What is an allele?
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a. an organism that lives in eukaryotes
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b. a group of chromosomes
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c. a gene that causes a specific trait
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d. a chromosome that determines gender

Just did the lesson on this, the answer is  C.

Explanation:An allele is one of the possible forms of a gene. Most genes have two alleles, a dominant allele and a recessive allele. If an organism is heterozygous for that trait, or possesses one of each allele, then the dominant trait is expressed. ... So a gene is a particular region of your DNA that controls a specific trait.

i would say the prokaryotic cells the prokaryotic cells are less complex, are usually found only in unicellular organisms and have more limited capabilities than eukaryotic cells.