Reading a Pedigree, and the Special Case of Mitochondrial Inheritance

The symbols of a pedigree

A pedigree is a standardized diagram of a family across generations, and it is the everyday tool of genetic counseling. Each generation is a row; squares are males, circles are females, and a horizontal line between them is a mating. Children hang below, oldest on the left. A filled-in shape means the person is affected; an empty shape, unaffected. A shape with a dot or half-fill often marks a known carrier.

Pedigree symbols

   [ ]  unaffected male       ( )  unaffected female
   [#]  affected male         (#)  affected female
   [.]  carrier male          (.)  carrier female

   [ ]---( )    a mating (horizontal line)
      |
   ---+---      sibship line; children hang below
   |     |
  [ ]   ( )     left = older

Generations are numbered with Roman numerals (I, II, III)
top to bottom; individuals left to right with numbers.

Deducing the pattern

Reading a pedigree is detective work. You scan for clues that fit one pattern and rule out others.

1. Does the trait appear in every generation, with affected people usually having an affected parent? That points to autosomal dominant (especially if it does not skip and affects both sexes equally).
2. Does it skip generations, appearing in children of two unaffected parents, affecting both sexes? That suggests autosomal recessive — the parents are carriers.
3. Are males affected much more than females, with no father-to-son transmission, often inherited through carrier mothers? That is the signature of X-linked recessive.
4. Is the trait passed only from mothers — never from an affected father — to all of their children? Suspect mitochondrial inheritance (the next section).

Mitochondrial inheritance: down the maternal line

One pattern ignores the nuclear chromosomes entirely. Mitochondria — the cell's energy organelles — carry their own small loop of DNA. At fertilization, the egg supplies essentially all the mitochondria; the sperm's are not transmitted. So mitochondrial inheritance runs strictly through mothers.

On a pedigree this is unmistakable: an affected mother can pass the trait to all of her children, sons and daughters alike, but an affected father passes it to none. The trait then continues only through his affected daughters' children, never his sons'. That asymmetry — every child of an affected woman at risk, no transmission through men — is the fingerprint of mitochondrial inheritance, and it breaks every autosomal and X-linked rule from earlier guides.

Mitochondrial inheritance on a pedigree

Gen I:    (#)affected mother ---- [ ]unaffected father
             |        |        |
Gen II:    (#)      [#]       (#)     <- ALL children affected
            ...                          (both sexes)

VS. affected FATHER:

Gen I:    ( )unaffected mother ---- [#]affected father
             |        |        |
Gen II:    ( )       [ ]       ( )    <- NO children affected

Rule: passed only by mothers; an affected father
      transmits it to none of his children.