* Alleles and Phenotypes: Alleles are alternative forms of a gene. Phenotypes are the observable characteristics resulting from the genotype (combination of alleles).

* Simple Inheritance: In basic Mendelian inheritance, two alleles at a single locus determine the phenotype. You have one allele from each parent.

* Possible Combinations: With two alleles, you have three possible genotype combinations:

* Homozygous Dominant: Two copies of the dominant allele (e.g., AA)

* Heterozygous: One dominant and one recessive allele (e.g., Aa)

* Homozygous Recessive: Two copies of the recessive allele (e.g., aa)

To get four phenotypes, you would need:

* Multiple Alleles: More than two alleles for the same gene. For example, the ABO blood group system has three alleles (IA, IB, i).

* Epistasis: When the expression of one gene masks or modifies the expression of another gene.

* Polygenic Traits: Traits influenced by multiple genes.

Example:

The ABO blood group system is an example of how multiple alleles can produce four phenotypes:

* IAIA or IAi: Blood type A

* IBIB or IBi: Blood type B

* IAIB: Blood type AB

* ii: Blood type O

Let me know if you'd like more examples!