achondroplasia treatmenthomozygous and heterozygous definitionhomozygous genotype definitionachondroplasia characteristicshomozygous alleles definitionachondroplasia hydrocephalushomozygous definition biologyhomozygous purebred definitionhomozygous dominant definitionheterozygous achondroplasiaachondroplasia descriptionachondroplasia complications