A particular disorder might be described as “running in a family” if more than one person in the family has the condition. A genetic counsellor can use a person’s family history to draw a pedigree (a record of health information about a person’s immediate and extended family) and to help determine whether a disorder has a genetic component.
A Representative Pedigree with affected (black) and unaffected individuals (white)
Some genetic conditions are caused by variations in a single gene, which is usually inherited in one of several straightforward patterns given below:
Autosomal Dominant Inheritance:
One variant copy of the gene is sufficient for a person to be affected with an autosomal dominant disorder. Each affected person usually has one affected parent. Autosomal dominant disorders tend to occur in every generation of an affected family and in some cases maybe denovo, then the disorder appears as sporadic in the family.
In this example, a man with an autosomal dominant disorder has two affected children and two unaffected children. These types of disorders can occur in both male and females and affected individuals have 50% chance of having affected offspring.
Autosomal Recessive Inheritance:
Both copies of the gene are required to be altered for a person to exhibit an autosomal recessive disorder. An affected person usually has unaffected parents who each carry a single copy of the variant gene (and are referred to as carriers). Autosomal recessive disorders are typically seen in consanguineous marriages (i.e., marriage between relatives) or marriages in the close-knit community.
Two unaffected carrier parents have a 25% risk of having affected children and 50% risk of having carriers. Both males and females are equally affected.
X linked dominant inheritance:
X-linked dominant disorders are caused by variations in genes on the X chromosome. Both females and males are equally affected and the chance of passing on an X-linked dominant disorder differs between men and women. A mother can pass it on to 50% of both male and female offspring, where as father can pass it on only to all his daughters. Families with an X-linked dominant disorder often have both affected males and females in each generation. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons (no male-to-male transmission).
A).
In this example, a man with an X-linked dominant condition has two affected daughters and two unaffected sons.
B)
In this example, a woman with an X-linked dominant condition has an affected daughter, an affected son, an unaffected daughter, and an unaffected son.
X linked recessive Inheritance:
X-linked recessive disorders are also caused by variations in genes on the X chromosome. Males are more frequently affected than females. Females are usually carriers and have 50% risk of having affected males and 50% carrier females.
Families with an X-linked recessive disorder often have affected males, but rarely affected females, in each generation. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons (no male-to-male transmission) but all daughters are carriers.
A)
In this example, a man with an X-linked recessive condition has two unaffected carrier daughters who each carry one copy of the gene variant, and two unaffected sons who do not have the variant.
B)
In this example, an unaffected woman carries one copy of a gene variant for an X-linked recessive disorder. She has an affected son, an unaffected carrier daughter, and two unaffected children who do not have the variant.
Codominance Inheritance:
In this both copies of the gene express and each makes a different protein. As a result, there are four types of inheritance. Both males and females contribute equally.
The ABO blood group is a major system for classifying blood types in humans. Blood type AB is inherited in a co-dominant pattern. In this example, a father with blood type A and a mother with blood type B have four children, each with a different blood type: A, AB, B, and O.
Mitochondrial Inheritance:
It is also known as maternal inheritance, as it is concerned with genes present on the mitochondrial DNA which is extra chromosomal i.e. not present in the nucleus within chromosomes but in the Mitochondria present in the cytoplasm. Therefore it is contributed only by the mother as the egg and not the sperm contribute mitochondria to the developing embryo.
Hence, only females can pass on mitochondrial variants to their children. Disorders resulting from these can appear in every generation of a family and can affect both males and females, but fathers do not pass these disorders to their children.
A woman with a mitochondrial DNA variant will have all affected children. While a man with a mitochondrial DNA variant will have all unaffected children.
Complex disease Inheritance:
Many other disorders are caused by a combination of the effects of multiple genes or by interactions between genes and the environment. Such disorders are more difficult to analyze genetically, and they do not follow the patterns of inheritance described above. But they have variants in multiple genes which together increase risk of the disease.