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.
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.
Let us begin by drawing the pedigree described below (which is not necessarily an autosomal dominant condition and which contains extraneous information).
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.
Draw the information one step at a time in a logical manner.
Step 1
Begin with Alice, Bob and Charles.
Here are three possible drawings of this nuclear family.
Click on the one which is correctly drawn.
Back to the beginning of the session 