Which statement about chromosomes is correct?

1.A gene is made up of one allele on a single chromosome. A person’s sex cells contain that single allele.

2.A gene is made up of two alleles on the same chromosome. A person’s sex cells contain one of the alleles.

3.A gene is made up of one allele spread over two chromosomes. A person’s sex cells contain that single allele.

4.A gene is made up of two alleles on separate chromosomes. A person’s sex cells carry one allele for each gene.