D3.2.17 Punnett grids for predicting genotypic and phenotypic ratios in dihybrid crosses (HL) Notes

Dihybrid Crosses Predict Independent Inheritance of Two Traits

1. Gregor Mendel's experiments with pea plants demonstrated how traits are inherited independently when controlled by unlinked genes.
2. But what about the inheritance of two traits simultaneously?
3. These experiments reveal how the Principle of Independent Assortment results in unique phenotypic ratios in the offspring.

Constructing a Punnett Grid for Dihybrid Crosses

1. Determine Parental Genotypes:

1. Identify the genotypes of the parent plants. In this example, both parents are heterozygous (RrYyRrYyRrYy).

2. Identify Gametes

1. Each parent produces four types of gametes (RY,Ry,rY,ryRY, Ry, rY, ryRY,Ry,rY,ry), following the FOIL method:
   1. First (RYRYRY)
   2. Outer (RyRyRy)
   3. Inner (rYrYrY)
   4. Last (ryryry)

3. Draw the Grid

1. Construct a 4×4 grid, placing one parent's gametes across the top and the other along the side.
2. Fill in each cell with the combination of alleles from the corresponding row and column.

Step 4: Determine Phenotypes

1. The genotypes in the grid correspond to four phenotypes:
   1. Round, yellow (\( R\_Y\_ \)) — 9 combinations
   2. Round, green (\( R\_yy \)) — 3 combinations
   3. Wrinkled, yellow (\( rrY\_ \)) — 3 combinations
   4. Wrinkled, green (\( rryy \)) — 1 combination

Step 5: Calculate the Phenotypic Ratio

1. The phenotypic ratio for the F2 generation is 9:3:3:1.

Why 9:3:3:1?

1. The 9:3:3:1 ratio arises from the independent assortment of alleles:
   1. Each trait follows a 3:1 ratio (dominant:recessive).
   2. When combined, the probabilities multiply: $3:1 \times 3:1 = 9:3:3:1$.

Test Crosses and the 1:1:1:1 Ratio

1. A test cross involves crossing an organism with a dominant phenotype (but unknown genotype) with a homozygous recessive organism.
2. This helps determine the genotype of the dominant organism.

Exceptions to Independent Assortment

1. Mendel's Second Law applies only to genes on different chromosomes or those far apart on the same chromosome.
2. Genes located close together on the same chromosome are linked and do not assort independently.

Practical Applications of Dihybrid Crosses

1. Agriculture: Breeding crops with multiple desirable traits, such as disease resistance and high yield.
2. Medicine: Understanding inheritance patterns of genetic disorders involving multiple traits.
3. Conservation Biology: Studying genetic diversity in endangered species.

Reflection

1. Dihybrid crosses reveal the complexity and elegance of inheritance.
2. While Mendel's laws provide a foundation, modern genetics shows us that exceptions, such as gene linkage, add depth to our understanding.