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Genes and Chromosomes


The Manual's Editorial Staff

Reviewed/Revised Jan 2024
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What are genes?

Your genes are the chemical codes that control everything about how your body works, how it's made, and what it looks like. People have over 20,000 different genes. Every cell in your body has a copy of each of your genes.

Cells are the microscopic building blocks of your body. Each of your organs is made of different types of cells. For example, you have nerve cells in your brain, liver cells in your liver, and cells in your stomach that make stomach acid. The way each cell grows and works is controlled by genes.

Some things in your body are controlled by just one gene. But most things, for example your height and weight, are controlled by many genes working together.

What are genes made of?

Genes are made of DNA. DNA is a long, long chemical chain that is twisted so it looks like a spiral staircase. The millions of steps on the staircase make up the genetic code.

Structure of DNA

DNA (deoxyribonucleic acid) is the cell’s genetic material, contained in chromosomes within the cell nucleus and mitochondria.

Except for certain cells (for example, sperm and egg cells and red blood cells), the cell nucleus contains 23 pairs of chromosomes. A chromosome contains many genes. A gene is a segment of DNA that provides the code to construct a protein or RNA molecule.

The DNA molecule is a long, coiled double helix that resembles a spiral staircase. In it, two strands, composed of sugar (deoxyribose) and phosphate molecules, are connected by pairs of four molecules called bases, which form the steps of the staircase. In the steps, adenine is paired with thymine and guanine is paired with cytosine. Each pair of bases is held together by a hydrogen bond. A gene consists of a sequence of bases. Sequences of three bases code for an amino acid (amino acids are the building blocks of proteins) or other information.

Structure of DNA

What are chromosomes?

Chromosomes are the structures that contain your genes and are within each cell. Each chromosome is a long string of DNA containing hundreds of genes all connected together. Cells contain 46 chromosomes arranged in 23 pairs. One of each pair of chromosomes comes from your mother and one comes from your father. That means half of your chromosomes (and half of your genes) came from each of your parents.

What is a genetic disorder?

A genetic disorder is a medical condition caused by a problem with your genes or chromosomes.

When you were conceived, an egg cell from your mother and a sperm cell from your father combined into one cell. That cell split into 2 cells. Then those 2 split into 4, and the 4 became 8, and so forth until a baby was formed of billions and billions of cells.

Each time the cells split, they had to make exact copies of each of your chromosomes with all their thousands of genes. Usually, the copies come out just fine, but sometimes there's an error. The error may be:

  • A mutation

  • A chromosome abnormality

A mutation is an error in the copy of a single gene. A mutation in sperm cells if you're a man, or egg cells if you're a woman, can be passed on to your children. A mutation in any other type of cell can cause you problems, but you can't pass the mutation to your children. Mutations can be:

A chromosome abnormality is an error copying a whole chromosome or a big piece of one. The result could be:

  • There's an extra copy of a chromosome

  • Part of a chromosome gets stuck to another one

  • Part of a chromosome is missing

  • Part of a chromosome is copied too many times

What causes genetic disorders?

Often, doctors don't know why a genetic disorder happened. But sometimes they're caused by:

  • Radiation

  • Certain chemicals

  • Certain medicines

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