Genetics

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Genetics

• Mendel And The Gene Idea

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Gregor Mendel

• Austrian Monk
• Background in mathematics and botany
• 1831 to 1853
• Studied inheritance patterns in peas.
• Discovered that
• Heritable factors called traits were passed on
  from one generation to the next.
• Traits are carried on chromosomes celled genes.
• Different forms of the gene are celled alleles.

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Pea Plants
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The Experiment

• Mendel cross pollinated pea plants and followed
  their traits.
• Stamen male organ of the pea plant
• Produces pollen - the male gamete
• Pollen produced at the terminus of the stamen in
  a structure called the anther.
• Carpel female organ of the pea plant
• Produces the ova female gamete.
• Union of pollen and ova creates a fertilized egg
  called the zygote which develops into a seed and
  eventually grows into an embryo.
• Mendel used True Breeders
• Self pollinating plants that produced clones of
  themselves.

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What did Mendel Find?

• Mendel found that the F1 generation (first
  filial) all of the flowers were the same color.
• There was no blending of parental
  characteristics.
• Only one trait was expressed in the hybrid
  offspring.
• When he crossed two flowers from the F1
  generation and found that a 31 ratio of purple
  to white flowers.
• This indicated that one trait was dominant over
  the other trait.

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Mendel Made Four Assumptions

• 1. There are alternative forms of heritable
  factors called alleles
• 2. The offspring inherit an allele from each
  parent.
• 3. The alleles separate during meiosis (Law of
  Segregation)
• 4.One allele can be masked by the presence of
  another allele.
• Phenotype
• Physical characteristics
• Purple flowers
• Genotype
• Genetic composition
• The genes on that particular locus on the
  chromosome.
• Locus location of the gene on the chromosome.
• Heterozygous
• Different alleles Aa
• Homozygous
• Same alleles AA

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Crosses and Following Traits

• Test Cross
• Used to find the genotype of an unknown
  individual
• Cross with a homozygous recessive to reveal the
  unknown genotype.
• Monohybrid Cross
• One trait is followed
• Dihybrid Cross
• Two traits are followed
• If all alleles segregate independently then there
  should be a 9331 phenotypic ratio in the
  offspring.
• 9 dominant/ dominant
• 3 dominant /recessive
• 3 recessive/dominant
• 1 recessive/ recessive

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Dihybrid Cross
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Inheritance, A Game of Chance

• Probability plays a role in the distribution of
  alleles in the off spring.
• One can use the laws of probability to predict
  the genotype and phenotype of the offspring.
• Probability is the ratio the a particular event
  will occur to the all the possible events that
  can occur.
• Toss a coin, there is ½ a chance of tossing heads
  and ½ chance of tossing tails.
• The outcome of previous events does not affect
  the future outcome of other events.

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Rule of Multiplication

• The probability of multiple events occurring at
  the same time is equal to the product of the
  probability of each event occurring
  independently.
• The chance of having a boy is ½.
• The chance of having two boys consecutively is
• ½ x ½ ¼

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Rule of Addition

• If a genotype can be formed in more than one way
  then the rule of addition applies.
• There are two ways to get a heterozygous genotype
  Aa.
• A can be maternal and a can paternal or
• A can be paternal and a can be maternal
• A a
• A a
• Rule of addition states that the probability an
  event will occur by two possible mechanisms is
  the sum of the those separate probabilities.
• Chance the heterozygote parents will produce a
  heterozygote offspring is
• ¼ chance A a
• ¼ chance A a
• Thus ¼ ¼ ½

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Not All Traits Exhibit Dominance and Recessive
Characteristics

• Incomplete Dominance
• The F1 hybrids express a phenotype that is
  somewhere in between the phenotypes of the
  parents.
• Snap dragons only express half the red pigment
  and appear pink.
• Human hypercholesterolemia the heterozygote on
  have ½ the LDL receptors and thus have high
  cholesterol.

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Incomplete Dominance
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Multiple Alleles

• More than two alleles may exist for one trait
• Both alleles may be codominant
• Both alleles are equally expressed
• Blood types are determined by proteins present on
  the surface of the blood cell.
• Types
• IA i or IA IA A blood type
• IB i or IB IB B blood type
• IA IB AB blood type (universal receiver)
• i i O blood type (universal donor)
• Receiving wrong blood type causes agglutination

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Agglutination
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Multiple Alleles

• Human blood types are ABO and are determined by
  three alleles at one locus. IA and Ib are
  dominant to I ( O).
• Is it possible for a normal individual to carry
  all three alleles? Explain. What gametes could
  be produced by someone with type AB? What
  gametes could be produced by someone with type O?
  Is it possible to contain more than one allele
  of a given gene?

