Biology meiosis Question!! HELPPP!!!

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Hannahxcx
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(c) An old form of wheat, emmer wheat (Triticum turgidum), has a diploid chromosome number of 28 (2n = 28). A wild wheat, einkorn wheat (Triticum tauschii), has a diploid chromosome number of 14 (2n = 14). These two species occasionally crossed and produced sterile hybrid plants. Due to an error during cell division, one of these hybrid plants formed male and female gametes with 21 chromosomes. Fusion of these gametes resulted in viable offspring. These plants were a new species, Triticum aestivum (2n = 42), our modern bread wheat.

(i) How many chromosomes would there have been in each of the cells of the hybrid plant produced by crossing Triticum turgidum with Triticum tauschii?


To me it seems like the answer should be 42 ... because if the haploid number is 21 then how can it be 21 in a normal cell as well?? But the mark scheme says 21 so now im really confused!!
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OxFossil
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The chromosome complement of offspring cells is formed by adding the chromosomes contained in the male gamete to the chromosomes in the female gamete.

In this case, the sterile hybrid would have been formed by the fusion of a gamete from T turgidum - which would be carrying its haploid number of 14 chromosomes - and a gamete from T tauschii - which would have been carrying its haploid number of 7 chromosomes.

The sterile hybrid therefore has 14+7 = 21 chromosomes.

The question describes T aestivum as arising from a cell division error - presumably when the developing gametes of the hybrid failed to split and all 21 chromosomes landed up in the one gamete instead of being divided in two. T aestivum has 42 chromosomes because it is made up of two gametes, each of which has the doubling error. Its gametes would have 21 chromosomes, but the question isn't asking about this new strain/species, it's only asking about the F1 sterile hybrid. Sneaky!
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