During fertilisation the egg cell merges with the sperm cell and a single cell results with - if all goes well - 46 chromosomes: 23 from the mother and 23 from the father. In the egg cell the 23rd chromosome is by definition an X; a sperm cell, however, has either an X or a Y. It is therefore the sperm cell that determines the gender of the embryo: if it adds an X, the embryo will be XX (a girl), and otherwise it will be XY (a boy).
We inherit our genes through the chromosomes, so logically we also have two copies of these. We refer to the two variants of the same gene as alleles.
If a particular characteristic or disorder is linked to a single gene, it is referred to as monogenic. If multiple genes are responsible we call it multigenic and if environmental factors also have an impact it is called multifactorial.
Many of our inherited characteristics are multigenic or even multifactorial in nature. In other words they depend on the interaction between multiple genes or between genes and external factors. This means that a disease that is essentially genetic can also be triggered by environmental factors.
If multiple elements are responsible for a characteristic or disorder this is known as multifactorial inheritance. Physical build and intelligence are examples of 'normal' multifactorial characteristics. Known multifactorial conditions include spina bifida, cleft lip and palate, club foot, congenital cardiac disorders and Crohn's disease.