Mendel and the Gene Idea

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Chapter 14 Mendel and the Gene Idea
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• A.  Gregor Mendels Discoveries
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•             Mendel brought an scientific and
  mathematical approach to studying heredity à this
  is the field of Genetics.
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•             He studied peas.  Why?
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• Peas have a variety of characters that were
  easily studied.  Characters are heritable
  features (eg. flower color).  Each variant of a
  character is called a trait (eg. purple or white
  flower).
• -Some selected traits used by Mendel were (See
  Table 14.1 for complete list) flower color, seed
  color, seed shape, and stem length.

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Mendel was extremely lucky in choosing the pea
plant with which to work. This is because, the
pea plant traits that he studied are all
discontinuous traits. This means that they are
either one way or the other, there is no in
between. For example, pea plants have either
purple or white flowers smooth or wrinkled
seeds.  These traits have no gradations. This is
important, because it allowed Mendel to discern
how traits are passed from one generation to the
next. There are many traits that have gradations
and we will see some of these later in the
Chapter. One example is the carnation flowers
colors (See next)
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There were other reasons that Mendel used pea
plants - Stamens (male reproductive organs)
could be removed to control mating.  (There would
be no self-fertilization.) Thus, he could mate
male and female gametes as he chose and could
control his experiments.    - This was be done by
taking pollen (sperm) from one plant, and adding
it to the carpel (female organ) of another plant
that had its stamen removed (Figure 14.1)    - By
fertilizing plants by hand, the parents of each
pea seed would be known.
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            - In addition, Mendel used only
true-breeding plants. With these plants, the
traits remain constant after self-fertilization.
(This means that the plants contain two identical
genes ? both genes encode the same trait.) For
example, because a pea plant has only genes for
white flowers, if it self-fertilizes, all the
offspring will only have genes for white flowers.
Thus, the trait is constant in each
generation.            
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• For his breeding experiments, Mendel did the
  following in which he tracked heritable
  characteristics for three generations (Figure
  14.3)
• - Produced offspring by hybridization. 
  Hybridization is the mating of two (2)
  true-breeding individuals.
• - True-breeding parents are called the P
  generation.
• - Hybrid offspring are called the F1 generation.
• He then allowed the F1 generation to
  self-pollinate, the offspring of this group are
  called the F2 generation.
• ? Note the ratio of three purple to one white
  flower!!

