Question

Explain the dihybrid cross with the help of a checkerboard.

Hint:

The dihybrid cross demonstrates the independent assortment of alleles for different traits, as proposed by Mendel's second law.

Answer 1

Step 1: Definition of Dihybrid Cross.
A dihybrid cross is a genetic cross between two individuals that differ in two traits controlled by two different genes. The purpose is to observe the inheritance of two traits simultaneously.
Step 2: Example and Genetic Setup.
For example, consider a cross between two pea plants, one with round yellow seeds (RRYY) and the other with wrinkled green seeds (rryy). In this cross, the two traits are seed shape (round or wrinkled) and seed color (yellow or green), where: - R = Round (dominant), r = Wrinkled (recessive) - Y = Yellow (dominant), y = Green (recessive)
Step 3: The F1 Generation.
The F1 generation from the cross between RRYY and rryy will all be heterozygous (RrYy), showing the dominant phenotypes (Round Yellow seeds).
Step 4: The F2 Generation (using a checkerboard).
To predict the possible genotypes and phenotypes of the F2 generation, we perform a Punnett square. The F1 generation (RrYy) is crossed with another F1 generation (RrYy). The Punnett square for the dihybrid cross is a 16-cell grid, showing the possible combinations of alleles for the two traits: \[ \begin{array}{|c|c|c|c|} \hline \text{RY} & \text{Ry} & \text{rY} & \text{ry} \\ \hline \text{RY} & RRYy & RRyy & RrYy & Rryy \\ \hline \text{Ry} & RRYy & RRyy & RrYy & Rryy \\ \hline \text{rY} & RrYy & Rryy & rrYy & rryy \\ \hline \text{ry} & RrYy & Rryy & rrYy & rryy \\ \hline \end{array} \]
Step 5: Phenotypic Ratios.
From the Punnett square, we can deduce the following phenotypic ratio for the F2 generation: - 9 Round Yellow - 3 Round Green - 3 Wrinkled Yellow - 1 Wrinkled Green
Final Answer: \[ \boxed{\text{Dihybrid cross predicts a 9:3:3:1 phenotypic ratio in the F2 generation for two traits.}} \]