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According to my textbook(NCERT)- out of two alleles one of them undergoes some changes(mutations I suppose) that leads to it making a normal/less efficient enzyme, a non functional enzyme or no enzyme at all. In the case of normal/less efficient enzyme both the alleles are equivalent. In the other two cases the modified alleles is recessive and the unmodified is dominant

I will add images of the text for reference.

a page from the textbook

I have two major questions

  1. How can in every organism, in every chromosome pair, the same changes happen leading to the alleles becoming recessive/staying dominant Or is this related to an early common ancestor? I do not understand how the concept of dominance here is achieved by every organism?

  2. So for a heterozygous pea plants flower colour ,the recessive allele does not make any functional enzyme/ no enzyme. Which is why only the effect of dominant allele is shown. So shouldn't this indicate every homozygous recessive flower be white? As no product is formed by the recessive allele.

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  • $\begingroup$ you are overthinking dominant and recessive, does it make something then it will be dominant to an allele that makes nothing as in if you have the allele that makes somthing then you will have that thing the other allele that does not make anything is irrelevent to whether you have something only the dominant (the functional making allele ) does. Its built inot the allele. this is also why codomiance exists. two allele that make two different things. and incompletel dominance one makes less than the other but both still make somthing. $\endgroup$ Commented Aug 2 at 14:39
  • $\begingroup$ @John of course I understand that. However this is barely an explanation. It is more of stating back the question. Still i appreciate any help :) $\endgroup$ Commented Aug 2 at 16:06
  • $\begingroup$ this is the comments section for clarification not answers. Maybe I musinderstand your first question, dominance is built into the allele, it does not "achieve" dominance the gene either makes somthing(dominant) (red stuff) or it does not (white). how many copies of the dominant allele controls how much "red" is made a small amount looks pink a lot looks red. Homozygous recessive snapdragons ARE white. You ask several questions so it is unclear where your misunderstanding is. $\endgroup$ Commented Aug 2 at 19:41
  • $\begingroup$ @John thank you $\endgroup$ Commented Aug 3 at 4:15
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    $\begingroup$ @RitvikBansal Yes, the idea is that the variant happened some time ago and is now present in the population at a certain frequency (not in every organism, as some organisms are still homozygous dominant). To your second question - the textbook does seem to indicate that homozygous recessive flowers are white, so I'm not sure why you are confused there. $\endgroup$ Commented Oct 20 at 3:22

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plant geneticist here. It is quite common for textbooks that explain basic genetics to give very confusing (and often inaccurate) "dominance" explanations. The idea of "dominance", "recessive" and further classifications (like incomplete dominance, codominance, overdominance...) only make sense phenotypically and when comparing one allele with another. They are also not very good ways of thinking about genetics if you ask me.

The example that you have in your book shows what is sometimes called the biochemical explanation of dominance (see here for a more specific biochemical explanation). With a biochemical point of view, all phenotypes should be due to a combination of (non)functional enzymes and the effects certain changes and combinations produce in a pathway. For your second question, Remember that some metabolic end-products may be colorless (white), so functional enzymes can produce white flowers by degrading intermediate metabolites that are colored.

As for your first question, mutations often do appear only once (with some exceptions) in the lineage of a species and then spread by reproduction through the population. Individuals with double-recessive mutations are therefore inheriting twice a recessive allele.

But for your own sake if you are studying genetics, I would take the whole thing of "recessive" and "dominant" with a big pinch of salt. Mendelian genetics is more commonly the exception than the rule.

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