About Pedigree Analysis
It is easy to represent a thing in a diagram manner than in terms of theory, which also helps in easy understanding.
Pedigree analysis is used to represent traits of an organism or an individual so that we can identify the genotypes and phenotypes of the future generation.
It also helps us to predict the characteristics of future generation by assuming with the present generation characteristics, which is very helpful for identifying the genetic diseases and thus we can provide counseling for the parents in such condition which helps them to take a better decision.
In human beings-controlled crosses cannot be made, so genetists discovered the method for undergoing crosses to find an individual with established mating’s.
The information that has obtained by established mating’s is by a chance or a hope. This method of identifying the possibility is referred to as pedigree analysis.
The individual in a family who comes first in attention of a genitists is known as propositus.
Generally, the propositus phenotype sometimes causes exceptional, where dwarf character appears in such cases the investigator traces the history of a character and analyses the change by tracing the family tree or pedigree chart.
Pedigree chart is drawn by using certain symbols. A pedigree helps us to represent the traits and it also gives an understanding of genotypes and phenotypes that will be passing to through the inheritance.
It also relates the relationship between the parents, offspring and their siblings.
It also helps in better understanding of the inheritance within the family of a particular individuals.
How to Read a Pedigree
Before knowing how to relate the traits in pedigree it is important to know the specific terms. And few of them are listed below
Thus, these symbols are commonly used to indicate the traits in a pedigree.
A family pedigree chart usually has circles to indicate the female individuals, squares to indicate the male individuals and perpendicular line drawn to the marriage bar indicates the off springs.
The symbols which are present on the same line are considered as they are from the same generation and the generations are denoted by roman numerical as I, II etc.
The unshaded symbols shows that they are unaffected individuals and the shaded symbols shows they are partially or fully affected depending upon their shaded area, the arrow indicates the specific person who is carrying the defective trait throughout his generations.
Considering an example of polydactyl, which means that some individuals have extra fingers in their limbs.
When the pedigree illustration is done for this, a marriage has been done between polydactyl man and a normal woman they reproduce three children’s where the first generation has a polydactyl daughter and a son and the other normal son.
In second generation when these polydactyl individuals marry a normal person their reproductive off springs are shown in the graph below.
However, from this polydactyl pedigree chart it is clear that polydactyl condition is playing a dominant role on recessive i.e., normal trait.
Determination of Particular Trait
If the trait is dominant then it will be sure that one of the traits will be obtained from the parents because dominant genes don’t skip the generation.
However, neither trait will be obtained from the parents if it is heterozygous.
Determination of Autosomal and Sex-linked Trait
Males are usually most affected than females in a sex-linked recessive inheritance. Where as in autosomal recessive conditions male send female are equally affected.
The other terms to be taken into consideration while reading a pedigree chart is;
Genotype: Genetic characteristics of an organism., where TT denotes homozygous tall.
Phenotype: Phenotype indicates the physical characters of an individual., where Tt heterozygous tall also considered as a tall.
Dominant Allele: An allele which is phenotypically dominant over the other allele.
Recessive Allele: This allele is expressed only in cases where the dominant alleles are absent or unable to express.
How to Read a Pedigree Chart?
Mendelian recessive alleles are very helpful in determining the human disease and other exceptional condition.
The pedigree chart can be read easily with above sorted clues. When the diseased condition appears in the progeny of the unaffected parents.
Also, at the same time the affected parents cannot have the unaffected individuals these chances are more when recessive alleles are revealed by consanguineous marriages where the best example is cousin marriages.
This also suits the conditions where the rate of mattings of heterozygotes are expected to be rare in cases of albinism, Tay- Sachs disease, cystic fibrosis and phenylketonuria.
There are also some conditions which are not caused by dominant alleles. This condition often occurs in every generations, where unaffected individual transmit the condition to their offspring in case if they are a carrier.
In other case when two parents are affected, they are also chancing of being a child to be unaffected where they get their trait from their grandparents and these unaffected individuals passes a diseased condition to their children’s.
Examples of abnormal conditions caused by dominant alleles are Huntington’s chorea and brachydactyly which means these individuals have very short fingers.
This pedigree analysis also helps in identifying the phenotypical and genotypical traits in plants as well as in insects as in humans to identify the inheritance passed to the upcoming generations.
Pedigree Analysis Citations
- Peditree: pedigree database analysis and visualization for breeding and science. J Hered . Jul-Aug 2005;96(4):465-8.
- Pedigree analysis for conservation of genetic diversity and purging. Genet Res (Camb) . 2009 Jun;91(3):209-19.
- Pedigree analysis of Czech Holstein calves with schistosoma reflexum. Acta Vet Scand . 2012 Apr 2;54(1):22.
- Enabling Pedigree Visualization and Analysis in tranSMART. Stud Health Technol Inform . 2018;253:75-79.