Heridity

Heredity is the passing of traits to offspring (from its parent or ancestors). This is the process by which an offspring cell or organism acquires or becomes predisposed to the characteristics of its parent cell or organism. Through heredity, variations exhibited by individuals can accumulate and cause some species to evolve. The study of heredity in biology is called genetics, which includes the field of epigenetics.

Types of Heridity
  
An allele is said to be dominant if it is always expressed in the appearance of an organism (phenotype). For example, in peas the allele for green pods, G, is dominant to that for yellow pods, g. Since the allele for green pods is dominant, pea plants with the pair of alleles 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).
The description of a mode of biological inheritance consists of three main categories:
1. Number of involved loci
  • Monogenetic (also called "simple") – one locus
  • Oligogenetic – 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
These three categories are part of every exact description of a mode of inheritance in the above order. Additionally, 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