Mendelian Genetics

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Mendelian Genetics
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History of Genetics

• Laws of inheritance and genetics were initially
  derived by the work of a 19th century monk name
  Gregor Mendel.
• He developed these laws by cultivating and
  breeding pea plant.

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History of Genetics

• Mendel's finding were largely neglected until the
  1900s when it was rediscovered and integrated
  into the chromosome theory of inheritance.

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Terms

• Genotype- the allelic makeup or genetic content
  of an individual.
• Phenotype- the physical manifestation of alleles
  in an individuals DNA. The way they look
• Allele- A version of a gene

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Terms

• Dominant trait- when two different versions of
  the same allele are present, the allele that is
  expressed physically is the dominant trait.
• Recessive trait- when two different versions of
  the same allele are present, the one that is not
  expressed physically is the recessive trait.

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Terms

• Homozygous- this means both alleles for a
  particular trait are the same AA.
• Heterozygous- this means that both alleles for
  the same trait are different Aa.
• Incomplete dominance- this means that neither
  allele for a particular trait is fully expressed.

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Terms

• Codominance- this means that both alleles for a
  particular trait are fully expressed.

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Mendel's laws

• Law of segregation, also known a Mendel's first
  law and has four essential parts.
• Law of independent assortment, also known as Law
  of Inheritance or Mendels second law.

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Law of Segregation

• Alternative versions of a gene account for
  variations in inherited characteristics.
• This is the concept of alleles.
• Alleles are different versions of a gene that
  impart the same characteristic.
• A good example is eye color.

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Law of Segregation Cont.

• For each characteristic, the organism inherits
  two alleles, one from each parent.
• This means one allele comes from the mother and
  one comes from the father.
• These alleles may be the same (homozygous) or
  they may be different (heterozygous).
• Homozygous means that both alleles are the same
• Heterozygous mean both alleles are different

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Law of Segregation Cont.

• If two alleles differ then one, the one that
  encodes the dominate trait will be expressed.
• The other allele, the recessive trait will have
  not noticeable effect.
• This means only the dominant trait is seen in the
  phenotype or outward appearance.
• This allows recessive traits to passed on to
  offspring even though they are not expressed.

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Law of Segregation Cont.

• Two alleles for each trait separate during gamete
  production.
• This means that each gamete will contain only one
  allele for each gene.
• This allows alleles from the mother and father to
  combine in offspring to ensure variation.

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Law of Segregation Cont.

• There are two other types of dominance,
  incomplete dominance and codominance.
• Incomplete dominance means that neither allele is
  fully express
• Codominance means both alleles are fully expressed

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Law of Independent Assortment

• This law states that the emergence of one trait
  will not effect the emergence of another.
• This means that your eye color will not have an
  effect on your hair color.
• Example Just because you have blue eyes that
  doesnt mean you will have blonde hair.

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Applying Mendels Laws

• Once an organisms alleles for a particular trait
  have been identified the probability of a certain
  phenotype and genotype in their offspring can
  begin obtained.
• This can be done by constructing a Punnet Square.
• When using a Punnet Square alleles are
  represented by letters

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Applying Mendels Laws

• Uppercase letters represent dominant traits .
• Lowercase letters represent recessive traits.
• Each allele or set of alleles are place above a
  column or beside a row on a grid.
• That allele is then placed in each box in its
  respective column or row.

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Applying Mendels Laws

• The resulting combination of traits represents
  then probable genotype or phenotype of the
  respective off spring.
• To obtain the probability of a particular
  offspring you simply divide the number of boxes
  by one hundred this will tell you what percent
  each box is worth.
• Then by adding boxes with identical traits in
  them you obtain what percent of offspring will
  have that genotype .

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Now for a Visual example of how a to use a Punnet
Square
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Types of crosses

• The above cross is called a monohybrid cross
  because it deals with two alleles for one trait.
• So dihybrid is two traits and trihybrid is three,
  naming these continues on in the same manner of
  adding numerical prefixes.
• Each time you add a trait you must also double
  the number of squares, so it is impractical to do
  more than two or three.

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Time for some questions

• How is genotype different from phenotype?
• How can woman with brown eyes and a man with
  brown eyes have a child with blue eyes?
• What is an allele?

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Time for some questions

• Why is it impractical to use the Punnet square to
  cross more than about three traits?
• What is codominance? Incomplete dominance?
• Give an example of an organism than exhibit
  either incomplete or codominance?