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CHAPTER 14 part B MENDEL AND THAT CRAZY GENE THING
1. Pedigree analyses follow Mendelian patterns in
human inheritance 2. Many human disorders follow
Mendelian patterns of inheritance 3. Genetic
testing and counseling whaddayathink?
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1. Pedigree analysis reveals Mendelian patterns
in human inheritance

• We dont purposely manipulate peoples mating
  patterns, but geneticists analyze the results of
  matings that have already occurred.
• Pedigree analysis collect info about a
  phenotypic trait from many individuals in a
  family and across generations.
• The distribution of these characters is then
  mapped on the family tree.

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• Example a person lacks a widows peak, but both
  her parents have widows peaks.
• Her parents must be h____zygous for that gene.

Fig. 14.14
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• A pedigree can help us understand the past and to
  predict the future.
• Predict the probability that a child with WwFf
  parents will have a widows peak and attached
  earlobes.
• Odds of having a widows peak ______
• (____ WW ____ Ww).
• Odds of having attached earlobes is ff _____
• Combination _____ X _____ _____.

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2. Many human disorders follow Mendelian patterns
of inheritance

• Thousands of genetic disorders are inherited as
  simple recessive traits.
• Relatively mild (albinism) to life-threatening
  (cystic fibrosis).
• The recessive allele codes for either a
  malfunctioning protein or no protein at all.
• Heterozygotes have a normal phenotype because one
  normal allele produces enough of the required
  protein.

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• A recessively inherited disorder shows up only in
  _____zygous individuals who inherit one recessive
  allele from each parent.
• Heterozygotes may have no clear phenotypic
  effects, but are carriers who may transmit a
  recessive allele to their offspring.
• Most people with recessive disorders are born to
  carriers 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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• Genetic disorders are not evenly distributed
  among all groups of humans.
• This results from the different genetic histories
  of the worlds people during times when
  populations were more geographically (and
  genetically) isolated.

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• Cystic fibrosis strikes one of every 2,500 whites
  of European descent.
• One in 25 whites is a carrier.
• Normal allele codes for a membrane protein that
  transports Cl- between cells and the environment.
• With defective protein channels, abnormally high
  extracellular levels of chloride accumulate and
  cause mucus coats to become thicker and stickier
  than normal.
• 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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• Tay-Sachs disease is another lethal recessive
  disorder.
• A dysfunctional enzyme fails to break down
  specific brain lipids.
• Symptoms begin with seizures, blindness, and
  degeneration of motor and mental performance a
  few months after birth.
• Affected children die after a few years.
• Among Ashkenazic Jews (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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• Sickle-cell disease - affects 1400 African-
  Americans.
• Cause substitution of a single amino acid in
  hemoglobin.
• Sickle-cell hemoglobin crystallizes into long
  rods.
• This deforms red blood cells into a sickle shape.

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• Unusually high frequency of sickle-cell trait,
  especially given severe detrimental effects in
  homozygotes.
• Individuals with one sickle-cell allele have
  increased resistance to malaria.
• Homozygous normal individuals die of malaria,
  homozygous recessive individuals die of
  sickle-cell disease, and carriers are relatively
  free of both.
• Its relatively high frequency in African
  Americans is a vestige of their African roots.

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• This sickling creates a cascade of symptoms,
  demonstrating the pleiotropic effects of this
  allele.
• Doctors can use regular blood transfusions to
  prevent brain damage and new drugs to prevent
  or treat other problems.

Fig. 14.15
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• At the molecule level, the two alleles are
  codominant as both normal and abnormal
  hemoglobins are synthesized.
• At the organismal level, Non-sickle allele is
  incompletely dominant to the sickle-cell allele.
• Carriers are said to have the sickle-cell trait.
• These individuals are usually healthy, although
  some suffer some symptoms of sickle-cell disease
  under blood oxygen stress.

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• Normally it is relatively unlikely that two
  carriers of the same rare harmful allele will
  meet and mate.
• Most societies and cultures have laws or taboos
  forbidding marriages between close relatives.
• This is also an argument for out-breeding.
• For example, why do cocker spaniels have hip
  problems?

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• Most harmful alleles are recessive, but some
  disorders are due to dominant alleles.
• For example, achondroplasia, a form of dwarfism,
  occurs in 110,000 people.
• Heterozygous individuals have the dwarf
  phenotype.
• Homozygous recessives (non-dwarfs) 99.99 of
  the population.
• Lethal dominant alleles are rare if the trait
  kills an offspring before it can mature and
  reproduce, the allele will not be passed on to
  future generations.

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• A lethal dominant allele can escape elimination
  if it causes death at a relatively advanced age,
  after the individual has already passed on the
  lethal allele to his or her children.
• Huntingtons disease a degenerative disease of
  the nervous system.
• No obvious phenotypic effect until 35-45 years
  old.
• Irreversible and inevitably fatal.

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• Any child born to a parent who has the allele for
  Huntingtons disease has a 50 chance of
  inheriting the disease and the disorder.
• Pedigree analysis locus for Huntingtons allele
  is near the tip of chromosome 4.

Fig. 14.15
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• Many other disorders are due to multiple factors.
• Genetic environmental influence.
• Heart disease, diabetes, cancer, alcoholism, and
  certain mental illnesses, (e.g., schizophrenia
  and manic-depressive disorder).
• The genetic component is typically polygenic.
• At present, little is understood about the
  genetic contribution to most multifactorial
  diseases

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3. Genetic testing and counseling
whaddayathink?

• A preventative approach to simple Mendelian
  disorders is sometimes possible.
• The risk that a particular genetic disorder will
  occur can sometimes be assessed before a child is
  conceived or early in pregnancy.
• Many hospitals have genetic counselors to provide
  information to prospective parents who are
  concerned about a family history of a specific
  disease.

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Scenario 1

• You and ____ are planning to have your first
  child.
• You both had siblings who died of cystic
  fibrosis.
• Neither you two nor your parents have the
  disease, but both sets of parents must have been
  carriers (Aa x Aa).
• You each have a 2/3 chance of being carriers and
  a 1/3 chance of being homozygous dominant.
• The probability that your first child will have
  the disease 2/3 (chance of being a carrier) x
  2/3 x 1/4 (chance that the child is homozygous
  recessive) 1/9.

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Scenario 2

• You and ____ are planning to have your first
  child.
• Your brother died of cystic fibrosis.
• Your parents must have been carriers (Aa x Aa),
  but your mates parents genotypes are unknown.
• You have a 2/3 chance of being a carrier and a
  1/3 chance of being homozygous dominant. Your
  mate ___?
• The probability that your first child will have
  the disease 2/3 (chance of being a carrier) x
  ___ x 1/4 (chance that the child is homozygous
  recessive) ___.