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Mendel did two-factor cross to answer this. We’ve called this a dihybrid cross .

Problem : Does segregation of one pair of alleles affect the segregation of another pair of alleles? Ex: does seed shape affect seed color ?. Mendel did two-factor cross to answer this. We’ve called this a dihybrid cross.

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Mendel did two-factor cross to answer this. We’ve called this a dihybrid cross .

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  1. Problem: Does segregation of one pair of alleles affect the segregation of another pair of alleles? Ex: does seed shape affect seed color? • Mendel did two-factor cross to answer this. We’ve called this a dihybrid cross.

  2. Step 1: Cross true breeding round yellow seed plants with wrinkled green seeds to produce hybrid F1Step 2: Cross these F1 seeds and observe F2 Results: 9:3:3:1 Ratio of phenotypes

  3. Conclusion: • Principle of Independent Assortment- genes for different traits can segregate independently during gamete formation. (Ex: genes controlling seed color are separate from seed texture.)

  4. Is leg length independent of coloration?

  5. Exceptions to Mendel’s RulesIncomplete Dominance • Neither allele is completely dominant • The heterozygous phenotype is a blend of the homozygous ones. • Ex: pink four o’ clock flowers

  6. Codominance • Both alleles contribute to phenotype-- both are seen and neither “covers” the other

  7. Multiple alleles • Individuals still only carry 2 alleles, but more than 2 exist in a population.

  8. Polygenic Traits • Traits controlled by interaction of several genes. Ex: human skin color controlled by more than 4 different genes • Polygenic traits tend to result in a distribution that resembles a bell-shaped curve, with few at the extremes and most in the middle.

  9. There may 4 or 6 or more alleles involved in the phenotype. At the left extreme, individuals are dominant for all alleles, and at the other extreme, individuals are completely recessive for all alleles. Individuals in the middle have various combinations of recessive and dominant alleles.

  10. Environmental Effects • Characteristics are determined by both genes and the environment. • Ex: temperature, nutrients, etc. can influence plant height or color

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