Heredity

Human genome sequencing has allowed for more than 25,000 genes on our 23 chromosomes to be named. Genes contain the genetic information for each individual. With the exception of a certain number of genes carried by the sex chromosomes X and Y, each gene is present in two copies in our cells. Each copy is carried on one homologous chromosome of one pair of chromosomes. One chromosome comes from our mother, the other from our father.

At the time of gamete formation, when a chromosome or a gene is altered, a genetic disease can occur and transmit through any descendants.

Two types of genetic anomalies to be considered are:

• The anomalies of number or structure of a chromosome (chromosomal aberrations like trisomy 21).
• The mutation of a gene (localised modification of the DNA). This gene can therefore exist as different forms such as deadly or deleterious alleles. The non mutated form is called the normal allele.

This animation illustrates many characteristic examples of the transmission of genetic anomalies due to the mutation of a gene. The study of a pedigree chart can determine the mode of transmission of a gene from one generation to another. It allows us to determine if the mutated allele of a gene responsible for the illness is recessive or dominant and if this gene is carried by an autosomal or a sex chromosome.