When the same gene was sequenced in a different person, the sequence was ATTTAGCC
Explain why this shows that the gene has more than one allele

The different DNA sequence for a gene shows that same character can be expressed in 2 or more forms. Alleles are present on homologous chromosome.

Explanation:

The gene is the DNA sequences on the chromosome coding for the particular trait in an organism.

The alleles are the variant of the gene. In a homologous chromosome the variant of the gene or allele is present. The homologous chromosome is present in progeny one from each parent.

For example the allele for tall as homozygous is present on one of the parent and shortness on the other parent in homologous chromosome. The trait of height is the gene and the way it expresses itself is the trait from either parent. The DNA sequence will also be different for the alleles because they will code for different proteins like short or tall phenotype.

The genotype will be different hence DNA sequence is also changed.

yes it is weird but true

you know they are heterozygous for a trait because they have it as jj or jj.