Mendel and the Gene Idea

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Chapter 14

• Mendel and the Gene Idea

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Mendel

• Mendel is considered to be the father of
  genetics.
• He conducted carefully controlled experiments
  with pea plants and discovered the basic
  principles of heredity.

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Useful Vocabulary

• Character- heritable trait such as flower color
  or hair color.
• Trait the variant for the character purple
  flowers, red hair.
• True-breeding for this instance, the plants
  produced offspring with the same characters when
  the parent plant self-pollinates.

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Generations

• P generation is the parent generation.
• F1 is the first filial generation or the
  offspring of the P generation.
• F2 is the second filial generation or the
  offspring of the F1 generation.

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Generations
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The Law of Segregation

• Alleles are alternate version of a gene. The two
  alleles are located on one of each homologous
  chromosome.
• The law of segregation is that two alleles for a
  character are packaged into separate gametes (one
  from the maternal line and one from the paternal
  line.)

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Alleles

• Alleles can be dominant or recessive.
• Individuals can be homozygous dominant (BB),
  homozygous recessive (bb) or heterozygous (Bb).
• In homozygous dominant and heterozygous
  individuals the dominant trait is expressed.
• For a recessive trait to be expressed, the
  individual must be homozygous recessive.

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Phenotype and Genotype

• The phenotype is the exhibited or expressed
  traits.
• The genotype is the genetic make-up.
• The genotype and phenotype are the same for
  homozygous individuals.
• Heterozygotes are more difficult to determine
  because they carry the recessive trait even
  though it is not expressed.

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Probability and Punnet Squares

• We can determine the probability of traits shown
  in potential offspring of two individuals by
  using a Punnet Square.
• The probabilities are just like tossing a coin.

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Probability

• Probability ranges from 1 very certain an event
  will happen to 0 the event will not happen.
• What is the probability that a coin when tossed
  will land on heads?
• What is the probability that when tossed two
  coins will both land on heads.

½ x ½ ¼
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Using Probability to Solve Genetic Problems

• We can use the Punnet square to calculate the
  number of offspring likely to result from the
  individuals of known genotype.
• FOIL is used to determine the genotypes.
• Draw a 4-square Punnet square and determine the
  likely numbers of offspring for the following
  cross.
• BB x Bb

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Dihybrid Trihybrid Crosses

• T tall, t short
• B brown eyes, b blue
• TtBB x ttBb
• Using the above parents, calculate the probable
  numbers of offspring with the traits that result
  from this cross.
• If R tongue roller, r non-tongue roller then
  calculate TTbbRr x Ttbbrr

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Other forms of Inheritance

• Genetics is not simple.
• There are more complex gene interactions that
  occur other than what we term simple Mendelian
  genetics

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Incomplete Dominance

• Incomplete dominance when neither allele is
  dominant over the other and a blending of the
  traits is the result. Example Hair Texture.
• Curly and Straight hair textures show incomplete
  dominance. CC curly, SS straight and CS
  wavy.

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Codominance

• Another example is codominance.
• Codominance is when both alleles are dominant and
  both traits show in the geneotype.
• Blood Groups are an example.
• I is used for blood.

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Blood Groups

• IA is for type A.
• IB is for type B
• i is for type 0
• Persons with type A can be IA i or IA IA
• Persons with type B can be IB i or IB IB
• Persons with Type AB are IA IB
• Persons with 0 are ii.

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Pleiotropy

• The ability of one gene to affect an organism in
  many ways is pleiotropy.
• The gene for Sickle-Cell Anemia causes multiple
  symptoms thus affects the organism in multiple
  ways.

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Epistasis

• Epistasis is when one gene at one locus affects
  the expression of another gene at another locus.

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Genetic Expression and the Environment

• Phenotypes are always affected by their
  environment. In buttercup (Ranunculus peltatus),
  leaves below water-level are finely divided and
  those above water-level are broad, floating,
  photosynthetic leaf-like leaves.
• Siamese cats are darker on their extremities, due
  to temperature effects on phenotypic expression.
  Expression of phenotype is a result of
  interaction between genes and environment.

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• Siamese cats and Himalayan rabbits both animals
  have dark colored fur on their extremities.
• This is caused by an allele that controls pigment
  production being able only to function at the
  lower temperatures of those extremities.
• Environment determines the phenotypic pattern of
  expression.

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Polygenic Inheritance

• Polygenic inheritance is a pattern responsible
  for many features that seem simple on the
  surface. Many traits such as height, shape,
  weight, color, and metabolic rate are governed by
  the cumulative effects of many genes.
• Polygenic traits are not expressed as absolute
  characters, as was the case with the pea plant
  traits.

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• Instead, polygenic traits are recognizable by
  their expression as a gradation of small
  differences (a continuous variation). The results
  form a bell shaped curve, with a mean value and
  extremes in either direction.
• Usually polygenic traits are distinguished by
• Traits are usually quantified by measurement
  rather than counting.
• Two or more gene pairs contribute to the
  phenotype.
• Phenotypic expression of polygenic traits varies
  over a wide range.

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Genetic Disorders
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Recessive Disorders

• These disorders only show up in individuals who
  are homozygous recessive for the trait.
• Cystic Fibrosis strikes 1 in 2,500 whites of
  European descent.
• Tay-Sachs disease is in high incidence in
  Ashkenazic Jews.
• Sickle Cell disease strikes 1 in 400 African
  Americans.

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Dominant Disorders

• The individual is heterozygous or homozygous
  dominant.
• Achondroplasia type of dwarfism.
• Huntingtons disease degenerative disease of
  the nervous system.