Alleles are various molecular forms of a gene for the same trait that arise by mutation. If homozygous, both alleles are the same. If heterozygous, the alleles differ.
When heterozygous, one allele is dominant (A), and the other is recessive (a). Homozygous dominant = AA, homozygous recessive = a a, and heterozygous = Aa. A dominant allele masks a recessive allele that is paired with it.
Genotype refers to particular genes an individual carries. Phenotype refers to an individual's observable traits. You cannot always determine genotype by observing phenotype.
REVIEW: A heterozygote has a _____ for the trait being studied.
REVIEW: The observable traits of an organism are its _____ .
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