What is a Pedigree?
A pedigree is a graphic method of illustrating inheritance of genetic traits within several generations of families. Geneticists, genetic counselors and animal breeders all use pedigrees to answer questions about inheritance using these tools.
Examine the pedigree below. Pay special attention to the symbols that are used to construct it. Think about what a pedigree might look like for your family. Sketch one out if you can.
- Males/females are represented by different shapes
- Matings are represented by horizontal lines between shapes
- Offspring are represented by vertical lines extending down from a mating
- Children are listed in birth order from left to right
- An affected individual is one who carries or expresses the trait of interest.
In addition to these standard symbols, some pedigrees will also include a half colored symbol to indicate that an individual has one dominant and one recessive allele for the trait. This individual is called a carrier.
Hemophilia: A Royal Disease
One of the most well known family pedigrees is that of the royal families of Great Britain, Russia and several other European countries. Queen Victoria(reign 1837-1901), unknowingly carried a single recessive allele for hemophilia.
Queen Victoria of the United Kingdom of Great Britain and Ireland
Hemophilia is a recessive, X-linked disease that results in the inability for blood to clot properly. When a person with hemophilia is injured, they can form a temporary scab, but the clot will not last very long. Persons with hemophilia do not bleed more, they bleed longer and without proper repair of blood vessels. Imagine if you were injured internally and had hemophilia – it could be permanently damaging or even fatal, as you slowly bleed to death.
Examine the pedigree of Queen Victoria’s bloodline below (you can also download a larger version in the sidebar). Can you see how the disease was transmitted from mothers to affected (hemophiliac) sons, and by way of daughters who were carriers? A key is located below the pedigree to help you.
With the mutation in Queen Victoria’s genes, and each of her nine children being married off into other noble or royal families, hemophilia continued to pop up throughout the Royal Houses of Europe (Spain, Russia, and Prussia). Each case of hemophilia can be traced back to Queen Victoria using our knowledge of pedigree analysis and inheritance.