Pedigree chart

A pedigree chart is a diagram that shows the occurrence and appearance of phenotypes of a particular gene or organism and its ancestors from one generation to the next,[1][2][3] most commonly humans, show dogs,[4] and race horses.

Example of a pedigree chart using Ahnentafel numbering

The word pedigree is a corruption of the Anglo-Norman French pé de grue or "crane's foot", either because the typical lines and split lines (each split leading to different offspring of the one parent line) resemble the thin leg and foot of a crane[5] or because such a mark was used to denote succession in pedigree charts.[6]

A pedigree results in the presentation of family information in the form of an easily readable chart. It can be simply called as a "family tree " Pedigrees use a standardized set of symbols, squares represent males and circles represent females. Pedigree construction is a family history, and details about an earlier generation may be uncertain as memories fade. If the sex of the person is unknown a diamond is used. Someone with the phenotype in question is represented by a filled-in (darker) symbol. Heterozygotes, when identifiable, are indicated by a shade dot inside a symbol or a half-filled symbol.

Relationships in a pedigree are shown as a series of lines. Parents are connected by a horizontal line and a vertical line leads to their offspring. The offspring are connected by a horizontal sibship line and listed in birth order from left to right. If the offspring are twins then they will be connected by a triangle. If an offspring dies then its symbol will be crossed by a line. If the offspring is still born or aborted it is represented by a small triangle.

Each generation is identified by a Roman numeral (I, II, III, and so on), and each individual within the same generation is identified by an Arabic numeral (1, 2, 3, and so on). Analysis of the pedigree using the principles of Mendelian inheritance can determine whether a trait has a dominant or recessive pattern of inheritance. Pedigrees are often constructed after a family member afflicted with a genetic disorder has been identified. This individual, known as the proband, is indicated on the pedigree by an arrow.[7] These changes may occur yearly or monthly.

• 
  In a Y-linked disorder, only males can be affected. If the father is affected all sons will be affected. It also does not skip a generation.
• 
  In mitochondrial disorders it is only passed on if the mother is affected. If the mother is affected, all offspring will be affected. If the father is affected, he does not pass it on to his offspring.
• 
  In an autosomal recessive disorder, both parents can not express the trait, however, if both are carriers, their offspring can express the trait. Autosomal recessive disorders typically skip a generation, so affected offspring typically have unaffected parents. With an autosomal recessive disorder, both males and females are equally likely to be affected.
• 
  Autosomal dominant disorders do not skip a generation, so affected offspring have affected parents. One parent must have the disorder for its offspring to be affected. Both males and females are equally likely to be affected, so it is an autosomal disorder.
• 
  In a X-linked recessive disorder, males are more likely to be affected than females. Affected sons typically have unaffected mothers. The father also must be affected for daughter to be affected and the mother must be affected or a carrier for the daughter to be affected. The disorder is also never passed from father to son. Only females can be carriers for the disorders. X-linked recessive disorders also typically skip a generation.
• 
  In a X-Linked dominant disorder, if the father is affected all daughters will be affected and no sons will be affected. It does not skip a generation and if the mother is affected she has a 50% chance of passing it onto her offspring.