example pedigree chart of an autosomal recessive disorder. an example pedigree chart of a sex-linked disorder (the gene is on the x chromosome). the description of a mode of biological inheritance consists of three main categories: 1. number of involved loci monogenetic (also called "simple") – one locus oligogenic – few loci polygenetic – many loci 2. involved chromosomes autosomal – loci are not situated on a sex chromosome gonosomal – loci are situated on a sex chromosome x-chromosomal – loci are situated on the x-chromosome (the more common case) y-chromosomal – loci are situated on the y-chromosome mitochondrial – loci are situated on the mitochondrial dna 3. correlation genotype–phenotype dominant intermediate (also called "codominant") recessive overdominant underdominant these three categories are part of every exact description of a mode of inheritance in the above order. in addition, more specifications may be added as follows: 4. coincidental and environmental interactions penetrance complete incomplete (percentual number) expressivity invariable variable heritability (in polygenetic and sometimes also in oligogenetic modes of inheritance) maternal or paternal imprinting phenomena (also see epigenetics) 5. sex-linked interactions sex-linked inheritance (gonosomal loci) sex-limited phenotype expression (e.g., cryptorchism) inheritance through the maternal line (in case of mitochondrial dna loci) inheritance through the paternal line (in case of y-chromosomal loci) 6. locus–locus interactions epistasis with other loci (e.g., overdominance) gene coupling with other loci (also see crossing over) homozygotous lethal factors semi-lethal factors determination and description of a mode of inheritance is also achieved primarily through statistical analysis of pedigree data. in case the involved loci are known, methods of molecular genetics can also be employed. dominant and recessive alleles an allele is said to be dominant if it is always expressed in the appearance of an organism (phenotype) provided that at least one copy of it is present. for example, in peas the allele for green pods, g, is dominant to that for yellow pods, g. thus pea plants with the pair of alleles either gg (homozygote) or gg (heterozygote) will have green pods. the allele for yellow pods is recessive. the effects of this allele are only seen when it is present in both chromosomes, gg (homozygote). this derives from zygosity, the degree to which both copies of a chromosome or gene have the same genetic sequence, in other words, the degree of similarity of the alleles in an organism. hereditary defects in enzymes are generally inherited in an autosomal fashion because there are more non-x chromosomes than x-chromosomes, and a recessive fashion because the enzymes from the unaffected genes are generally sufficient to prevent symptoms in carriers. on the other hand, hereditary defects in structural proteins (such as osteogenesis imperfecta, marfan's syndrome and many ehlers–danlos syndromes) are generally autosomal dominant, because it is enough that some components are defective to make the whole structure dysfunctional. this is a dominant-negative process, wherein a mutated gene product adversely affects the non-mutated gene product within the same cell.