Basic Genetics

The genetic code for every feature of each human is contained within the nucleus of each cell of the body. The genetic code is contained within a long string of nucleotides on a molecule called deoxyribonucleic acid (DNA). Each gene is the result of a string of DNA that contains the instructions for how to form specific proteins. These proteins are responsible for how our body functions. 

DNA codes for hundreds to thousands of genes are bundled into individual chromosomes. Chromosomes are single molecules of DNA. Chromosomes come in pairs. Therefore, each human has two copies of each gene, one on each chromosome in a pair. One chromosome in each pair comes from each parent. We have 23 chromosome pairs or 46 chromosomes in total.

One of the pairs of chromosomes are the sex chromosomes. The sex chromosomes are the X chromosome and the Y chromosome. Males have an X and a Y chromosome and females have two X chromosomes. The other 44 chromosomes are called autosomes. They are the non-sex chromosomes.

Genotype refers to the genes that we have. For example, having the gene that codes for brown eyes.

Phenotype refers to the physical expression of the genes that we have. For example, actually having brown eyes.

 

Inheritance

A person’s DNA is the result of half the genes from their mother combined with half the genes from their father.

A mother’s egg contains half her DNA, and a father’s sperm contains half his DNA. The DNA that makes up the egg or sperm is selected at random and is impossible to predict. The process of creating a gamete (egg or sperm) that contains half the genetic material of a normal cell is called meiosis.

 

Dominant versus Recessive Genes

When there is a single gene that codes for a physical feature, the phenotype is the result of the combination of the two genes (one on each chromosome). Some genes are more dominant than others, and the phenotype will reflect the more dominant gene. This is called Mendelian inheritance.

Genes that are more dominant over other genes are known as dominant, and genes that aren’t expressed when paired with more dominant genes are called recessive.

Consider a very simplistic example, the genetic inheritance of eye colour. If the mother passes on the gene for brown eyes and the father passes on the gene for blue eyes, the child will have one gene for each eye colour. If the gene for brown eyes is more dominant, the child will have brown eyes. The gene for brown eyes overpowers the gene for blue eyes. In order for the child to have blue eyes, they would need two copies of the gene for blue eyes, one from their mother and one from their father. In this example, the gene for blue eyes would be described as “recessive”.

(The genes for eye colour are not as simple as the example above, but this does provide a useful illustration to help us understand the topic.)

 

Last updated January 2020
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