Editing Heredity (section)

===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.
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