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Understanding Monohybrid and Dihybrid Crosses in Genetics

Explore the concepts of monohybrid and dihybrid crosses through practical examples involving pea plants. This guide covers the genetic probabilities related to traits such as widow's peak and seed shape/color. Learn how to establish parental genotypes, determine gametes, and construct Punnett squares. Gain insights on calculating genotypic and phenotypic ratios for various offspring. This resource is ideal for students of genetics looking to solidify their understanding of inheritance patterns.

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Understanding Monohybrid and Dihybrid Crosses in Genetics

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  1. DO NOW Get out your monohybrid crosses HW A woman with a widow’s peak (heterozygous) has children with a man who does not have a widow’s peak. What is the probability they will have a child without a widow’s peak?

  2. Dihybrid Crosses • Crosses that involve two traits • Similar to monohybrid crosses, but HARDER! 

  3. A pea plant that is heterozygous for round, yellow seeds is self fertilized . What are the genotypic and phenotypic ratios of the plant’s offspring?

  4. A pea plant that is heterozygous for round, yellow seeds is self fertilized. What are the genotypic and phenotypic ratios of the plant’s offspring? • Step 1: Write the symbols R = Round r = smooth Y = Yellow y = green

  5. A pea plant that is heterozygous for round, yellow seeds is self fertilized. What are the genotypic and phenotypic ratios of the plant’s offspring? • Step 2: Determine the parental genotypes and write the parental Cross.

  6. A pea plant that is heterozygous for round, yellow seeds is self fertilized. What are the genotypic and phenotypic ratios of the plant’s offspring? • Step 3: Determine the gametes. (kind of like the FOIL method from math class) • Combine the R’s and Y’s for each parent to determine the alleles for each egg or sperm.

  7. A pea plant that is heterozygous for round, yellow seeds is self fertilized. What are the genotypic and phenotypic ratios of the plant’s offspring? • Step 4: Set up a large 4x4 Punnet square, place one gamete set from parent 1 on the top and one gamete set from parent 2 on the side. Gametes – Parent #1 ry RY Ry rY RY Ry Gametes – Parent #2 rY ry

  8. A pea plant that is heterozygous for round, yellow seeds is self fertilized. What are the genotypic and phenotypic ratios of the plant’s offspring? • Step 5: Fill in the Punnett Square. Gametes – Parent #1 ry RY Ry rY RY RRYY RRYy RrYY RrYy RrYy Ry RRYy Rryy RRyy Gametes – Parent #2 rY RrYY RrYy rrYy rrYY ry rryy RrYy Rryy rrYy

  9. A pea plant that is heterozygous for round, yellow seeds is self fertilized. What are the genotypic and phenotypic ratios of the plant’s offspring? • Step 6: Determine the number of offspring for each phenotype (phenotypic ratio). 3 9 1 3 ___ round & yellow: ___round & green:___ wrinkled & yellow: ___ wrinkled and green Gametes – Parent #1 ry RY Ry rY RY RRYY RRYy RrYY RrYy RrYy Ry RRYy Rryy RRyy Gametes – Parent #2 rY RrYY RrYy rrYy rrYY ry rryy RrYy Rryy rrYy

  10. Dihybrid Cross

  11. Practice! • Cross a plant that is heterozygous round and heterozygous yellow with a plant that is wrinkled and green.

  12. Cross a plant that is heterozygous round and heterozygous yellow with a plant that is wrinkled and green. Parental Cross: • SYMBOLS • R = Round • r = smooth • Y = Yellow • y = green Phenotypic Ratio: 4 round & yellow 4 round & green 4 wrinkled & yellow 4 wrinkled & green 1:1:1:1

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