How do you know that individual 1 is heterozygous for the trait?

Individual 2 is homozygous recessive, so they can pass only a recessive allele to their children. Individuals 5 and 8 do not show the dominant trait, so they must have received a recessive allele from individual 1 as well. Since individual 1 shows the dominant trait but carries a recessive allele, individual 1 is heterozygous. The fact that individuals 3 and 7 show the dominant trait alone would not establish this.

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