Down syndrome -karyotyping involves taking photographs of chromosomes during metaphase in mitosis; normal and abnormal chromosomes can then be identified by appearance and numbers. down syndrome in humans can be readily diagnosed, since it is a result of having a third chromosome number 21 accompanying the normal pair of chromosomes. this is termed a trisomy, specifically trisomy 21, referring to the three chromosomes. other examples of trisomy are patau syndrome (trisomy 13) and edwards syndrome (trisomy 18). while a genetic factor is hypothesized as a factor in alcoholism, it is not proven, and in any case it does not change the karyotype. hemophilia, huntingtons chorea, and phenylketonuria are known inherited disorders, but again, they do not affect the karyotype.