First: Look for Mitochondrial Inheritance
- Female transmits disease to all the offsprings (both males and females).
- Male doesn’t transmit the disease and only the females transmit the disease.
If Mitochondrial inheritance is absent, go to second step.
Second: Look if the gene is Dominant, Recessive
- Dominant:
- Atleast one member in all generations will have the disease.
- Both affected parents can produce normal offsprings. Parents must be heterozygous.
- Recessive inheritance:
- There will be some generations without the disease also (skips generation).
- Both normal parents can produce affected offsprings. In such case, parents must be heterozygous.
Third: Look if the disease is X-linked or Autosomal or Y-linked
- X-linked dominant:
- Male transmits disease only to the daughters (all daughters).
- Female transmits disease to half of sons and half of daughters.
- X-linked recessive:
- If only males are affected, it is likely to be X-linked recessive.
- Female only acts as carriers and remain unaffected.
- Autosomal dominant:
- No sex predilection
- Affected individual transmits disease to 1/2 of offsprings.
- Autosomal recessive:
- No sex predilection
- Affected individual transmits disease to 1/4 of offsprings.
- Consanguinity increases risk of autosomal recessive disorders.
- Y-linked (Holandric):
- Never skips generation
- Only males are affected (father transmits the disease to son).
Codominant Inheritiance
This is similar to autosomal dominant inheritance but in codominant inheritance, two different versions (alleles) of a gene can be expressed, and each version makes a slightly different protein. e.g. Blood group, HLA.
