In a dihybrid test cross,if the parental types exceed the recombination types among the resultant progeny, it is due to

Options

A. Linkage
B. Complete linkage
C. Independent assortment
D. Crossing -over

Answer

**Step-by-Step Solution:**

1. **Understanding Dihybrid Test Cross**: A dihybrid test cross involves two traits being studied simultaneously. In this case, we will consider two traits in garden peas: seed shape (round vs. wrinkled) and seed color (yellow vs. green).

2. **Identifying Parental and Recombination Types**: In a typical dihybrid cross, we expect to see parental types (the combinations that resemble the parents) and recombinant types (new combinations that do not resemble the parents). For example, if we have a round yellow parent (RRYY) and a wrinkled green parent (rryy), the parental types would be round yellow (R_Y_) and wrinkled green (r_y_y).

3. **Performing the Test Cross**: In a test cross, we cross the F1 generation (which is heterozygous for both traits) with a homozygous recessive individual (rryy). This helps us determine the genotype of the F1 hybrids based on the phenotypes of the offspring.

4. **Analyzing Offspring Ratios**: In a typical dihybrid cross, we would expect a 9:3:3:1 ratio of phenotypes in the F2 generation if the genes assort independently. However, when parental types exceed recombinant types, it indicates a deviation from this expected ratio.

5. **Understanding Linkage**: If the parental types exceed the recombinant types, it suggests that the two genes are linked. This means they are located close to each other on the same chromosome and tend to be inherited together, reducing the frequency of recombination.

6. **Conclusion**: Therefore, the reason parental types exceed recombinant types in a dihybrid test cross is due to linkage between the two genes, which prevents them from assorting independently.

**Final Answer**: In a dihybrid test cross, if parental types exceed recombinant types among the resultant progeny, it is due to linkage between the two genes.

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