Medical Genetics | The Key to Understand Chromosomes, DNA, Genes

Medical genetics chromosome dna genes

The Key Notions of Medical Genetics

Understanding genetics and how certain diseases are transmitted and develop requires some knowledge of biology. Here is a summary of basic knowledge of medical genetics that will help you better understand the link between humans, genetics and heredity.


The human body is made up of billions of “cells” each with a nucleus. This nucleus contains all our genetic information. This is contained in our chromosomes which themselves contain our DNA.

What is a chromosome?

Chromosomes are made up of DNA that carries genes (about 20,000).
The genetic information is distributed over the 46 chromosomes (23 pairs). For each pair, there is a chromosome of paternal origin and a chromosome of maternal origin. Thus, for the same pair, the two chromosomes will not be identical. The first 22 pairs are called autosomes. The 23rd pair is the one that determines the sex of the person. These are the X and Y chromosomes. Women have two X chromosomes, while men have one X chromosome and one Y chromosome.

Human chromosome

The number of human chromosomes are 23 pairs.
One copy inherited from the mother and the other inherited from the father. The 22 pairs of identical chromosomes are called “autosomes” and the 23rd pair, which determines the person’s sex, is made up of the X and/or Y sex chromosomes, or “gonosomes”: XX for a woman and XY for a man.

Here are the details:

base pairs
% of basesSequenced
base pairs
% sequenced
base pairs
X (sex chromosome)800154,913,7545.0151,058,75497.51%
Y (sex chromosome)20057,741,6521.925,121,65243.51%

Base pairs are nucleobase : nucleobase  are parts of RNA and DNA that may be involved in pairing These include cytosine, guanine, adenine, thymine (DNA) and uracil (RNA). These are abbreviated as C, G, A, T, and U, respectively. They are usually simply called bases in genetics. Because A, G, C, and T appear in the DNA, these molecules are called DNA-bases; A, G, C, and U are called RNA-bases, respectively.

The number of chromosomes is always the same for all individuals of the same species, but it can vary from one species to another
SpeciesNumber of chromosome pairs
Drosophila (fruit flies)4

What is DNA? what is a gene?

The DNA, which makes up our chromosomes, carries the genes (about 20,000)
The genes are therefore also present in 2 copies (maternal/paternal).
The DNA molecule, also known as deoxyribonucleic acid, is found in all of our cells. It is the “detailed plan” of our body, also called the genetic code: it contains all the information necessary for the development and functioning of the body. DNA is made up of four complementary elements, nucleotides: adenine, thymine, guanine and cytosine: A, T, G, C.

The gene is a piece of this DNA that corresponds to specific genetic information that codes for a unique protein. It is therefore a very small portion of chromosome.
As we have duplicate chromosomes, each gene is also duplicated in our cells. These two copies of the same gene, called alleles, are most often different: one of paternal origin and one of maternal origin.

DNA animation
Animation of a rotating DNA structure. A section of DNA. The bases lie horizontally between the two spiraling strands. brian0918, Public domain, via Wikimedia Commons


Genes tell each cell its role in the body. At their command, they synthesize proteins: this is the translation of the genetic code. We produce tens of thousands of proteins. Each protein has a different role to play.
For example, hemoglobin is the protein used to transport oxygen in the blood. Actin and myosin are proteins that help muscles contract. Antibodies protect the body from foreign bodies. Still other proteins are involved in defining the color of the eyes or the shape of the face.

A genetic abnormality (mutation or chromosomal abnormality) can disrupt protein production. In a way, it gives “wrong orders” to manufacture them with the consequence: absence of manufacture, excess of manufacture or abnormal manufacture. The protein can therefore no longer play its role, which leads to a genetic disease.

Nevertheless, a genetic anomaly does not always lead to a disease, certain manufacturing errors can go completely unnoticed or only be expressed depending on the environment. Indeed, the environment plays a role in the expression of genes.

Please note that we are not “all genetics”!
In some cases, the role of genes is decisive in the risk of developing a genetic disease. For others, environmental factors (diet, tobacco, lifestyle, etc.) play a more or less important role, sometimes reducing the weight of the genetic heritage.

DNA, GENES and CHROMOSOMES, what are their relationships and functions?

DNA, chromosomes and genes are working together to make who wre are. Chromosomes will carry DNA in cells. DNA is responsible to build and maintain our human structure. While genes are segments of our DNA, this is to give you physical characteristics that make us unique. All together our body has a complete instruction manual that tells our cells how to behave individually.


The DNA (chromosomes, genes) of each human is half inherited from his father and half from his mother. This is why genetic diseases have the particularity of affecting not only the affected person but often also their family (grandparents, parents, uncles/aunts, brothers/sisters, children, nephews/nieces, cousins/cousins).
Depending on the genetic mechanism, the disease is not transmitted in the same way. There are three main modes of inheritance: autosomal dominant, autosomal recessive, X-linked.

A genetic disease is not systematically inherited. The genetic anomaly can occur accidentally, not inherited, during the production of gametes (spermatozoon, oocyte) or very soon after fertilization. This is called de novo mutations. In this case, the disease can only affect the (possible) descendants of the patient.

The genetic abnormality can occur in a cell other than a gamete. It only affects a few cells within the body. The mutation is acquired, it is not transmissible and has not been inherited. These cells can then be at the origin of cancers. We then speak of a somatic genetic disease, acquired by the patient during his life.

Genetic Diseases | Explanations and Examples

Glycogen Storage Disease | Disorders of the sugar metabolism, cannot properly broken down or processed

Sources: PinterPandai, Genetic Alliance UK, East London Genes & Health, National Human Genome Research Institute

Photo credit (main picture): Sponk, Tryphon, Magnus Manske, User:Dietzel65, LadyofHats (Mariana Ruiz), Radio89, CC BY-SA 3.0, via Wikimedia Commons

Photo explanation: The nuclear DNA of eukaryotic cells is located on the chromosomes in the nucleus.

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