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Dominant and Recessive genes

  1. Jan 7, 2014 #1
    What makes a gene dominant over another? Like why is the gene for brown eye dominant over the one for blue eyes? My teacher says it is "just like that" but I'm not so sure. IS there a reason for this?
  2. jcsd
  3. Jan 7, 2014 #2


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    A quick note on terminology: rather than saying genes are dominant over others it's best to say that different versions of genes (called alleles) are dominant or recessive over other versions. Dominance is not inherent to an allele meaning that there is nothing special about any allele that determines whether it is dominant or recessive. Rather it's the relationship between alleles because in some cases the phenotype of one allele "overrides" that of the other.

    In the case of eye colour it's complicated because whilst students are taught in school that it's an example of simple dominance it isn't. Eye colour is determined by multiple genes which makes it more complicated. Keeping it simple though to illustrate the concept it's important to understand that pigment in the eye can accumulate both at the front and back of the iris. Genes that give rise to dark eyes cause pigment to appear in both layers however genes that cause light coloured eyes only allow pigment to form at the back. So if you imagine that you have one allele saying pigment should be at the back and one saying pigment should be at the front and back you end up with pigment in both areas.

    Does this make sense?
  4. Jan 7, 2014 #3


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  5. Jan 8, 2014 #4
    Okay, but what causes this difference? I mean why do the genes for light eyes cause pigment to appear in just the back layer. And thanks for correcting my terminology!
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