Conditions caused by a mutation in the mitochondrial DNA have unusual patterns.
- both males and females are affected
- the condition is transmitted through the female to her offspring
- if a male has the trait and his spouse doesn’t, their offspring won’t have the trait
In the first pedigree, you will see that when a female has the trait, all of her offspring have the trait, but in the second pedigree, this is not necessarily the case. The expression of mitochondrial conditions is variable. Inside each cell, there are several mitochondria. The number of mitochondria that carry the mutation can vary. A certain proportion of mutant mitochondria within a cell can be tolerated and the disease will not be expressed in the organism. A larger proportion of mutant mitochondria however, may cause the disease to be expressed in the organism.
The first pedigree shows how inheritance is transmitted through the female, in the cytoplasm of her egg cell. The second pedigree shows how the expression can vary depending on the proportion of mitochondria carrying the mutation.