Patterns of inheritance

As described in the section on What are genes?, the human body is made up of billions of tiny building blocks called cells. Inside each cell are genes, which are the instructions which guide our development and physical characteristics. People have two copies of each gene, one copy inherited from the mother and the other copy inherited from the father. (An exception is the genes on the X and Y chromosomes, which will be described below in the section on X-linked inheritance. The two copies of the gene may be exactly the same or be slightly different versions, called alleles. Examples of alleles can be found in the genes for eye colour and hair colour. These genes are present in all people, but slightly different versions, or alleles, account for the variations seen in eye and hair colour.

Genotype is a term that refers to the an individual's genetic make up. Phenotype is a term that refers to the traits or features that a person displays. The relationship between genotype and phenotype is often complicated. Although a child has half his or her genetic information from each parent, he or she is not necessarily a simple combination of the parents' traits. This is because of the complex interaction between the two alleles for each gene, between different genes, and between genes and the environment.

While most characteristics and conditions have a complex genetic and environmental basis, there are a few traits or diseases for which the role of genes is well understood. In most cases, these traits or diseases are due to different alleles of a single gene. As described below, the alleles can be dominant or recessive. The concept of dominant and recessive alleles was first described in the 1860s by a monk named Gregor Mendel who studied the relationship between genotype and phenotype in pea plants. Therefore, traits or diseases which are due to single genes are sometimes called Mendelian. top

Autosomal dominant inheritance

Autosomal recessive inheritance

Cystic fibrosis is caused by changes in a gene called CFTR. Everyone has two copies of the CFTR gene, one inherited from each parent. Some people have inherited one copy of the gene which has a change in it that stops it from working properly. They are called carriers of cystic fibrosis. Being a carrier does not cause health problems, because the second, working, copy of the gene acts as a "back-up." This type of genetic change is called recessive, because the second, working, copy of the gene can compensate for the non-working copy. All people are carriers for 5 to 10 different recessive gene changes. About 1 in 25 caucasian people are carriers for cystic fibrosis. If two people who are carriers for cystic fibrosis have children, there is a 25% (1 in 4) chance that they could both pass on the copy of the CFTR gene which is not working. When a child inherits two non-working copies of the CFTR gene, one from each parent, then there is no "back-up," so he or she would have cystic fibrosis. When a child is diagnosed with cystic fibrosis, the parents are presumed to be carriers of this condition. If one parent is a carrier of cystic fibrosis and the other is not, there is a 50% (1 in 2) chance for each of their children to be carriers of cystic fibrosis, but none of their children would have this disease.

Other conditions which are inherited in an autosomal recessive pattern include thalassemia, a severe form of anemia, sickle cell anemia, and Tay-Sachs disease.

X-linked recessive inheritance

Recessive alleles on the X chromosome show a special pattern of inheritance called X-linked inheritance, because females have two X chromosomes while males have only one. A recessive allele on the X chromosome does not influence the phenotype in females if it is present in only one copy, because there is a second copy to block it. However, the same allele present in one copy in males does influence the phenotype, because there is no second allele to block it. For this reason, the phenotype resulting from X-linked recessive alleles appears more frequently in males than in females. For example, certain types of color blindness are X-linked and are more common in males.

X-linked dominant inheritance

Punnet's squares

How to interpret this diagram - This diagram is called a Punnet's square. It is used to determine the possible combination of alleles for a certain gene. The parents' alleles are listed on the sides of the box, and all the possible combinations of alleles for the children are listed in the compartments of the box. Important observations: The dominant allele produces a phenotype, even when it is present in only one copy. The recessive allele only produces a phenotype when it is present in both copies (i.e. there is no dominant allele to block it). Parents who have one dominant and one recessive allele are called carriers of the recessive trait. They have the phenotype of the dominant allele, but approximately 1/4 of their children will have two recessive alleles and display the recessive phenotype.

Many of the genetic diseases that we know of are inherited in either a dominant or recessive fashion. For example, Cystic Fibrosis (CF) is a recessive disease. When two people who are carriers of CF have children, each child has a 1/4 (25%) chance of getting the recessive allele from both parents and therefore have the CF phenotype. Huntington disease is a dominant condition. People who have the disease allele of the Huntington gene, even in one copy, exhibit the phenotype of the disease. A database of the human traits and conditions that are inherited in a Mendelian fashion is available through Online Mendelian Inheritance in Man (OMIM).

While the model of dominant and recessive alleles is useful for understanding many traits that are determined by a single gene, most human characteristics are actually determined by interactions between many genes, as well as environmental factors. For more discussion of these topics, see the section on The role of genes in determining our characteristics.

Note: Human genetics is an extremely complex topic. This website is meant only as an introduction and overview. If you are concerned about how genetics may affect your health, always consult your physician.