Chromosomes determine an individual's phenotype because they contain the genes that code for the individual's traits. Genes are segments of DNA that provide instructions for the development and functioning of an organism. Each chromosome carries multiple genes, and the combination of genes on a chromosome determines the individual's phenotype.

For example, the gene for eye color is located on chromosome 1. If an individual has two copies of the brown eye gene, they will have brown eyes. If an individual has one copy of the brown eye gene and one copy of the blue eye gene, they will have hazel eyes. The phenotype of an individual is the result of the interaction of all of the genes on their chromosomes.

In addition to genes, chromosomes also contain other DNA sequences that are involved in regulating gene expression. These sequences can turn genes on or off, and they can also control how much of a protein is produced. This means that the phenotype of an individual can also be influenced by the way in which their genes are regulated.

The study of chromosomes and genes is called genetics. Genetics is a complex field, but it is essential for understanding how individuals inherit their traits and how those traits are expressed.