The Basics of Inheritance
Inheritance is the process by which a parent passes copies of his or her genes to an offspring. Genes cluster together to form chromosomes, which are located in a cell’s nucleus. There are two different types of chromosomes: sex chromosomes and autosomes. Sex chromosomes are what make up egg and sperm cells (or germ cells) that are responsible for reproduction, or creating new life (offspring) and passing on parent genes that will make up the offspring. Autosomes are the chromosomes that make up all of the rest of our cells (called somatic cells).
Humans have a total 46 chromosomes in almost all of their cells. The only cells that do not have 46 chromosomes are germ cells. Germ cells have 23 chromosomes, which is half the number of chromosomes in somatic cells. This is necessary so that the total number of chromosomes from the egg (23) and sperm (23) cells adds up to 46 chromosomes when creating an offspring.
All chromosomes are comprised of pairs of genes, with each gene in the pair coding for the same set of functions. For example, blood type is determined by the combination of two genes that code for blood type. Even though each gene obtained may code for a different blood type, blood type is the common function coded for.* The combined pair of genes from the parents is called a genotype.
Through a special replication process called meiosis, each germ cell obtains one gene (or an incomplete pair) that will form one complete gene pair in the offspring when the egg and sperm cells combine. Therefore, each egg cell and each sperm cell have only one gene for a given function from each parent.
Each new offspring receives one egg cell from his or her mother and one sperm cell from his or her father. This means that we receive one copy of a gene from our mother and one copy from our father for any given trait. Together, this gives us one complete pair of genes for that trait.
Even though we receive a copy of one gene from each parent, it does not mean that we will necessarily express (exhibit the trait for) both. Some genes will be expressed and some will not be expressed, depending on the dominant and recessive patterns of those genes.
The genes we receive from each parent may be recessive or dominant. The gene pair could have two recessive genes, two dominant genes, or one recessive and one dominant gene. The combination of recessive and dominant genes determines the trait that will be expressed for any given gene pair.
Typically, dominant genes will be the ones that are expressed when a gene pair has one dominant and one recessive gene, but gene expression could also fall somewhere in between the two genes. For example, we receive one gene for hair color from our mother and one from our father, but our hair color may be different from one or both of our parents depending on the how the genes express themselves as a pair.
The way that the pair of genes gets expressed is called a phenotype. Regardless of an offspring’s phenotype, it is always true that each parent contributes one half of the offspring’s genes, even when genes from one parent may not be expressed. The combination of genes from each parent is what makes each person a unique combination of the two parents, and genes inherited from these cells are present in all of a person’s cells.
Hereditary Cancer Syndromes
Most hereditary cancer syndromes are inherited in an autosomal dominant fashion. This means that only one gene (from either the mother or father) is required for a particular trait to be expressed. Each offspring (or child) has a 50 percent chance of inheriting the gene. In autosomal dominant inheritance, many generations express the trait without skipping generations. This is known as complete penetrance.
In some cases, however, someone who inherits a dominant gene may not express the trait. They still have the gene as part of their genetic code, inherited from a parent, and are still capable of passing the gene to their offspring. This is known as incomplete penetrance, and the person who doesn’t express the trait is called a carrier. In the case of hereditary cancer syndromes, the inherited gene is a mutation that is passed on from one generation to the next.
* When a gene that codes for a specific function has several different expressions, those different expressions are called alleles. There are different alleles for blood type, such as A type, B type, and O type.