Modes of inheritance
Modes of inheritance
Genetic disorders can be passed on from generation to generation in several ways.
Autosomal Dominant Inheritance
There are two copies of every autosomal gene. When the presence of one faulty gene causes a problem despite the presence of a normal one the condition is said to be dominant. Conditions with dominant inheritance show what's known as vertical transmission, this is based on the appearance of the family pedigree. When only 1 parent is affected there is a 50% chance of each child expressing the condition. Autosomal dominant conditions often show pleiotropy. This refers to the situation where a single gene influences several characteristics. An example is Marfan's syndrome where the hypermobility of the joints, elongation of the limbs, and heart defects are caused by a single gene. Make sure not to confuse this with polyploidy which refers to a situation where there are more than 2 sets of chromosones (for example 69 chromosones in a cell).
Autosomal Recessive Inheritance
In an autosomal recessive condition a person requires two faulty copies of a gene to manifest a disease. A person with one healthy and one faulty gene will generally not manifest a disease and are labelled a carrier. Autosomal recessive conditions demonstrate horizontal transmission.
X-linked (sex-linked) Inheritance
In X-linked conditions the problem gene lies on the X chromosome. This means that all males are affected. Like autosomal conditions they can be dominant or recessive. Affected males are unable to pass the condition on to their sons.
In X-linked recessive conditions, the inheritance pattern is characterised by transmission from affected males to male grandchildren via affected carrier daughters, also known as knights move transmission.
Note: Fragile X is X-linked but cannot be classified simply as dominant or recessive. Technically it is dominant because if you have a mutated FMR1 gene then you could potentially present with features of fragile X syndrome. Having said that as it is a trinucleotide repeat disorder many people with the mutation will not have features if there are not enough repeats.
Multifactoral Inheritance
Multifactoral conditions result from interaction between genes from both parents and the environment
Genetic disorders can be passed on from generation to generation in several ways.
Autosomal Dominant Inheritance
There are two copies of every autosomal gene. When the presence of one faulty gene causes a problem despite the presence of a normal one the condition is said to be dominant. Conditions with dominant inheritance show what's known as vertical transmission, this is based on the appearance of the family pedigree. When only 1 parent is affected there is a 50% chance of each child expressing the condition. Autosomal dominant conditions often show pleiotropy. This refers to the situation where a single gene influences several characteristics. An example is Marfan's syndrome where the hypermobility of the joints, elongation of the limbs, and heart defects are caused by a single gene. Make sure not to confuse this with polyploidy which refers to a situation where there are more than 2 sets of chromosones (for example 69 chromosones in a cell).
| Autosomal dominant conditions |
|---|
| Huntington's |
| Neurofibromatosis |
| Marfan's |
| Achondroplasia |
Autosomal Recessive Inheritance
In an autosomal recessive condition a person requires two faulty copies of a gene to manifest a disease. A person with one healthy and one faulty gene will generally not manifest a disease and are labelled a carrier. Autosomal recessive conditions demonstrate horizontal transmission.
| Autosomal recessive conditions |
|---|
| Homocystinuria |
| Phenylketonuria |
| Wilson's disease |
X-linked (sex-linked) Inheritance
In X-linked conditions the problem gene lies on the X chromosome. This means that all males are affected. Like autosomal conditions they can be dominant or recessive. Affected males are unable to pass the condition on to their sons.
In X-linked recessive conditions, the inheritance pattern is characterised by transmission from affected males to male grandchildren via affected carrier daughters, also known as knights move transmission.
| X-linked dominant conditions |
|---|
| Vitamin D resistant rickets |
| Rett syndrome |
Note: Fragile X is X-linked but cannot be classified simply as dominant or recessive. Technically it is dominant because if you have a mutated FMR1 gene then you could potentially present with features of fragile X syndrome. Having said that as it is a trinucleotide repeat disorder many people with the mutation will not have features if there are not enough repeats.
| X-linked recessive conditions |
|---|
| Duchenne muscular dystrophy |
| Red green colour blindness |
| Hemophilia A and B |
Multifactoral Inheritance
Multifactoral conditions result from interaction between genes from both parents and the environment
