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My textbook says:

genes which code for a pair of contrasting traits are known as alleles.

Then that means T and t are alleles. Further, it says:

Mendel also proposed that in a true breeding, tall or dwarf pea variety the allelic pair of genes for height are identical or homozygous, TT and tt respectively.

This line confused me, it will be of great help if you can help me out.

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    $\begingroup$ It would be helpful if you mention the full name of the textbook rather than saying "my textbook". $\endgroup$ Commented May 29, 2023 at 12:16
  • $\begingroup$ @NilayGhosh i am sorry, here is the link of the pdf of the textbook: ncert.nic.in/ncerts/l/lebo105.pdf see the page number 72 of the textbook, definition of allele is given there and allele is written is bold letters. $\endgroup$
    – Charles
    Commented Jun 4, 2023 at 10:10

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That's a poor definition of allele. Alleles don't need to have anything to do with traits - different alleles can code for the same or different traits.

Alleles are simply different versions of a genetic variant. The most simple way to think of what an allele is the base pair (A, C T or G) which exists at a particular place in the genome. You have 2 alleles at any place, which can be the same allele or different. In your case, the alleles are coded as T and t - if both are the same, e.g. TT, then it's called a homozygous genotype, if they are different then it's a heterozygous genotype.

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  • $\begingroup$ Thank you sir, i really appreciate your knowledge, i didn't knew what exactly is allele, but your answer made everything clear, thank you again. $\endgroup$
    – Charles
    Commented May 27, 2023 at 12:49
  • $\begingroup$ sir is it possible to have a gene which is exact copy of another gene (same in everything, including bases) or only different versions of a gene are possible ? $\endgroup$
    – Charles
    Commented May 27, 2023 at 12:54
  • $\begingroup$ I don't know if I'm understanding the question correctly, but gene duplication is a common process. Immediately after duplication there would be two identical copies of a gene in the same genome, although those two copies would usually diverge quickly. $\endgroup$
    – Ben Bolker
    Commented May 31, 2023 at 13:58
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Basically allele is an alternative form of gene. For example let us consider a ball. A ball has a property called color. Color has some alternative forms like it can be red,blue,black,etc. So in this case gene is the element which define the ball's color that is the property defining element of our body and it can have many different variants of properties defined in different individuals like the colors red,blue,black,etc and these respective variants of a single property are called allele. Let us consider a gene defining our skin colour(the property) then it can define blackish or white skin color(variants) then the gene has 2 alleles. Homozygous gene means that it has 2 same alleles or alternative form of genes on the homologous chromosomes like having 2 same alleles on homologous chromosomes defining oval face.

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  • $\begingroup$ Thanks a lot! I wish I could upvote, but I don't currently satisfy the requirements to upvote. $\endgroup$
    – Charles
    Commented May 27, 2023 at 12:43

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