Mendel proposed that each individual carries forms of a gene for a given character, which are called alleles, and that the alleles of a gene separate during gamete formation so that each gamete receives allele(s) ___.

either one of the two alleles  in a diploid individual.

Explanation:

A trait is expressed only when two alleles pair up. Thus, an allele pair during cell division assort independently into two individual alleles so that each gamete receive one of the two allele

For example  

During gamete formation XX and XY allele pairs divide to contribute one allele to each gamete respectively.

Thus, XX allele pair will divide to give X gene to both the gamete

And XY pair will divide to give X gene to one gamete and Y gene to other gamete  

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