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Pleiotrophy

• The inheritance of a single gene can have
  multiple effects on the individual.
• Sickle Cell Anemia

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Sickle Cell Anemia
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Epistasis

• An allele one chromosome can affect the
  expression of the allele on another chromosome.
• Example
• Epistasis occurs in mice.
• One allele determines the coat color.
• Dominant B for black
• Recessive b for brown
• The other allele determines whether or not
  pigment is deposited at all.
• Dominant C for pigment deposition.
• Recessive c for no pigment deposition

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Epistasis
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Polygenic Inheritance

• More than one copy of the allele determines the
  degree in which the allele is expressed.
• Pigment deposition in human skin cells.
• Dominant alleles cause the pigment melanin to be
  deposited.
• Multiple copies of the dominant allele cause more
  melanin to be deposited.
• AABBCC very dark and aabbcc very light.
• AaBbCc is intermediate shade

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Polygenic Inheritance
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Environmental Factors

• The environment can have an impact on phenotypic
  characteristics.
• Exercise changes the build of a person.
• The product of a genotype is not rigidly defined
  by a phenotype but rather a range of
  possibilities.
• This range is known as the norm of the reaction.
• Blood a particular locus may determine the blood
  type, however the number of blood cells from one
  individual from the next varies and there affect
  physical fitness at certain altitudes.

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Multifactorial Influence of Phenotype

• Phenotype can refer to a specific characteristic.
• Flower color
• Phenotype can also refer to the phenotype of the
  organism as a whole in which many characteristics
  determine the overall phenotype.
• A combination of environmental and genetic
  factors influence phenotype.

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In Hydrangea an acidic soil produces a pink flower
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Pedigree Charts

• Pedigrees are used to track the inheritance
  patterns of previous generations to predict the
  future to determine what characteristics are
  likely to be inherited in future generations.
• Probabilities and Mendelian genetics are used to
  determine the genotypes of various family members

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Recessive Disorders

• The allele is recessive and in most cases codes
  for a protein that is malfunctioning.
• The result can be a syndrome such as
• Tay Sachs Disease
• Malfunctioning protein that breaks down lipids in
  the brain
• Higher occurrence in Ashkenazic Jews due to
  ancestral heritage.
• Infant experiences seizures, blindness and
  degeneration of motor and mental performance.
• The heterozygote is the carrier.
• May produce enough normal protein to compensate
  and the individual is normal.

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Penetrance

• Proportion of individuals who show the phenotype
  expected from their genotype.
• 100 means all individuals with genotype show
  phenotype.
• Tay-Sachs disease shows complete or 100
  penetrance as all homozygote for the allele
  develop disease and die.

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Dominant Allele Disorders

• The heterozygote has the disorder.
• Dominant Lethal disorder
• Huntingtons disease
• Eventual deterioration of the nervous system.
• Remains in the gene pool because the onset is in
  individuals that are 35 years or older.
• Any child born to the parent carrying this allele
  has a 50 chance of having the disease.
• Achondroplasia
• A type of dwarfism

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Multifactorial Disorders

• Heart disease, cancer, diabetes manic depression,
  schizophrenia and alcoholism
• Increased propensity due to heredity can be
  circumvented by diet, exercise and behavior.
• Carriers for certain diseases can be identified
  through the genetic testing.
• Genetic counseling may help couples decide if
  they want to have children based on the
  probability the disorder will be inherited.

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Phenocopy

• An environmentally produced phenotype that
  stimulates the effects of a particular gene.
  Normal genotype but show trait.
• Thalidomide causes same effects of birth defect
  ( mutation) phocomelia
• Babies have flipper like limbs.

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Expressivity

• The degree to which a particular gene is
  expressed in individuals showing the trait.
• Example Retinoblastoma ( type of eye tumor)
• Not all individuals who inherit the allele
  develop the tumor ( incomplete penetrance) and in
  those who do develop the tumor the severity
  varies.

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Fetal Testing

• Amniocentesis
• Amniotic fluid is extracted from the womb during
  the 14th to 16th week of pregnancy.
• The presence of certain chemicals can determine
  whether or not a particular genetic disorder is
  present immediately.
• Cystic Fibrosis
• Cells can be grown in vitro fro 2 weeks and
  chromosomal abnormalities can be determined
  through a karyotype.
• Down syndrome
• Tay Sachs
• Chorionic Villis Sampling (CVS)
• A needle is inserted into the cervix and
  placental (fetal) is extracted.
• Karyotype can be done in 24 hours
• Can be performed in the 8th week of pregnancy
• More risky than amniocentesis
• Less available than amniocentesis
• Cannot be done to look for abnormalities in
  amniotic fluid.

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What genetic disorder will this patient have?
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