A pedigree is an ancestral history of a person or animal, written as an organized diagram showing breeding events and traits of the individuals involved. A pedigree is a tool geneticists use, when studying the relationship between a gene and observed traits. By studying several generations in a pedigree, a geneticist can determine whether a trait is genetically determined. They can also determine from a pedigree if a trait is caused by multiple alleles of a single gene, multiple genes, or if the trait is mainly influenced by environmental factors.
How to Read a Pedigree
Above is a typical pedigree. The males are represented by squares, the females by circles. Sometimes a diamond will be used in instances where the sex of an individual is unknown. Deceased individuals will have a slash through them, while living individuals do not. The above pedigree tracks a single trait, indicating with a black fill if the individual is affected or not.
The vertical lines represent offspring. The horizontal lines between individuals represent breeding events. In most human pedigree charts, the horizontal lines often indicate marriages, because many humans practice monogamy. In other species, the lines may cross and jump across the chart. This is especially true of animals that do not practice any sort of monogamy.
Many more traits can be indicated by a pedigree. A double horizontal line between individuals represents a consanguineous mating, or a mating between related individuals. Though this is less common in humans, it happens consistently in certain groups of organisms.
While the above pedigree is relatively simple, geneticists can incorporate a variety of information into a pedigree, to understand complex genetic conditions. Typically, this means comparing the frequency of a trait within a family to the general population. Further, the more generations a geneticist can observe, the more accurate their interpretation of the underlying genetics. Studying the patterns of a pedigree, a geneticist can tell if a trait is based on a recessive or dominant allele. They can also tell if the trait is on an autosomal chromosome, or if it is sex-linked.
Below are some examples of a pedigree, and how a geneticist would use the information to understand
Pedigree of Cystic Fibrosis
Cystic fibrosis is a genetic disease affecting the chloride ion channels within cells of a person’s mucous membranes. Without this protein to shuffle chloride ions, a thick, salty mucous builds up on the inside of many organs including the lungs and pancreas. The reduced functioning of these organs leads to a variety of terrible symptoms, and can lead to a much shortened lifespan.
Seen above is a pedigree for a family with cystic fibrosis. Using charts like this, geneticists were able to determine that cystic fibrosis is an autosomal recessive disorder. Take a look at the pedigree. First, you will notice that two affected parents always pass the disorder onto their children. This can be seen by all the black circles and squares in the first and second generation. These parents both had the disorder, as did all of their children.
In the second generation, one of the affected children marries a non-carrier of the disease. None of their children have the disease but are carriers of the recessive allele. This is identified by the half black squares and circles. This pattern is called horizontal inheritance. These facts, in combination, show two things.
First, the disease is recessive, and not dominant. If it were dominant, children could not be “carriers” of the disease. This is only possible in a recessive condition because you can have one functioning allele and one diseased allele. In a dominant condition, the “carriers” would have the disorder. Second, the geneticists can tell that the disorder is autosomal (resides on an autosomal chromosome). This can be done by comparing the rate at which males and females get the disease. In this pedigree, it is equal, therefore it can be assumed that the disorder is not on one of the sex chromosomes.
Pedigree of Huntington Disease
Another genetic disorder which shows a different pedigree is Huntington disease. The disorder causes a deterioration of the nervous system, which cumulates in a loss of intellect, emotions, and physical control over the body. This is a late-onset disorder has been shown to be genetically inherited. Look at the following pedigree and see for yourself:
In this pedigree, you see that in a combination of an affected individual and a non-affected individual leads to three affected offspring. If this were a recessive disease, the offspring would at most be half-affected, and some would be carriers. This means that Huntington disease is a dominant disorder, and a single non-functional allele can cause the disease. If this were a common allele, such as eye color, the evidence would not be as strong. However, because the allele is so rare, it can be proven that the disease is inherited in familial lines. This form of inheritance is known as vertical inheritance, because typically at least one child in every generation will be affected, if not more.
Other Uses of a Pedigree
Pedigrees have been used for centuries, not by geneticists, but by animal breeders. These individuals track the lines and traits of the animals they are breeding. Knowing these traits allows them to artificially select for traits, and cultivate the characteristics they are seeking. It is in this way that all modern dogs and agricultural animals have been breed. In the case of the dog, this has resulted in hundreds of different forms, all which come from an ancestor not much different from a wolf.
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