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Dihybrid Crosses - looking at 2 traits

Dihybrid Crosses - looking at 2 traits. Mendel’s dihybrid crosses: Mendel also performed crosses involving two pairs of traits , e.g., seed shape (smooth vs. wrinkled) and color (yellow vs. green).

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Dihybrid Crosses - looking at 2 traits

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  1. Dihybrid Crosses - looking at 2 traits

  2. Mendel’s dihybrid crosses: • Mendel also performed crosses involving two pairs of traits, e.g., seed shape (smooth vs. wrinkled) and color (yellow vs. green). • If alleles sort independently, four possible phenotypes (2n) appear in the F2 generation in a 9:3:3:1 ratio. “Mendel’s Principle of Independent Assortment”: Alleles for different traits assort independently of one another. Modern formulation of independent assortment: Genes on different chromosomes behave independently in gamete production.

  3. 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.

  4. Possible Gametes: To determine the number of different gametes a parent can have use the FOIL method. RREE RE RE RE RE All gametes are RE

  5. Possible Gametes: To determine the number of different gametes a parent can have use the FOIL method. RREe Re RE RE Re You have 2 possible gametes: Re and RE

  6. Possible Gametes: To determine the number of different gametes a parent can have use the FOIL method. rrEe re rE rE re You have 2 possible gametes: rE and re

  7. How many gametes? To determine the number of different gametes a parent can have use the FOIL method. Number and kind of gametes TTYY = one TY TtYY = two TY and Ty TtYy = four Ty, tY, ty, TY

  8. Possible Gametes (sperm/egg): If I cross: What are the different possible gametes for these parents?? RRTT x RrTt RT RT, Rt, rT, rt There are 4 total different gametes

  9. If I cross: What are the possible gametes? RRTT x RrTt RT Rt rT rt RRTT RRTt RrTT RrTt RT I have 4 possible offspring

  10. If I cross: What are the possible gametes and offspring? rrTt x RRTt RT Rt RrTT RrTt rT RrTt Rrtt rt I have 4 possible offspring

  11. If I cross: What are the possible gametes? RrTt x RrTt RT Rt rT rt RRTT RRTt RrTT RrTt RT RRTt RRtt RrTt Rrtt Rt I have 16 possible offspring 9:3:3:1 rT RrTT RrTt rrTT rrTt rt RrTt Rrtt rrTt rrtt

  12. Why Did Mendel Conclude That The Inheritance of one Trait is Independent of Another? Phenotype Ratio: 9 yellow/smooth 3 yellow/wrinkled 3 green/smooth 1 green/wrinkled Because it’s the only way to explain the pattern of inheritance.

  13. What Works for Peas Also Works for 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.

  14. Exceptions to the rule…

  15. Some alleles are related through - Incomplete Dominance Dominance relationships may differ, but the Law of Segregation remains the same.

  16. Multiple Alleles Many genes are present in 3 or more versions (alleles) – this is known as multiple alleles. The human ABO blood group is determined by three alleles (IA, IB, and i) of a single gene. The AB phenotype (genotype IA IB) is an example of codominance

  17. Codominance The human ABO blood group illustrates another genetic phenomenon – codominance. Codominance occurs when the phenotype associated with each allele is expressed in the heterozygote. Neither allele is dominant or recessive

  18. Incomplete or Codominant? +

  19. Pleiotropy – When One Allele Influences Many Traits • Examples: • Sickle Cell Anemia • Albinism

  20. Anemia, infections, weakness, impaired growth, liver and spleen failure, death. Traits (phenotypes) associated with the sickle cell allele. Pleiotropy in Action

  21. Polygenic Inheritance – when a Single Trait is influenced by many genes • Examples: • Height 2. Hair texture • 3. Skin color 4. eye color

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