KEY CONCEPT Mendel

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KEY CONCEPT Mendels research showed that traits
are inherited as discrete units.
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Heredity patterns can be calculated with
probability.

• Probability is the likelihood that something will
  happen.
• Probability predicts an average number of
  occurrences, not an exact number of occurrences.
  Predictions are just the likelihood that a
  particular outcome will occur.
• The larger the sample, the more accurate the
  predictions will be.
• Probability applies to random events such as
  meiosis and fertilization.

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• Probability Review
• Flipping a coin
• Chances of landing on heads 1/2 or 50.
• Flip a coin 3 times
• Probability of each toss ½, so you multiply
  this by the number of tosses ½ ½ ½ ?
• This shows an important fact about probabilities
  past outcomes do not affect future ones. Each
  time we flipped the coin there was a ½ chance of
  landing on heads.
• This is important to Mendel, because the
  principles of probability can be used to predict
  the outcome of genetic crosses.

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Mendel laid the groundwork for genetics.

• Traits are distinguishing characteristics that
  are inherited.
• Genetics is the study of biological inheritance
  patterns and variation.
• Gregor Mendel showed that traits are inherited as
  discrete units.
• Many in Mendels day thought traits were blended.

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Mendels data revealed patterns of inheritance.

• Mendel made three key decisions in his
  experiments.
• use of purebred plants
• control over breeding
• observation of seveneither-or traits

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• Mendel used pollen to fertilize selected pea
  plants.

• P generation (parent generation) crossed to
  produce F1 generation
• interrupted the self-pollination process by
  removing male flower parts

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• Mendel allowed the F1 generation to
  self-pollinate.
• F2 generation is the offspring from the F1
  generation.

• Results
• -- Among the F1 generation, all plants had purple
  flowers
• F1 plants are all heterozygous
• Among the F2 generation, some plants had purple
  flowers and some had white

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• Mendel observed patterns in the first and second
  generations of his crosses.

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• Mendel drew three important conclusions.

1. Traits are inherited as discrete units. 2.
Organisms inherit two copies of each gene, one
from each parent. 3. The two copies segregate
during gamete formation. Conclusions 2 and 3 make
up the Law of segregation.
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• A gene is a piece of DNA that directs a cell to
  make a certain protein.
• Each gene has a locus, a specific position on a
  pair ofhomologous chromosomes.

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• An allele is any alternative form of a gene
  occurring at a specific locus on a chromosome.

• Each parent donates one allele for every gene.
• Homozygous describes two alleles that are the
  same at a specific locus.
• Heterozygous describes two alleles that are
  different at a specific locus.

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• Alleles can be represented using letters.

• A dominant allele is expressed as a phenotype
  when at least one allele is dominant.
• A recessive allele is expressed as a phenotype
  only when two copies are present.
• Symbols
• Dominant alleles are represented by uppercase
  letters recessive alleles by lowercase letters.

round
rr
RR
wrinkled
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• All of an organisms genetic material is called
  the genome.

• A genotype refers to the makeup
• of a specific set of genes.
• Ex. (RR) homozygous dominant
• (Rr) heterozygous dominant
• (rr) homozygous recessive
• A phenotype is the physical
• expression of a trait.
• Ex. Round or wrinkled

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• The Punnett square is a grid system for
  predicting all possible genotypes resulting from
  a cross.
• The axes representthe possible gametesof each
  parent.
• The boxes show thepossible genotypesof the
  offspring.

• The Punnett square yields the ratio of possible
  genotypes and phenotypes.

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• Mendels Crosses
• P heterozygous x heterozygous
• F1 31 dominant recessive phenotypes/
• 121 homozygous dominant heterozygoushomozygou
  s recessive genotypes

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• A testcross is a cross between an organism with
  an unknown genotype and an organism with the
  recessive phenotype.

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A dihybrid cross involves two traits.

• Mendels dihybrid crosses with heterozygous
  plants yielded a 9331 phenotypic ratio.

• Mendels dihybrid crosses led to his second
  law,the law of independent assortment.
• The law of independent assortment states that
  allele pairs separate independently of each other
  during meiosis.

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Genetic Variability

• The major advantage of sexual reproduction is
  that it gives rise to a great deal of genetic
  variation within a species.
• Two Sources of Genetic Variability
• Independent assortment of chromosomes during
  meiosis.
• Random fertilization of gametes.