Dominant inheritance:
A characteristic is considered dominant if it manifests itself in a heterozygous situation. It is the easiest type of Mendelian inheritance to recognize in a pedigree as only one mutant allele is needed for the phenotype to become evident. For the traits that are exhibiting dominant gene action:
- Apart from the extremely rare cases where mutations happen by themselves, the affected individual must have at least one parent with the disorder - the disorder cannot skip a generation.
- The phenotype will appear in every generation.
- If both parents are unaffected, then their offspring will also be unaffected.
- Males and females have an equal chance of getting the disease. The recurrence risk of each child of an affected parent is 50%.
- Siblings of the affected individual that do not have the disease do not transmit the trait to their children.
- The defective product in the gene is usually a structural protein.
People with genetic disorders that have been inherited with a dominant trait are usually heterozygous. Although they can be homozygous, those that are are much more severely affected than the heterozygous individuals, Examples of such disorders are achondroplasia, Peutz-Jeghers syndrome and Waardenburg syndrome. Also, there are reasons why an individual with this type of disorder should always be considered to be heterozygous until they are proven to be homozygous:
- These types of diseases are fairly rare, with an incidence rate of 1 in 10 000. This ratio is for a heterozygous individual. In order to produce a homozygous patient, two heterozygous people with the disease would have to mate - the chances of this happening is 1 in 1 000 000. Even after they had mated, the couple still only have a 25% chance of producing a homozygous individual.
- In the rare case that two heterozygous individuals have mated and produced a homozygous sufferer, the child is usually so severely affected by the disease and its symptoms that their chances of survival are very slim - the only exception to this being Huntington's disease. Also, dominant diseases possess much more extreme symptoms than recessive disorders do, thus, people with these conditions usually die before they have the opportunity to reproduce.
- The most likely way in which two heterozygous individuals with the disease will mate is if they are closely related - this is forbidden in our society.
- Apart from the extremely rare cases where mutations happen by themselves, the affected individual must have at least one parent with the disorder - the disorder cannot skip a generation.
- The phenotype will appear in every generation.
- If both parents are unaffected, then their offspring will also be unaffected.
- Males and females have an equal chance of getting the disease. The recurrence risk of each child of an affected parent is 50%.
- Siblings of the affected individual that do not have the disease do not transmit the trait to their children.
- The defective product in the gene is usually a structural protein.
People with genetic disorders that have been inherited with a dominant trait are usually heterozygous. Although they can be homozygous, those that are are much more severely affected than the heterozygous individuals, Examples of such disorders are achondroplasia, Peutz-Jeghers syndrome and Waardenburg syndrome. Also, there are reasons why an individual with this type of disorder should always be considered to be heterozygous until they are proven to be homozygous:
- These types of diseases are fairly rare, with an incidence rate of 1 in 10 000. This ratio is for a heterozygous individual. In order to produce a homozygous patient, two heterozygous people with the disease would have to mate - the chances of this happening is 1 in 1 000 000. Even after they had mated, the couple still only have a 25% chance of producing a homozygous individual.
- In the rare case that two heterozygous individuals have mated and produced a homozygous sufferer, the child is usually so severely affected by the disease and its symptoms that their chances of survival are very slim - the only exception to this being Huntington's disease. Also, dominant diseases possess much more extreme symptoms than recessive disorders do, thus, people with these conditions usually die before they have the opportunity to reproduce.
- The most likely way in which two heterozygous individuals with the disease will mate is if they are closely related - this is forbidden in our society.