Types of Inheritance in Medical Genetics - Autosomal Dominant and Autosomal Recessive Inheritance
The main types of inheritance in Medical genetics are as follows: Autosomal recessive, Autosomal dominant, X-linked recessive, and X-linked dominant.
The pattern of inheritance is determined by taking the family history to cover there generations in full and constructing a pedigree.
Autosomal recessive inheritance 1.
The affected individuals are in one generation.
This is, therefore, called horizontal transmission.
2.
Males and females are equally affected.
3.
There is a 25% chance of recurrence in siblings.
In other words, on an average, one-fourth of the children are affected.
4.
Both parents are carriers of the disease.
In the affected offspring one mutant gene comes from one parent, the other mutant gene in the pair is from the other parent.
5.
There is often consanguinity in the parents.
The more likely it is that the parent are consanguineous.
Each individual is the carrier of 6-8 harmful autosomal recessive genes.
In a consanguineous marriage, there is a greater likelihood that the offspring would inherit the same mutant gene from the parents, as a proportion of the genes are common in consanguineous parents, for example-Friedreich's ataxia, Phenylketonuria, Fanconi's syndrome.
Autosomal dominant inheritance 1.
The affected individuals are in more than one generation, and if, in the pedigree one draws a line through the affected individuals, it will be a vertical line.
Hence it is also called 'vertical transmission'.
2.
Males and females are equally affected.
3.
There is a 50% chance of recurrence in siblings, irrespective of sex.
In other words, on an average, one-half of the children are affected.
4.
A male-to-male transmission provides evidence that the disease is not X-linked but is autosomal dominant.
5.
Occasionally, both the parents of an affected child are normal.
In such cases, it is presumed that the offspring has acquired the disease by a new mutation in the gametes of one of the parents.
The more an autosomal disorder limits reproduction, the higher is the rate of new mutation for that disorder.
Where the disorder arises by a new mutation, the risk of recurrence in future siblings is negligible.
Occasionally, an offspring affected with an autosomal dominant disorder is born to normal parents, while one of the grandparents is affected.
In such cases, it is presumed that the parent on the side of the affected grandparent has the gene but is not manifesting the disease.
Such genes are considered non-penetrant.
6.
The clinical manifestations in an autosomal dominant disorder vary considerably in the same family.
This is partly explained by the fact that only one o the genes of a particular pair is abnormal.
while the other gene is normal.
The degree to which the presence of a gene is expressed clinically is termed as the expressivity.
If the expressivity is reduced to an extent so that there is no clinical manifestation, such a gene is called non-penetrant, e.
g, Achondroplasia, Huntington's chorea, Marfian's syndrome.
The pattern of inheritance is determined by taking the family history to cover there generations in full and constructing a pedigree.
Autosomal recessive inheritance 1.
The affected individuals are in one generation.
This is, therefore, called horizontal transmission.
2.
Males and females are equally affected.
3.
There is a 25% chance of recurrence in siblings.
In other words, on an average, one-fourth of the children are affected.
4.
Both parents are carriers of the disease.
In the affected offspring one mutant gene comes from one parent, the other mutant gene in the pair is from the other parent.
5.
There is often consanguinity in the parents.
The more likely it is that the parent are consanguineous.
Each individual is the carrier of 6-8 harmful autosomal recessive genes.
In a consanguineous marriage, there is a greater likelihood that the offspring would inherit the same mutant gene from the parents, as a proportion of the genes are common in consanguineous parents, for example-Friedreich's ataxia, Phenylketonuria, Fanconi's syndrome.
Autosomal dominant inheritance 1.
The affected individuals are in more than one generation, and if, in the pedigree one draws a line through the affected individuals, it will be a vertical line.
Hence it is also called 'vertical transmission'.
2.
Males and females are equally affected.
3.
There is a 50% chance of recurrence in siblings, irrespective of sex.
In other words, on an average, one-half of the children are affected.
4.
A male-to-male transmission provides evidence that the disease is not X-linked but is autosomal dominant.
5.
Occasionally, both the parents of an affected child are normal.
In such cases, it is presumed that the offspring has acquired the disease by a new mutation in the gametes of one of the parents.
The more an autosomal disorder limits reproduction, the higher is the rate of new mutation for that disorder.
Where the disorder arises by a new mutation, the risk of recurrence in future siblings is negligible.
Occasionally, an offspring affected with an autosomal dominant disorder is born to normal parents, while one of the grandparents is affected.
In such cases, it is presumed that the parent on the side of the affected grandparent has the gene but is not manifesting the disease.
Such genes are considered non-penetrant.
6.
The clinical manifestations in an autosomal dominant disorder vary considerably in the same family.
This is partly explained by the fact that only one o the genes of a particular pair is abnormal.
while the other gene is normal.
The degree to which the presence of a gene is expressed clinically is termed as the expressivity.
If the expressivity is reduced to an extent so that there is no clinical manifestation, such a gene is called non-penetrant, e.
g, Achondroplasia, Huntington's chorea, Marfian's syndrome.