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I was reading Genetic Analysis (Sanders, Bowman) and saw the following figure summarizing the 1909 work of Herman Nilsson-Ehle in describing the genetic control of kernel color in wheat:

Figure 21.1, pg. 711 in 2012 ed.

How did Nilsson-Ehle determine that the original pure-breeding parents were in fact homozygous for each allele?

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    $\begingroup$ Welcome to Biology.SE. They were probably inbred lines for many generations. $\endgroup$
    – Remi.b
    Aug 1 '16 at 18:10
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If they are "pure-breeding", that means that they are homozygous. If they were heterozygous, the offspring would not all have the same color as the parents. But tree-breeding" means they all do.

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  • $\begingroup$ These genes are additive. No dominance relationship is assumed to exist between allele $A_1$ and $A_2$ or between allele $B_1$ and $B_2$. $\endgroup$
    – Josh Gray
    Aug 2 '16 at 19:53
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In Campbell Biology (2021) 12th edition it says that

"True-breeding" (or pure lines) can be verified over many generations of self-pollination. If they produce only the same variety as the parent plant, then they are homozygous.

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    $\begingroup$ There are more complicated situations, such as if being homozygous at a locus is fatal. You then end up with pure-breeding heterozygotes. $\endgroup$
    – Armand
    Sep 4 at 8:12

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