Genetics is the study of our genes and how we inherit features of our health from generation to generation. A genetic condition is a health concern caused by one or more differences in a person’s genetic information. Genetic conditions can affect any body system, and happen to any age group.
Many common conditions, such as cancer and heart disease, are related to our genetics. Most genetic conditions are rare and affect 1 in every several thousand or million people. Understanding genetics and genetic terms can be confusing. To understand genetics, we need to learn about the DNA (stands for deoxyribonucleic acid), genes, and chromosomes found inside human cells.
We can think about DNA, genes, and chromosomes as the letters, words and sentences on this page. DNA is like a set of letters but the DNA “alphabet” only has 4 letters. When you put letters together, you make words. When you read words together in a sentence, you get some meaning. Genes are like the words that come together in a specific way to give instructions, like when words are joined to form a sentence.
A cell is a small container of chemicals, smaller structures, and water that is wrapped in a membrane. Cells are the basic building blocks of all living things. Cells have many parts that work together to give living things their structure (how they look) and function (how they work). Our bodies are made up of trillions of cells, and different cells in the body have different functions.
The centre of most cells have a nucleus. The nucleus is the part of the cell that holds quite a bit of the genetic material (DNA) needed to make that particular living creature. There is also some separate DNA in a different cell structure called the mitochondria.
DNA stands for deoxyribonucleic acid. Each human cell has the right amount of DNA to make a person one of a kind. DNA is made up of a code, using 4 small units called bases or nucleotides. The names of these bases are adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA has about 3 billion bases, so there is an unlimited number of ways these 4 bases can be arranged into genes.
The bases are arranged in a chain called a DNA strand. Each base forms a chemical bond with a second base. Adenine (A) always matches up with thymine (T), and guanine (G) always matches up with cytosine (C). A set of 2 bases is called a base pair. Once each base has a match, the set of base pairs on the 2 strands of DNA twist around, and form the final DNA structure called a double helix.
A gene is made up of a specific piece of DNA having a very specific order of base pairs. In general, each gene has the code to make one thing or do one job. The size of a gene can be quite small, from a few hundred DNA base pairs, up to more than 2 million base pairs.
Genes give a cell instructions to make molecules called proteins. Proteins are important to how a cell functions, and how our bodies work and develop. A gene’s instructions are not used by all cells. Each cell has its own specific set of instructions that it needs to get from the genes to work normally.
Humans have about 20,000 genes. We have 2 copies of each gene, one we inherit from each parent. Our genes give us the features and traits we inherit from our birth parents, such as our eye colour and blood type, as well as the chance of developing certain diseases.
Human genes are packed tightly into structures called chromosomes inside each cell nucleus. Most human cells have 23 pairs of chromosomes, making a total of 46 chromosomes in each cell. There are 22 pairs called autosomes that are the same in males and females. The last pair are sex chromosomes that decide a person’s biological sex (the sex you are given at birth that may appear on proof of identity documents). In general, males have 1 X and 1 Y chromosome, and females have 2 X chromosomes.
We inherit one of each of the chromosome pairs from our biological mother and the other from our biological father. This means that half of our genetic material is from one parent, and the other half is from our other parent.
Genome and Exome
The genome is the complete set of genes or genetic material in a human. The human genome is made up of the DNA in the 46 chromosomes and the DNA in the mitochondria. Most of the DNA that makes up the genome is not used by the body as instructions for anything.
Exome refers to the smaller amount of DNA that we know the body uses to code for specific instructions. The exome is thought to only represent about 1% to 3% of all the DNA in the genome. Some doctors and researchers only study the exome since we think this is the DNA that is most important for our growth and development and giving other instructions to the body.