How is CDH1 Inherited
CDH1 is a gene which provides instructions for making a protein within the membranes that surround epithelial cells called epithelial cadherin, or E-cadherin. E-cadherin is just one of many cadherin proteins whose function is to help cells adhere to one another (cell adhesion) in an organized fashion. E-cadherin also transmits chemical signals within cells, controls cell maturation and movement, and regulates certain gene activities.
When there is an error (mutation) in the DNA sequence of the CDH1 gene, the mutation prevents the E-cadherin protein from functioning properly. This can cause the cells impacted to act in nonsensical ways, and can lead to health conditions related to the genetic changes.
A CDH1 mutation can lead to:
Hereditary diffuse gastric cancer
HDGC stands for Hereditary Diffuse Gastric Cancer. It is a syndrome...
Knowing your Risks
"A gene mutation is a permanent alteration in the DNA sequence that makes up a gene, such that the sequence differs from what is found in most people. Mutations range in size; they can affect anywhere from a single DNA building block (base pair) to a large segment of a chromosome that includes multiple genes.
Gene mutations can be classified in two major ways:
Hereditary mutations are inherited from a parent and are present throughout a person’s life in virtually every cell in the body. These mutations are also called germline mutations because they are present in the parent’s egg or sperm cells, which are also called germ cells. When an egg and a sperm cell unite, the resulting fertilized egg cell receives DNA from both parents. If this DNA has a mutation, the child that grows from the fertilized egg will have the mutation in each of his or her cells.
Acquired (or somatic) mutations occur at some time during a person’s life and are present only in certain cells, not in every cell in the body. These changes can be caused by environmental factors such as ultraviolet radiation from the sun, or can occur if an error is made as DNA copies itself during cell division. Acquired mutations in somatic cells (cells other than sperm and egg cells) cannot be passed to the next generation." 
CDH1 is considered a hereditary mutation, although some de novo (new) mutations have been discovered recently. This happens either "when the mutation occurs in the fertilized egg shortly after the egg and sperm cells unite,"  or "the mutation occurs in a person’s egg or sperm cell but is not present in any of the person’s other cells."  Meaning neither of the parents have the mutation, but the child does (this is VERY rare).
"Everyone has two copies of the CDH1 gene — a copy they inherited from their mother and a copy they inherited from their father. If one of these CDH1 genes has a mutation, then the person has hereditary diffuse cancer syndrome. If someone has hereditary diffuse gastric cancer syndrome, then each child he or she has will have a 50% chance of inheriting the CDH1 mutation (and therefore hereditary diffuse gastric cancer syndrome) and an equal 50% chance of NOT having inherited the mutation (and therefore NOT having hereditary diffuse gastric cancer syndrome)."