Introduction to the amazing world of genetics

Genetics is a fascinating branch of biology that uncovers the secrets of life hidden in the cells of our bodies. The key to understanding genetics is DNA, which carries genetic information. By looking inside the cell, we begin to discover the amazing mechanisms that shape our physical nature and determine heredity. 

A few words about DNA

Deoxyribonucleic acid, or DNA for short, is the molecular building block of life. It is a long double-stranded chain (double helix) composed of nucleotides (the basic building blocks of nucleic acids, which are molecules containing genetic information), each of which contains a sugar, a phosphate, and one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C), or guanine (G). The combination of these bases forms the genetic code, which carries the information necessary for the development and functioning of the organism. DNA is found in the nucleus of cells. The structure of DNA is incredibly elegant and efficient and allows for the precise preservation and transmission of genetic information from generation to generation. It is, in short, a miracle of evolution.

Introduction to the amazing world of genetics

The location of DNA in the cell and the role of chromosomes

Introduction to the amazing world of genetics

In the nucleus of cells, DNA is organized into so-called chromosomes, which are rigid structures made up of coiled DNA. Humans have 23 paired chromosomes, meaning that each parent passes on one set of chromosomes to their offspring, that is, 46 chromosomes in total, half of which come from the father and the other half from the mother.

  • This combination of chromosomes forms the genotype, the genetic basis of each individual. One of the most amazing aspects of genetics lies in the way chromosomes work together to ensure the transmission of genetic information. During cell division, chromosomes double, separate, and form two identical copies, which are then evenly divided between the two new cells.
  • This process, called mitosis, is crucial for the growth, repair, and maintenance of body tissues. In a different process, called meiosis, chromosomes combine and then divide into four cells with half the number of chromosomes. This allows the formation of sex cells, sperm in males and eggs in females, which have half the genetic material.
  • The joining of the sperm and egg at fertilization recreates the complete genetic code that creates the new individual.

Figure 1: Simplified structure of a human cell

Introduction to the amazing world of genetics

What are alleles

Introduction to the amazing world of genetics

Alleles are variants of a gene that affect a particular trait or characteristic of an organism. Genes are located on chromosomes that are in the nucleus of the cell. Each individual receives one allele from the mother and one from the father. Alleles are inherited independently of each other if they are on different chromosomes.The combination of alleles in a genotype influences whether dominant or recessive traits are expressed. This is how genetic information is passed on and affects the appearance, behaviour and other characteristics of offspring. Paired chromosomes contain the same genes but may have different alleles.

In the context of alleles, it is important to know the terms phenotype and genotype.

  • Phenotype is a set of observable traits or characteristics of an individual that result from the interaction of its genotype (genetic material) with its environment. Simply put, a phenotype includes all the external manifestations of an organism that we can see or measure, such as eye colour, height, body shape, behaviour, and other physical and behavioural characteristics.
  • In contrast, a genotype is the set of all the genes contained in an organism's DNA that can influence its development and functioning.

Dominant and recessive alleles are two basic categories of genetic variants that affect the expression of a trait or characteristic of an organism.

Figure 3: principle of allele functioning using blood group as an example

Introduction to the amazing world of genetics

What are genetic mutations and why genes mutate:

Introduction to the amazing world of genetics

What we look for when we examine your DNA are genetic mutations. Creating a mutation on a gene can happen in several ways.

In layman's terms, this can be explained as follows.

  • Incidental DNA replication errors:During each cell division, DNA replication occurs, a process in which genetic material is copied. Sometimes mistakes can occur in this process, which can cause changes in the genetic code and the development of mutations.
  • Influence of external factors: Mutations can be caused by external factors such as chemicals, radiation or viruses. For example, some chemicals can damage DNA and cause mutations.
  • Genetic recombination: In a process called recombination, genetic material can be moved between chromosomes. This natural genetic variation can lead to new combinations of genes and potential mutations.
  • Mutagens are substances or factors that increase the likelihood of mutations. Chemicals, such as some carcinogens, can act as mutagens and cause changes in DNA.
  • Inheritance of mutations:Some mutations can be inherited, meaning that they are passed from one generation to the next. These mutations can arise either by chance or can be caused by genetic errors passed from parent to offspring (see next section).

Figure 5: Graphical representation of a gene without a mutation and a gene with a mutation

Introduction to the amazing world of genetics

Example of hereditary disease transmission to offspring for a specific hereditary disease

Introduction to the amazing world of genetics

Cystic fibrosis (CF) is a genetic disease that is transmitted according to the laws of heredity. CF is caused by mutations in the Cystic Fibrosis Transmembrane conductance Regulator (CFTR) gene.

Here are the basic principles of inheritance for this disease:

Recessive Inheritance:

  • CF is a recessive genetic disease. This means that two mutated copies of the CFTR gene (one from the mother and one from the father) are required to manifest the disease.
  • Persons who have one normal and one mutated copy of the gene are called carriers. They have no symptoms of the disease because the normal copy of the gene compensates for the mutated one.

Probability of offspring being affected:

When two carriers have offspring, there is a 25% chance that the offspring will inherit two mutated copies of the CFTR gene and be affected by cystic fibrosis. There is a 50% chance that the offspring will be a carrier (one mutated copy) and a 25% chance that the offspring will neither be a carrier nor affected.

Carriers do not have symptoms:

Carriers of a mutation in the CFTR gene usually have no symptoms of cystic fibrosis because they still have a functional copy of the gene. Overall, the manifestation of cystic fibrosis in an offspring requires both parents to be carriers of the mutation in the CFTR gene. If one parent is a carrier and the other has a normal gene, their offspring will not be affected but may be a carrier.

Figure 7: Graphical representation of the principle of inheritance