The trait of an organism is primarily determined by its phenotype, which is the observable expression of its traits. However, it is influenced by several factors, including:
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The combination of its alleles: The specific allele pairs (genotype) that an organism possesses will determine its phenotype. Dominant and recessive alleles play a significant role in this.
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The number of copies of the gene can also affect traits, especially in polygenic traits or in cases of gene dosage effects.
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The presence of dominant alleles can influence the phenotype since dominant alleles mask the effect of recessive alleles in heterozygous combinations.
So while the phenotype is what we observe, it depends on the underlying genetic makeup, specifically the combination of alleles present in the organism.