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By observing those three generations, Mendel laid
the foundation for two important
principles             1.  Law of
Segregation 2.  Law of Independent
Assortment Lets look at the tenets of the Law
of Segregation in detail a.  There are multiple
versions of the same gene (each version is a
different allele See Figure 14.4). b.  Each
organism inherits two (2) alleles for each
character one allele from each parent.  c.  If
the two alleles are different, then the dominant
allele is fully expressed the recessive allele
has no noticeable effect on the organisms
appearance. d.  The two alleles for each
character separate during gamete production
(Occurs during meiosis) ? Segregation
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A Punnett Square is a device for predicting the
results of a genetic cross between two
individuals of known genotypes.  It is used to
illustrate the 31 ratio that Mendel observed in
the F2 generation.   The Punnett Square and how
to use it is described in Figure 14.5 (p. 255)
Mendels law of segregation. Remember the
following vocabulary words and apply them to Fig.
14.5 and 14.6 - Homozygous  contains identical
alleles for a character - Heterozygous  contains
two different alleles for a character -
Phenotype  an organisms traits - Genotype  an
organisms genetic makeup
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         One of the things we can do with a
Punnett Square is to devise a way to reveal the
genotype of an unknown organism. This is done by
doing a Testcross ?                               
      - By breeding an organism of unknown
genotype with an organism with a homozygous
recessive individual, we can determine the
genotype of the unknown individual.  The ratio of
phenotypes in the offspring is used to determine
unknown genotype. For example                   
        Figure 14.7 (p. 256) A testcross.
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            2.  Law of Independent
Assortment                         Mendel did
his experiments by following only a single
character at a time. Instead, one can follow two
characters at a time, to demonstrate the Law of
Independent Assortment ? Each allele pair
segregates independently from other allele pairs
during gamete formation. These experiments use
whats called a dihybrid cross.   Figure 14.8 (p.
257) Testing two hypotheses for segregation in
a dihybrid cross. Note that the combination of
two traits gives a 9331 ratio!
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B.  Extending Mendelian Genetics ? There are many
factors that make genetics not as straight
forward as Mendel saw. These include            
   1.  Incomplete dominance                      
   - F1 hybrids have a phenotype somewhere in
between the phenotypes of the two
parents.   Figure 14.10 (p. 261) Incomplete
dominance in snapdragon color. ? Can you
determine how one can see if its incomplete
dominance?
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2.  Dominance vs. co-dominance                    
     - Both alleles are equally expressed. 
Example  Tay-Sachs disease   Homozygous
recessive do not produce an enzyme to metabolize
lipids that accumulate in brain cells, which
causes the cells to die. Heterozygotes produce
the enzyme but only at half the amount produced
in homozygous dominants. You dont see the
symptom of the disease, because half the normal
amount of enzyme is sufficient.            
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3.  Multiple alleles - Most genes have more than
two (2) alleles.  Example  Blood type A B
AB O
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4.  Pleiotropy                         - Most
genes affect an organism in many ways they dont
affect just one phenotypic character. Example ?
The many effects of sickle cell anemia  
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5.  Epistasis                         - One gene
affects the expression of another
gene.                          Figure 14.11 (p.
263) An example of epistasis. In this case,
the gene for color is B where BB black, and bb
brown. But a second gene, C, determines
whether pigment can be produced. C allows for
pigment to be produced, c does not allow pigment
to be produced (albino).
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            6.  Polygenic inheritance            
             - Two or more genes affect one
phenotypic character the opposite of
pleiotropy.  These are called quantitative
characters. Example, skin color, where three
genes impart color.                       Figure
14.12 (p. 263) A simplified model for polygenic
inheritance of skin color.
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7.  Environmental impact on gene
expression                         -
Environmental factors/conditions may alter gene
expression.   Figure 14.13 (p. 264) The effect
of environment on phenotype.
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C.  Mendelian Inheritance in Humans               
            - We are unable to manipulate mating
patterns of humans for experimentation. For this
reason,   - Traits are studied by gathering
information and placing it into a family
tree.   - The interrelationships between parents
and children are called the family
pedigree.                           Figure 14.14
(p. 265) Pedigree analysis.
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Many Human Disorders Follow Mendelian Patterns of
Inheritance   1.  Recessively Inherited
Disorders - Heterozygous individuals exhibit
normal phenotype because one copy of the normal
allele is typically sufficient.   -
Heterozygotes, who are phenotypically normal, are
called carriers.  They may transmit the recessive
allele to their offspring.                        
                         - Cystic
fibrosis                                          
       - Tay-Sachs disease                        
                         - Sickle-cell disease
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2.  Dominantly Inherited Disorders                
                     - Lethal dominant alleles
are uncommon because, if they cause death before
maturity, then the allele will not be passed to
future generations.   - Huntingtons disease
(nervous disorder) is caused by a late-acting
allele and is sometimes passed to future
generations.   3.  Multifactorial
Disorders                                     -
Most diseases are influenced not only by
genetics, but also by environmental
factors                                           
      - Heart disease, diabetes, cancer,
alcoholism, mental illnesses
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            Technology  for Genetic Testing and
Counseling    1.  Carrier Recognition             
                        - Determine whether
prospective parents are heterozygous carriers of
a recessive trait                         -
Identify carriers of diseases such as Tay-Sachs,
sickle cell, or cystic fibrosis   - Ethical
issues?
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2.  Fetal Testing                                 
    a.  By inserting a needle into the uterus,
physicians can extract amniotic fluid.  In some
cases, the fluid can be used to detect genetic
disorders.  The technique is known as
amniocentesis. In rare cases, amniocentesis
can result in complications or fetal death. 
Therefore, it is reserved for cases in which the
risk for defect is greatest.   b.  An alternative
technique is called chorionic villus sampling
(CVS).  A tube is inserted through the cervix and
fetal tissue from the placenta is
extracted.   c.  Other techniques, such as
ultrasound, can be used to examine the fetus
directly for physical abnormalities.    Figure
14.17 (p. 270) Testing a fetus for genetic
disorders.
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  3.  Newborn Screening                           
          - Some genetic disorders can be
detected by simple tests perfomed soon after
birth.                                           
       - Phenylketonuria (PKU) inability to
break down phenylalanine
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• Chapter 15
• You should review and understand Figure 15.1

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