KEY CONCEPT The inheritance of traits follows the rules of probability.

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KEY CONCEPT The inheritance of traits follows
the rules of probability.
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Punnett squares illustrate genetic crosses.

• 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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A monohybrid cross involves one trait.

• Monohybrid crosses examine the inheritance of
  only one specific trait.
• homozygous dominant-homozygous recessive all
  heterozygous, all dominant

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• heterozygous-heterozygous121 homozygous
  dominant heterozygoushomozygous recessive 31
  dominantrecessive

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• heterozygous-homozygous recessive11, 2 out of
  4, heterozygoushomozygous recessive11, 2 out
  of 4, 50 dominant 50
  recessive

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

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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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Mendels Second Law

• Law of Independent Assortment states that the
  presence of one trait does not affect the
  presence of another trait. The alleles separate
  and are inherited independently of each other.
• For exampleseed color and seed shape
• Y-yellow y-green R-round r-wrinkled
• Possible combinations monohybrid121
• YR-yellow round dihybrid 9331
• Yr-yellow wrinkled
• yR-green round
• Yr-green wrinkled

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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.

• Probability applies to random events such as
  meiosis and fertilization.