In humans, each cell contains 23 pairs of chromosomes, 22 are called autosomal chromosomes and the 23rd pair is the sex chromosome (XX for women, XY for men). In the autosomal chromosomes, each individual carries two genes, one from the father and the other from the mother. When the gametes form, a copy from each parent is passed on at random to the offspring. For example, in the case of a recessively inherited disease, if an individual carries two copies of the normal version of the gene, this individual will be healthy. A carrier of two mutated copies will be affected, and an individual carrying a mutated and a healthy copy will not develop the disease, and be an asymptomatic carrier. The affected individual with two mutated copies will pass one copy of the mutated gene to all of his/her offspring, and an asymptomatic carrier will pass the mutated gene to 50% of his/her offspring.
Not all hereditary diseases are autosomal recessive. There are diseases that depend on a single gene and exhibit dominant inheritance. The affected parent passes the mutated gene (and the disease) to 50% of his/her offspring. Finally, in the case of gender-related diseases (the gene is located on the X chromosome), a carrier mother will pass the mutated X chromosome to 50% of her girls (who will be healthy carriers) and 50% of her boys. Those who inherit it will be affected.