Positive selection of t cells occur in .the thymus
        
             
        
        
        
Answer: A gene is a sequence of nucleotides in DNA or RNA that encodes the synthesis of a gene product, either RNA or protein. A chromosome is a long DNA molecule with part or all of the genetic material of an organism. A allele is one of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.
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 I'm pretty sure the answer is Sound Waves