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THE DIHYBRID CROSS

THE DIHYBRID CROSS. Studying the inheritance of two characters simultaneously SBI3U. Independent Assortment. Are Different Characters Like Color and Shape Inherited Together or Inherited Independently?. Mendel performed dihybrid crosses to find out.

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THE DIHYBRID CROSS

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  1. THE DIHYBRID CROSS Studying the inheritance of two characters simultaneously SBI3U

  2. Independent Assortment Are Different Characters Like Color and Shape Inherited Together or Inherited Independently? Mendel performed dihybrid crosses to find out. Mendel’s conclusion: Different characters are inherited independently.

  3. Why Did Mendel Conclude That The Inheritance of one Trait is Independent of Another? The alternative and incorrect hypothesis: dependent inheritance. Because it’s the only way to explain the pattern of inheritance.

  4. The Punnett Square Note that we’re simultaneously applying the Principles of Segregations and Independent Assortment.

  5. The alignment of one pair of homologs is independent of any other. The Independent Alignment of Different Pairs of Homologous Chromosomes At Meiosis Accounts for the Principle of Independent Assortment Principle of Independent Assortment: The assortment of one pair of genes into gametes is independent of the assortment of another pair of genes.

  6. Peas & Humans Consider a cross between parents heterozygous for both deafness and albinism. This is the same 9:3:3:1 ratio seen for Mendel’s cross involving pea color and shape.

  7. Mendel’s peas

  8. Combinations

  9. Is the inheritance of one character affected by the inheritance of another?

  10. Is the inheritance of one character affected by the inheritance of another?

  11. A dihybrid cross can be treated as two separate monohybrid crosses The expected probability of each type of seed can be calculated: • Probability of an F2 seed being round = 75% or ¾ • Probability of an F2 seed being wrinkled = • Probability of an F2 seed being yellow = • Probability of an F2 seed being green =

  12. A dihybrid cross can be treated as two separate monohybrid crosses The expected probability of each type of seed can be calculated: • Probability of an F2 seed being round = 75% or ¾ • Probability of an F2 seed being wrinkled = 25% or ¼ • Probability of an F2 seed being yellow = 75% or ¾ • Probability of an F2 seed being green = 25% or ¼

  13. Therefore • Probability of an F2 seed being round and yellow = ¾ x ¾ = 9/16 = 56.25% • Probability of an F2 seed being round and green = • Probability of an F2 seed being wrinkled and yellow = • Probability of an F2 seed being wrinkled and green =

  14. Therefore • Probability of an F2 seed being round and yellow = ¾ x ¾ = 9/16 = 56.25% • Probability of an F2 seed being round and green = ¾ x ¼ = 3/16 = 18.75% • Probability of an F2 seed being wrinkled and yellow = ¼ x ¾ = 3/16 = 18.75% • Probability of an F2 seed being wrinkled and green = ¼ x ¼ = 1/16= 6.25%

  15. Predicting how many seeds we could expect to get in a sample

  16. Predicting how many seeds we could expect to get in a sample

  17. THE LAW OF INDEPENDENT ASSORTMENT • It appears that the inheritance of seed shape has no influence over the inheritance of seed colour • The two characters are inherited INDEPENDENTLY • The pairs of alleles that control these two characters assort themselves independently

  18. Mendel & Meiosis • The pairs of chromosomes could orientate in different ways at Anaphase 1

  19. Dihybrid cross genetic diagram

  20. Dihybrid cross genetic diagram

  21. Dihybrid cross genetic diagram

  22. Dihybrid cross proportions

  23. Dihybrid cross proportions

  24. Dihybrid test cross • In monohybrid crosses, to know if a dominant trait is homozygous (RR) or heterozygous (Rr) it is necessary to carry out a test cross • This is done with a homozygous recessive (rr) individual • The same is true for a dihybrid cross where the test cross is made with an individual which is homozygous recessive for both characters (rryy)

  25. Dihybrid test cross

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