The nucleotide segments of DNA molecules that make up genes and are expressed in the phenotype of an organism are called exons.

Here's a breakdown:

* Genes: Segments of DNA that contain the instructions for building and maintaining an organism.

* Exons: Coding regions within a gene that are transcribed into messenger RNA (mRNA) and ultimately translated into proteins. These proteins are responsible for various functions within the organism, contributing to its observable characteristics (phenotype).

* Introns: Non-coding regions within a gene that are removed during RNA processing before the mRNA is translated.

Here's how it works:

1. Transcription: The DNA sequence of a gene is copied into mRNA. This includes both exons and introns.

2. RNA Processing: Introns are removed from the mRNA molecule, leaving only the exons.

3. Translation: The exons in the processed mRNA are translated into a chain of amino acids, forming a protein.

4. Protein Function: The protein carries out its specific function within the cell, contributing to the overall phenotype of the organism.

In summary: Exons are the crucial parts of a gene that hold the instructions for building proteins, which directly influence the organism's traits.