The number and arrangement of loci on homologous chromosomes are usually identical. The specific nucleotide sequence of the genes that occupy the two homologous loci along the two chromosomes of a pair are called alleles. If a trait or disorder occurs when only one allele is abnormal, the disorder is said to be dominant. A disorder is said to be recessive if it occurs only when both alleles at the loci on both chromosomes are abnormal. More recently, mitochondrial and nontraditional patterns of inheritance have been recognized.
Humans vary phenotypically and genotypically.
Heterogeneity results from different alleles and mutations of the multiple genes, which are part of almost all biochemical pathways. Mutations in different parts of a gene may cause different disorders. For instance, mutations in different locations along the gene for one type of collagen can produce tall stature, arthritis, and deafness or lethal dwarfism. Concern about the teratogenic effects of drugs taken during pregnancy is growing. Thus, phenotypically similar conditions may be due to mutations in different genes, nongenetic factors, or both.