Phenotypic concordance in monozygotic(MZ; identical) and dizygotic(DZ; fraternal)twins MZ twins arise when a developing embryo(derived from one zygote fertilization of one egg by one sperm) splits into two parts each giving rise to a baby DZ twins arise from two separate but nearly simultaneous fertilization events Relationship degree Parent-child 1st 1/2 Siblings 1st 1/2 Aunt/niece 2nd 1/4 First Cousins 3rd 1/8 亿 Z twins 0 DZ twins 1st 1/2Phenotypic concordance in monozygotic (MZ; identical) and dizygotic (DZ; fraternal) twins MZ twins arise when a developing embryo (derived from one zygote; fertilization of one egg by one sperm) splits into two parts, each giving rise to a baby DZ twins arise from two separate, but nearly simultaneous fertilization events. Parent-child 1st 1/2 Siblings 1st 1/2 Aunt/niece 2nd 1/4 First Cousins 3rd 1/8 DZ twins MZ twins Relationship degree r 0 1 1st 1/2