Editing Heredity (section)

==Types==
[[File:Autosomal dominant.png|thumb|An example pedigree chart of an autosomal dominant disorder]]
[[File:Autosomal recessive.png|thumb|An example pedigree chart of an autosomal recessive disorder]]
[[File:Sex linked inheritance.png|thumb|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 [[Locus (genetics)|loci]]'''
:*[[Mendelian inheritance|Monogenetic]] (also called "simple") – one [[Locus (genetics)|locus]]
:*[[Oligogenic]] – few loci
:*[[Polygene]]tic – many loci

:'''2. Involved [[chromosome]]s'''

:*[[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|X-chromosome]] (the more common case)
:**Y-chromosomal – loci are situated on the [[Y chromosome|Y-chromosome]]
:*[[Mitochondrial]] – loci are situated on the [[mitochondrial DNA]]

:'''3. Correlation [[genotype]]–[[phenotype]]'''
:*[[Dominance relationship#Dominant allele|Dominant]]
:*Intermediate (also called "[[Codominance|codominant]]")
:*[[Recessive]]
:*[[Overdominance|Overdominant]]
:*[[Underdominance|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 (genetics)|Expressivity]]
:**Invariable
:**Variable
:*[[Heritability]] (in polygenetic and sometimes also in oligogenetic modes of inheritance)
:*Maternal or paternal [[imprinting (genetics)|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 chromosome|Y-chromosomal]] loci)

:'''6. Locus–locus interactions'''
:*[[Epistasis]] with other loci (e.g., [[overdominance]])
:*Gene coupling with other loci (also see [[Chromosomal crossover|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.
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File:Autosomal recessive inheritance for affected enzyme.png|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.
Autosomal dominant inheritance for structural protein.png|On the other hand, hereditary defects in structural proteins (such as [[osteogenesis imperfecta]], [[Marfan's syndrome]] and many [[Ehlers–Danlos syndrome]]s) are generally autosomal dominant, because it is enough that some components are defective to make the whole structure dysfunctional. This is a [[Dominant negative mutation|dominant-negative]] process, wherein a mutated gene product adversely affects the non-mutated gene product within the same cell.
</gallery>