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

KEY CONCEPT The inheritance of traits follows the rules of probability. Punnett squares illustrate genetic crosses. The Punnett square is a grid system for predicting all possible genotypes resulting from a cross. The axes represent the possible gametes of each parent.

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

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

  2. 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.

  3. A monohybrid cross involves one trait. • Monohybrid crosses examine the inheritance of only one specific trait. • homozygous dominant-homozygous recessive: all heterozygous, all dominant

  4. heterozygous-heterozygous—1:2:1 homozygous dominant: heterozygous:homozygous recessive; 3:1 dominant:recessive

  5. heterozygous-homozygous recessive—1:1, 2 out of 4, heterozygous:homozygous recessive;1:1, 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.

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

  7. A dihybrid cross involves two traits. • Mendel’s dihybrid crosses with heterozygous plants yielded a 9:3:3:1 phenotypic ratio. • Mendel’s 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.

  8. Mendel’s 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 example:seed color and seed shape • Y-yellow y-green R-round r-wrinkled • Possible combinations monohybrid1:2:1 • YR-yellow round dihybrid 9:3:3:1 • Yr-yellow wrinkled • yR-green round • Yr-green wrinkled

  9. Probability = number of ways a specific event can occur number of total possible outcomes 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.

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