pedigrees:
There is a large number of genes that regulate the growth and development of the body. Also, one gene may influence more than one trait, thus making it very difficult to clearly determine if a trait is dominant or recessive. However, one of the easiest ways to determine inheritance within a family is through the employment of a pedigree. A pedigree is simply a family tree which shows the history of a particular genetic disease. The construction of pedigrees must adhere to certain rules to ensure that it is clear and the history of the disease is able to be followed with ease. A number of symbols are used to help aid this process:
- Males = squares.
- Females = circles.
- Those that possess the characteristic in question = shaded.
- Union/marriage = horizontal line joining symbols.
- Union between close relatives/consanguineous union = double horizontal lines.
- Children = horizontal line extending from union.
- Generational numbers = indicated by Roman numerals on the left hand side.
- Individuals = numbered left to right using normal numbers.
The pedigree can then be constructed according to the situation given.
Pedigrees can also be used to work out the probability of inheritance. Probability is the chance that something will happen, and pedigrees are a visual representation of what could potentially occur within a family. Once the pedigree is constructed and the rules have been adhered to, one is able to calculate their chance or the chance of someone they love inheriting the disorder. However, in doing so, it is important to remember that genetic disorders are highly variable and thus difficult to predict. The prediction derived from a pedigree should only be taken as a potential indicator of what may happen in the future. The proportion of affected children predicted by a pedigree may not be the best indicator however, as there is not a large enough sample space.
However, pedigree analysis has lead to the discovery of certain patterns of inheritance:
- Every person that shows the dominant characteristic needs only one allele for that characteristic and every person possessing such an allele must exhibit the characteristic.
- A person with a dominant characteristic must have at least one parent with that characteristic - it cannot skip a generation.
- Two people with a dominant characteristic can have a child without that characteristic.
- Two people that do not possess a dominant characteristic cannot have a child with the characteristic.
- Every person that exhibits a recessive characteristic must have two alleles of that characteristic (homozygous).
- A person with a recessive characteristic does not have to have a parent with that characteristic - it can skip a generation.
- Two people without a recessive characteristic can have a child with the characteristic.
- Two people with the recessive characteristic cannot have a child without it.
- Characteristics that are linked to the X chromosome cannot be passed on from father to son - fathers can only continue their characteristics through their daughters.
- Two people without an X-linked characteristic can have a son with the characteristic, but no daughter.
- Males = squares.
- Females = circles.
- Those that possess the characteristic in question = shaded.
- Union/marriage = horizontal line joining symbols.
- Union between close relatives/consanguineous union = double horizontal lines.
- Children = horizontal line extending from union.
- Generational numbers = indicated by Roman numerals on the left hand side.
- Individuals = numbered left to right using normal numbers.
The pedigree can then be constructed according to the situation given.
Pedigrees can also be used to work out the probability of inheritance. Probability is the chance that something will happen, and pedigrees are a visual representation of what could potentially occur within a family. Once the pedigree is constructed and the rules have been adhered to, one is able to calculate their chance or the chance of someone they love inheriting the disorder. However, in doing so, it is important to remember that genetic disorders are highly variable and thus difficult to predict. The prediction derived from a pedigree should only be taken as a potential indicator of what may happen in the future. The proportion of affected children predicted by a pedigree may not be the best indicator however, as there is not a large enough sample space.
However, pedigree analysis has lead to the discovery of certain patterns of inheritance:
- Every person that shows the dominant characteristic needs only one allele for that characteristic and every person possessing such an allele must exhibit the characteristic.
- A person with a dominant characteristic must have at least one parent with that characteristic - it cannot skip a generation.
- Two people with a dominant characteristic can have a child without that characteristic.
- Two people that do not possess a dominant characteristic cannot have a child with the characteristic.
- Every person that exhibits a recessive characteristic must have two alleles of that characteristic (homozygous).
- A person with a recessive characteristic does not have to have a parent with that characteristic - it can skip a generation.
- Two people without a recessive characteristic can have a child with the characteristic.
- Two people with the recessive characteristic cannot have a child without it.
- Characteristics that are linked to the X chromosome cannot be passed on from father to son - fathers can only continue their characteristics through their daughters.
- Two people without an X-linked characteristic can have a son with the characteristic, but no daughter.