Human Pedigrees Pedigrees are a convention for keeping track of human genetic traits used to infer genotype. Pedigrees are the human equivalent of test crosses. In a visualization of a pedigree: males are designated with square symbols. females with round symbols lines are drawn to indicated matings, parent-offspring relationships, and relationships between siblings.

Drawing Pedigrees requires an old fashioned pencil and paper

Symbols Before we consider human inheritance we need to learn the symbols used to draw pedigrees. The figure below shows some of the commoner symbols:

Note that the symbols for non-identical twins and for identical twins differ by whether they descend from a common vertical before bifurcating

Pedigrees Generations are numberered from the top of the pedigree in uppercase Roman numerals, I, II, III etc. Individuals in each generation are numbered from the left in arab numberals as subscripts, III1 , III2, III3 etc. For example, here is a typical autosomal dominant pedigree with numbered generations and individuals.

Sample Pedigree

Try It! Let us begin by drawing the pedigree described below (which is not necessarily an autosomal dominant condition and which contains extraneous information).

The Scenario Alice and Bob have a two year old son, Charles, who is showing mental retardation, short stature, micropenis, and cryptorchidism. Alice has two living, unaffected, brothers but her eldest brother died at age 9 and a second brother died aged 10 months. Both had similar problems to Charles. Alice's father, David, who was symptomless, has a sister, Ethel, who has an unaffected boy and girl, and a brother, Fred, who also has two unaffected children. Alice's mother, Gertrude, has two living sisters and had a brother who had died in childhood and who, she remembers, had been mentally retarded. Bob has two brothers, Henry and Ignatius, who are still unmarried. His parents, John and Kate, had tragic lives, both were adopted and never knew their biological parents and both died as the result of a road accident.

Step 1 Begin with Alice, Bob and Charles. Here are three possible drawings of this nuclear family.

Correct Solution Alice and Bob are connected by a horizontal line to show that this is a mating. Charles is connected to that horizontal line to show that he is a product of that mating.

Step 2 Now add Alice's siblings and parents to the pedigree.

Choose One…

Correct Solution was Possiblilty 3 Alice and her four brothers are connected vertically to a horizontal line which is, in turn, connected to the line drawn between her parents David and Gertrude. Her two dead brothers (whom we presume died of the same genetic disease - though this can sometimes be a foolish assumption without medical evidence) are shaded in (to show that they suffered from the disease) and are crossed through (to show that they are dead).

Step 3 Now add Gertrude's siblings to the pedigree. And David's siblings and his nephews and nieces Finally add Bob's side of the family Try Drawing it!!!

Correct Drawing!