that John and Carols genotype must be Aa
  and they both are carriers.
• The chance that their next child will also have
  the disease is 1/4.

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• In utero testing for a disorder.
• 1. ____________- beginning at the 14th to 16th
  week of pregnancy to assess the presence of a
  specific disease.

Fig. 14.17a

• Fetal cells extracted from amniotic fluid are
  cultured and ____________.

• 2. _____________________- performed as early as
  the eighth to tenth week of pregnancy.
• Extracts a sample of fetal tissue from the
  chrionic villi of the placenta are karyotyped.

Fig. 14.17b
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• Other techniques-_________________________ allow
  fetal health to be assessed visually in utero.
• usually reserved for cases in which the risk of a
  genetic disorder or other type of birth defect is
  relatively great.
• If fetal tests reveal a serious disorder, the
  parents face the difficult choice of terminating
  the pregnancy or preparing to care for a child
  with a genetic disorder.

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• Some genetic tests can be detected at birth by
  simple tests that are now routinely performed in
  hospitals.
• One test can detect the presence of a recessively
  inherited disorder, _______________________
• This disorder occurs in one in 10,000 to 15,000
  births.
• Accumulate the amino acid phenylalanine in the
  blood to toxic levels.
• Leads to mental retardation.
• If the disorder is detected, a special diet low
  in phenyalalanine usually promotes __________
  development.