Pedigree analysis is a record of inheritance of a genetic trait for two or more generations. It is presented in the form of a diagram or chart or family tree. It is employed in case of human beings and domesticated animals, especially pets.
Controlled crosses performed in pea plants or some other organisms are not possible in case of human beings.
Long generation time (twenty years or more) and few offspring’s per couple makes man unsuitable for studying Mendelian inheritance for certain trait. In human genetics, pedigree study is a strong tool utilised to trace the inheritance of a specific trait, abnormality or disease.
By pedigree analysis the mode of inheritance of a number of diseases and traits such as hemophilia, colour blindness, syphilis, insanity, rheumatism, hysteria etc. among children can be predicted. In a pedigree chart, it is conventional to use circles for females and square for males (Fig.5.25).
The normal character is represented by hollow symbols, bold symbol shows the trait under study and a cross or shade (of any type) is the symbol that signifies the carrier of a recessive allele. Matings or marriages are shown by horizontal line while their offspring are connected to it by a vertical line. Each generation is listed on a separate row labeled with Roman numerals. Individuals within a generation receive Arabic numerals.
Hemophilia or the “royal disease” is the most notorious of all sex-linked diseases. It is also commonly called as “bleeder’s disease” as the person suffering from this disease (hemophiliacs) bleed for a very long period (from 1/2 to 22 hours or more) without coagulation of blood on receiving a minor injury.
Thus, the hemophiliacs are in constant danger of death from excessive bleeding. A specific substance (antihaemophilic globulin in haemophilic. A and plasma thromboplastin in haemophilic B) which is present in the plasma of normal individuals is absent in the plasma of hemophiliacs.
As a result the coagulation time prolongs (the normal coagulation time is 2-8 minutes). It is found that a gene located on the X chromosome (in normal individual) is responsible for the production of coagulating substance. A mutation of this gene causes the coagulating substance not to be produced in proper quantity resulting in haemophilia.
A statistical analysis reveals that the frequency of haemophilic male birth is about only 1 in 10,000 and the frequency of homozygous female birth is about only 1 in 100,000.000. This reveals that haemophilia in females is very rare compared to their male counter parts. A girl with severe bleeding dies before adolescence.
Figure 5.26 shows the pedigree of haemophilia in the descendants of Queen Victoria of England. It is believed that the gene for haemophilia arose as a mutation in a reproductive cell in one of the parents of Queen victoria. She had one haemophilic son (Leopold) and two carrier daughters (Alice and Beatrice).
Leopold, the Duke of Albany had one carrier daughter (Alice). Alice had one haemophilic son (Federick William) and two carrier daughters (Irene and Alexandra). Beatrice had two haemophilic sons (Leopold and Maurice) and one carrier daughter (victoria Eugenie).
The crown prince of Spain, Prince Alfonso (haemophilic son of victoria Eugenie) died in Miami, Florida as a result of a minor injury received in an automobile accident. This death brought to mind other untimely deaths of members of royal families of Europe because of haemophilia. This disease had a great influence in shaping the history of the world.