Galactosemia is a genetic condition, meaning that it is passed down through a child's parents.
When a baby is conceived, it receives genes from each parent, creating thousands of pairs of genes that determine things like the color of their eyes and hair. There is also a gene pair that enables the production of an enzyme that helps our bodies process galactose—this is the GALT gene.
Galactosemia occurs when each of the infant's parents carry one form of the gene that makes a working GALT enzyme and one nonworking gene that does not make the GALT enzyme. When a child inherits a nonworking form of the GALT gene from both parents to form a nonworking GALT gene pair, their GALT enzyme will not work as it should and causes Galactosemia. This kind of genetic inheritance is known as "autosomal recessive" inheritance.