Chapter 13: Mendel and the Basics of Genetics

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Chapter 13 Mendel and the Basics of Genetics
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Terminology

• Chromosome structure containing a single, linear
  DNA molecule that physically transmits hereditary
  info from one generation to the next.
• Chromatin the DNA and protein of the chromosome
• Nucleosome a bead-like structure made of a
  single DNA molecule spooled around a histone
  protein core (necessary for fit in the nucleus)

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Terminology

• Gene sequence of DNA nucleotides (sugar,
  phosphate, nitrogenous base) on a chromosome that
  encodes a specific protein
• Alleles alternative forms of a gene (Ex. brown
  vs. black for hair color)
• Codon 3 DNA nucleotides in sequence that code
  for a particular amino acid (combine to form
  proteins)

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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 in structures
  called genes.
• Different forms of the gene are called alleles.
• Autosomes 22 matched chromosomes
• Sex Chromosomes pair 23 (XX, XY)

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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 (pistil
  plus ovary)
• 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 in 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 he found a 31 ratio of purple to
  white flowers.
• This indicated that one trait was dominant over
  the other trait.

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Mendels Two Laws of Heredity

• 1. Law of Segregation members of each pair of
  alleles separate when gametes are formed (get ONE
  allele from mom and ONE allele from dad for each
  trait)
• 2. Law of Independent Assortment when two or
  more pairs of alleles are located on different
  chromosomes or far apart on the same chromosome,
  they separate independently of one another during
  gamete formation (just cause you got moms eye
  color does not mean you will also get her hair
  color, even if found on her same chromosome)

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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 is 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.
• If all white, unknown must be homo recessive
• If get some white, unknown must be hetero
• Monohybrid Cross
• One trait is followed (generations)
• Dihybrid Cross
• Two traits are followed (generations)
• According to the laws of probability and if all
  alleles segregate independently then when
  crossing two hetero parents (for both traits)
  there should be a 9331 phenotypic ratio in the
  offspring.
• 9 dominant/ dominant
• 3 dominant /recessive
• 3 recessive/dominant
• 1 recessive/ recessive
• EXPECTED, BUT NOT NECESSARILY OBSERVED!

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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
  have ½ the LDL receptors and thus have high
  cholesterol.

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Incomplete Dominance
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• Codominance
• Both alleles are equally expressed
• Ex. Black feathered chickens and white feathered
  chickens when mated can produce chicks with BOTH
  white feathers and black feathers.

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Multiple Alleles

• More than two alleles may exist for one trait
• (not SOLELY a dominant and a recessive)
• Both alleles may be codominant
• Both alleles are equally expressed
• Ex. 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) BOTH
  PARENTS MUST BE TYPE O or at least heterozygotes
  for type A or B TO HAVE A TYPE O CHILD

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Rh factor

• Rh factor is an additional antigen found on the
  red blood cells and it is a separate gene from
  the ABO gene.
• If a person has two () genes for Rh or one ()
  and one (-) they will test ().
• A person will be (-) ONLY if they have two (-)
  because Rh() is dominant.
• Positive means you have the Rh antigen, negative
  means you dont.
• Other minor antigens include Kell, Lewis A, Lewis
  B, rho, P etc. and doctors also attempt to match
  as many of those minor antigens as possible when
  selecting blood for transfusions.

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Blood Types

• Types Red blood cells Plasma
• IA i or IA IA A A antigen B
  antibodies
• IB i or IB IB B B antigen A
  antibodies
• IA IB AB A and B antigens NO antibodies
• i i O NO antigens A and B antibodies
• Universal donor Type O-negative (no antigens) is
  compatible with all blood types because it has no
  antigens for other blood types to recognize with
  their antibodies
• Universal Recipient Type AB-positive has no
  antibodies in its plasma so it can accept any
  type without destroying those foreign cells

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Pleiotrophy

• The inheritance of a single gene can have
  multiple effects on the individual.
• Ex. Sickle Cell Anemia
• Causes defective hemoglobin molecules but that
  single gene can ALSO cause
• 1. heart failure
• 2. anemia
• 3. susceptibility to pneumonia
• 4. kidney failure
• 5. enlarged spleen

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Sickle Cell Anemia
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Epistasis

• An allele on 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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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 to the next varies and this affects
  physical fitness at certain altitudes.

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Ex. 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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• SquareMale CircleFemale

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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

• The 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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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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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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Nondisjuntion

• Non-disjunction
• Failure of the chromosomes to separate properly
• Ex. Extra chromosome 21 Downs Syndrome
• XXY Klinefelter
• XYY Jacobs Syndrome
• Single X (XO) Turner Syndrome

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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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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 for 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) fluid 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?