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(genetics) Of, or pertaining to an individual (or a condition in a cell or an organism) containing two copies of the same allele for a particular trait located at similar positions (loci) on paired chromosomes


The term homozygous is used to describe that which has the same or identical alleles for a particular trait located at similar loci on paired chromosomes (i.e. homologous chromosomes). In a diploid organism, there are two sets of chromosomes. One of the sets comes from the mother and the other set from the father. Each maternal chromosome has a corresponding paternal chromosome to match with based on their loci. When the loci in the matching chromosomes bear the same alleles, it is described as homozygous. It means that the alleles code for the same trait.

A ‘homozygous’ organism for a particular trait is described to possess either a pair of dominant alleles (e.g. AA) or a pair of recessive alleles (e.g. aa). True breeding organisms are homozygous since the trait in question can be held constant as they produce the same phenotypic result.

Word origin: Greek homo ("same") + zygous (of a zygote)


See also:

Related form(s):

  • homozygosity (noun)
  • homozygous (adjective)