The role of genes and chromosomes in heredity. Number and types of chromosomes humans have 46 chromosomes (23 pairs) Cell division meiosis = cell division of sex cells mitosis = cell division of somatic cell division. Introduction genetic disorders can be found on either type of chromosome (see information on alleles, homozygous, heterozygous, multifacotral transmission, dominant, and recessive. Tuberous sclerosis cause: nonworking gene on the 9th or 16th chromosome results: tumour growth anywhere in the body, shaped like tubes & calcify can lead to seizures and cids common: skin lesions. Edwards syndrome (trisomy 18) causes: additional 18th chromosome results: physical appearance (microcephaly, low-set malformed ears, cleft lip), cardiovascular and neurological problems treatment: most die within the rst year of life or miscarried. Palau syndrome (trisomy 13) causes: additional 13th chromosome results: physical malformation (sloping forehead, microcephaly, extra digits, small/absent eyes), incomplete forebrain, auditory, and optic nerves treatment: most will miscarry or die within the rst year of life.