Why does a gene have two alleles? When there is a gene for producing the color pigment for a flower, why are there there two alleles, producing either same color or different color (homozygous and heterozygous)?


closed as unclear what you're asking by March Ho, AliceD, rg255, WYSIWYG Jul 11 '15 at 19:09

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  • $\begingroup$ Why does sexual reproduction exist? You need to answer that first, alleles are just a consequence. $\endgroup$ – mistermarko Jul 11 '15 at 9:15
  • $\begingroup$ To sustain our species $\endgroup$ – Naveen Kumaar Jul 11 '15 at 9:22
  • $\begingroup$ I'm not sure exactly what question you're asking. $\endgroup$ – James Jul 11 '15 at 11:14
  • $\begingroup$ Have you done any research at all? $\endgroup$ – SolarLunix Jul 11 '15 at 16:09
  • $\begingroup$ So which factor decides a specific allele to be expressed and the other allele to remain repressed $\endgroup$ – Naveen Kumaar Jul 11 '15 at 16:14

Allele is a variant form of a gene. How many alleles there are depends on number of copies of the gene and number of variants. In theory you can have 5 identical copies or single copy and single allele (Y-linked genes come to mind).

Regulation is very complicated, I'll refer you to reading on recessiveness, dominance. Barr bodies also have to do with various expression profiles in same thing.


A flower (or a person) has two alleles for a gene because it inherits one set of chromosomes from one parent and another, comparable, set of chromosomes from the other parent. Hence there are two copies of each gene, and so there are two alleles for each gene.

  • $\begingroup$ So which factor decides a specific allele to be expressed and the other allele to remain repressed $\endgroup$ – Naveen Kumaar Jul 11 '15 at 16:13
  • $\begingroup$ In a heterozygous diploid cell, typically both alleles are expressed (parental imprinting is a well-known exception to this statement). However, if, as in Mendel's pea plants, one allele is wild type and one is mutant, than whichever allele is dominant will be expressed (or in other words have a visible phenotype that you can score). Co-dominance is a rare exception to this statement--if two alleles are co-dominant then each homozygote has a different phenotype, but the heterozygote has a third phenotype. For example, the mating type locus in baker's yeast. $\endgroup$ – mdperry Jul 12 '15 at 20:29

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