This was a project I worked on while completing Codecademy's Full Stack Engineer career path. Context: You're part of a research team researching the P. aequor organism but it is difficult to study the organism in real life so you create a simulation of it.

pAequor.js

// Returns a random DNA base
const returnRandBase = () => {
  const dnaBases = ['A', 'T', 'C', 'G']
  return dnaBases[Math.floor(Math.random() * 4)] 
}

// Returns a random single stand of DNA containing 15 bases
const mockUpStrand = () => {
  const newStrand = []
  for (let i = 0; i < 15; i++) {
    newStrand.push(returnRandBase())
  }
  return newStrand
}

let dnaStrand = mockUpStrand();

//Creates an instance of P. aequor. Contains many functions.
const pAequorFactory = (specimenNum,dna) => {
  return {
    specimenNum: specimenNum,
    dna: dna,
// Mutates a random base in the 15 base strand of DNA. Does not return the same base it mutated.
    mutate(dna){
    var randomIndex = Math.floor(Math.random() * 15);
    let mutatedBase = this.dna[randomIndex];
      switch (mutatedBase) {
        case 'A':
        this.dna.splice(randomIndex,1,['T', 'C', 'G'][Math.floor(Math.random() * 3)]);
        break;
        case 'T':
        this.dna.splice(randomIndex,1,['A', 'C', 'G'][Math.floor(Math.random() * 3)]);
        break;
        case 'C':
        this.dna.splice(randomIndex,1,['T', 'A', 'G'][Math.floor(Math.random() * 3)]);
        break;
        case 'G':
        this.dna.splice(randomIndex,1,['T', 'C', 'A'][Math.floor(Math.random() * 3)]);
        break;
      };
      return this.dna
    },

// Compares DNA with another specimen's DNA and logs a message stating the percentage of DNA they share.
    compareDNA(pAequor){
      let dnaCounter = 0;
      for (let i = 0; i< this.dna.length; i++){
        if (pAequor.dna[i] === this.dna[i]){
          dnaCounter++;
        }
      }
      let percentage = (dnaCounter/15)* 100;
      console.log (`Specimen ${this.specimenNum} and specimen ${pAequor.specimenNum} have ${percentage}% DNA in common.`);

    },
//Returns true if the object’s .dna array contains at least 60% 'C' or 'G' bases. Otherwise willLikelySurvive() returns false.
    willLikelySurvive(){
     let gCounter = 0;
     let cCounter = 0;
      for (let i= 0; i< this.dna.length; i++){
        if (this.dna[i] === 'C'){
          cCounter++;
        }
        if (this.dna[i] === 'G'){
          gCounter++;
        }
      }
      if (gCounter >= 9 || cCounter >= 9){
        return true;
      }
      else {
        return false;
      }
    },
  };
};
// Holds all 30 surviving instances of P. aequor
let survivalArray = [];

// Creates 30 instances of P.aequor likely to survive (i.e. contain at least 60% of 'G' or 'C' in their DNA strands)
const createSurvivors = () => {
  let id = 1;
  do {
    let newStrand = pAequorFactory(id,mockUpStrand());
    id++;
    if (newStrand.willLikelySurvive() === true){
      survivalArray.push(newStrand);
    }
  } while (survivalArray.length < 30);
  return survivalArray;
};

console.log(createSurvivors())

I really liked your work, but you can improve this few lines:

if (this.dna[i] === 'C'){
cCounter++;
}
if (this.dna[i] === 'G'){
gCounter++;
}

by joining them and make the code cleaner and easier to read, like this:

if (this.dna[i] === 'C' || this.dna[i] === 'G' ){
gCounter++;
}