Simple Medelian Genetics

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Simple Medelian Genetics

• Competency 12.00

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Genetic Terminology
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DNA

• Deoxyribonucleic Acid
• The major nucleic acid in organisms
• Carries genetic information and is responsible
  for the transmission of traits.

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Gene

• A segment of DNA that codes for a specific trait
  in an organism.

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Allele

• An alternative form of a gene/trait.
• Example Eye color alleles are blue, green,
  hazel, brown
• Can be homozygous or heterozygous

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Homozygous Allele

• Organism with identical alleles for a given trait
• Can be dominant or recessive
• Example TT or tt

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Phenotype

• The physical appearance of a trait in an organism
• Determined but not always indicative of the
  genetic makeup of the organism
• Example tall or short

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Genotype

• The genetic composition of an organism for a
  given trait
• Often cannot be determined by looking at an
  organism
• Example Tt or TT, both are tall

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Recessive Gene/Allele

• Variation of a trait that can only be expressed
  in the absence of a dominant allele
• Heterozygous individuals are carriers for
  recessive alleles.

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Dominant Gene/Allele

• Variation of a trait that is expressed over other
  variations of the same trait
• Most common forms in natural populations
• Some traits can be co-dominant or exhibit
  incomplete dominance

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Chromosome

• Long condensed strand of DNA forming in the
  nucleus of a cell prior to cell division
• Form cells that when split, create an exact copy
  of DNA in the daughter cell

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Chromatid Pairs

• X-shaped structures that serve as the mechanism
  for the transmission of genetic material during
  cell division.
• They are pulled apart in the process of mitosis
  and meiosis.

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Heterozygous Allele

• Organism with different alleles for a given trait
• Example Tt

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Heredity
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Gregor Mendel

• 1863
• Austrian monk who conducted the first genetics
  experiments using pea plants in the mid 1800s.
• Often considered the founder of genetics and
  heredity.

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What is Heredity?

• Heredity is best described as the manner in which
  inheritable characteristics (traits) are passed
  from parents to offspring.

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Heredity

• A direct outcome of the RANDOM genetic
  recombination resulting during reproduction
• Ensures genetic diversity

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Heredity

• Determines the genetic potential of an animal,
  but . . .
• Heredity and environmental influences determine
  the overall quality of the animal.
• Nature versus Nurture

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Types of Heredity

• Simple Heredity
• Complex Heredity
• Polygenic Inheritance
• Incomplete Dominance
• Codominance

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Simple Heredity

• One gene controls one traitalleles are either
  dominant or recessive.
• Example height and color in pea plants

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Complex Heredity

• Polygenic Inheritance
• One trait is controlled by several genes and
  possibly environmental factors
• Genes may even be located on different chromosomes

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Complex Heredity

• Polygenic Inheritance
• This is a slow process requiring many generations
  to achieve desired results.
• Example Height in humans

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Complex Heredity

• Incomplete Dominance
• Multiple alleles for a given trait are not
  expressed over one another, but in combination.
• Example RR (Red Flower) x WW (White Flower)
  RW (Red and White Striped Flower)

X

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Codominance

• Similar to incomplete dominance, except
  characteristics of alleles blend instead of
  remaining distinct
• Example RR (Red Flower) x WW (White Flower)
  RW (Pink Flower)

X

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Heredity in Agriscience

• Heredity is a huge factor in successful
  agricultural selective breeding programs.
• Heredity is manipulated to create high quality
  HYBRID offspring.

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Heredity in Agriscience

• Plants and animals are inbred through several
  generations to isolate a specific trait or
  traits.
• No more than 7 generations are inbred to prevent
  genetic disorders.

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Heredity in Agriscience

• The final generation of two different lines
  inbred for different traits are crossed producing
  offspring with the beneficial traits of both
  lines.
• Resulting offspring possesses hybrid vigor

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Hybrid Vigor
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Heredity in Agriscience

• Hybrid vigor usually lasts only one generation,
  as hybrid organisms rarely express traits true to
  type in offspring.
• Alternative forms of the gene resurface in the
  new cross.

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Heredity in Agriscience

• Punnett Squares, Pedigree Charts, Genetic Mapping
  and DNA analysis can be used to predict heredity.

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Punnett Squares
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What is a Punnett Square?

• A method utilizing the known genotypes of parent
  offsprings to predict the expression of a given
  trait or traits in offspring.
• Must know the genotype of parents and the
  inheritance pattern of the trait.

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Using Punnett Squares

• When using Punnett Squares the Dominant Trait is
  always represented by an uppercase letter
• Recessive is lowercase
• Example TTall, tshort

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Using Punnett Squares

• A box should be drawn with one space for each
  allele expressed by both parents.
• In simple heredity boxes are 2x2.

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Using Punnett Squares

• The alleles for one parent should be placed above
  each column at the top, with the alleles for the
  other placed beside each row on one side.

T
T
T
t
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Using Punnett Squares

• The alleles of each parent should be distributed
  across and down the box.

T
T
TT
TT
T
t
Tt
Tt
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Using Punnett Squares

• TT-Homozyous Dominant
• Tt-Heterozygous
• Genotype Ratio
• 310
• Phenotype Ratio
• 40

T
T
TT
TT
T
t
Tt
Tt
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Using Punnett Squares

• When crossing using complex heredity boxes are
  4x4 and two different traits are being crossed.

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Using Punnett Squares
TG
Tg
tG
tg
Genotype Ratio-044440 Phenotype Ratio-
0880
Tg
TTGg
TTgg
TtGg
Ttgg
TTGg
TTgg
TtGg
Ttgg
Tg
Tg
TTGg
TTgg
TtGg
Ttgg
Tg
TTGg
TTgg
TtGg
Ttgg
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Assignment

• Go to the career center and look up the following
  website. Complete the assignments listed on the
  corresponding worksheet.
• http//www2.edc.org/weblabs/WebLabDirectory1.html