What causes individuals to have a trait such as curly hair?


Their DNA codes (genotypes) are ultimately translated in the ribosomes into proteins, which are expressed as curly hair (phenotype).
Their DNA codes (phenotypes) are ultimately translated in the nucleus into proteins, which are modified by the environment to produce curly hair (phenotype).
Their RNA codes (phenotypes) are transcribed into DNA strands, which are translated in the ribosomes into proteins that code for curly hair (phenotype).
Their RNA codes (genotypes) are transcribed in the nucleus to amino acids, which are translated in the ribosomes into proteins that code for curly hair (phenotype).