The Zariski closure of the Kimura 2-parameter (K2P) model is a well-known object in the study of algebraic statistics, particularly in phylogenetics. The K2P model is used to describe the substitution rates between different nucleotides in a DNA sequence, where the transition rates between purines (A↔G) and between pyrimidines (C↔T) are equal, but different from the transversion rates (A↔C, A↔T, G↔C, G↔T).

Generators of the Ideal Defining the Zariski Closure of the K2P Model

The ideal defining the Zariski closure of the K2P model can be described using a set of polynomials. These generators correspond to the phylogenetic invariants of the model. Here are the steps to find these generators:

1. Setup the Model:

   • Consider a DNA sequence of length n, with nucleotides A, G, C, T represented by {A, G, C, T}.
   • The K2P model has two parameters, say α for transitions (A↔G, C↔T) and β for transversions (A↔C, A↔T, G↔C, G↔T).

2. Construct the Model Matrix:

   • The transition matrix for the K2P model is a 4x4 matrix describing the rates of substitution from one nucleotide to another.

3. Compute the Ideal:

   • The ideal $I$ corresponding to the Zariski closure is computed by finding the relations among the entries of the joint distribution matrix, which can be parameterized by the K2P model.

4. Generators of the Ideal:

   • For a sequence of length 2 (i.e., considering pairs of nucleotides), the ideal is generated by a set of polynomial relations known as phylogenetic invariants.
   • These invariants for the K2P model can be found by determining the vanishing ideal of the projective variety associated with the K2P model.
   • The generators of this ideal typically involve quadratic relations that describe the constraints on the probabilities under the K2P model.

   A key set of generators for the K2P model involves polynomials such as: $p_{AC}p_{GT} - p_{AT}p_{GC}, \quad p_{AG}p_{CT} - p_{AC}p_{GT}, \quad \text{and others derived similarly}.$ These are just example forms, the exact set of generators depends on the length of the sequence and the number of states.

5. Conclusion:

   • The ideal generators are those polynomials that describe the affine variety defining the Zariski closure of the model. They are derived using the relations among the entries of the transition matrix that are consistent with the model assumptions.

Finding these generators requires algebraic computations, often done using software like Macaulay2, Singular, or other algebraic geometry tools.

Would you like to see the computation of these generators for a specific case (like a sequence of length 2 or 3)?

Here are some related questions:

1. What is the importance of the Zariski closure in algebraic geometry?
2. How are phylogenetic invariants used in statistical models?
3. Can you explain the difference between transitions and transversions in nucleotide sequences?
4. How does the K2P model compare with the Jukes-Cantor model?
5. What software tools are commonly used to compute ideals and their generators?
6. Can the generators of the ideal be interpreted biologically?
7. How do these invariants help in the reconstruction of phylogenetic trees?
8. What are the applications of algebraic statistics in phylogenetics?

Tip: Understanding the structure of phylogenetic models requires both a biological background and familiarity with algebraic geometry, particularly the concept of ideals and varieties.