Pick any free letter for each trait. For example, "A" for the dominant allele and "a" for the recessive allele for the number of arms. Assuming complete dominance, the genotype for the dominant trait will always have at least one "capital" allele, so a creature with 2 arms would have genotype either AA (homzygous dominant) or Aa (heterozygous), while a creature with the recessive trait of having 4 arms will always be homzygous recessive with genotype aa.
