**3.2 Segregation analysis**

They are used to study the inheritance of disease within the families. Genes are passed from parents to kids in an exceedingly foreseeable manner, and typically segregate in families as foretold by Mendel's laws [15]. Pattern of transmission of disease through generations is analysed in several families and compared with those expected under different models of inheritance to choose the best fitting model.

In this way, segregation analysis helps to identify the best model that simulates the ascertained transmission of a trait in a given population by sequential comparison with all the available models. Segregation analysis is applied by geneticists to determine whether a trait transmission belongs to Mendelian mode of genetic transmission [16].

The pattern by which disease is transmitted across generations depends on whether or not disease alleles:


Genetic characteristics involving mode of transmission (e.g. autosomal, X-linked, dominant, recessive, complex, multi-locus, or random environmental), penetrance, phenocopy rates and frequencies for disease and non-disease alleles are some of the characteristics assessed in the various models. Phenotype of individual will be determined by the dominant allele. The recessive allele can be inherited only if it is located at both loci on homologous chromosomes. Penetrance refers to the possibility that a particular phenotype will rise from a genotype. Partially penetrant explains that only few individuals who inherit the disease alleles will be affected. The power of segregation analysis was dependent upon the size of population to study the observed pattern of disease.

#### *3.2.1 Advantages*

Segregation analysis helps to assess whether the disease gene is autosomal or sex linked, recessive or dominant.

#### *3.2.2 Limitations*


#### **3.3 Twin studies**

Twin studies are commonly used to study the influence of genetic and environmental factors on the complex diseases like periodontitis with multifactorial aetiology. Studying phenotypic traits of twins is a method of differentiating variations due to environmental and genetic factors [17]. Sir Francis Galton in 1875 was the first scientist to use this concept. The subject of interest in twin studies can be monozygotic or dizygotic twins.

Monozygotic twins ascend from single fertilised ovum and are therefore genetically identical and always the same sex. Dizygous twins arise from the fertilisation of two separate ovum and share one half of their descendent genes in the same way as siblings do. Concordance refers to the degree of similarity between twins in one or more characteristics whereas discordance refers to the degree of dissimilarity between twins in one or more characteristics.

*Genetics and Periodontal Disease: An Explicit Insight DOI: http://dx.doi.org/10.5772/intechopen.99266*

Only environmental factors might account for any discordance in disease between monozygotic twins [18, 19]. Environmental and genetic variation might account for any discordance in disease between dizygotic twins. Presuming that the environmental influence is constant, the effects of excess shared genes in monozygotic twins measures the difference in discordance between monozygotic (MZ) and dizygotic (DZ) twins [20, 21].

For binary traits (present or absent), a genetic effect is inferred if the positive concordance rate, or percentage of twin pairs in which both twins are affected, is greater for MZ than DZ twins.

There are two types of twin studies:


Heritability, which refers to the proportion of phenotypic variation attributed to genetic variation, can be evaluated efficiently by twin data. 50% heritability clearly states that half of phenotypic variance in the population is attributed to genetic variance and it does not imply that a child of an affected parent has a 50% chance of inheriting the disease.

It was proposed that 38–82% of the population variance for probing depth (PD), attachment loss (AL) and dental plaque may be attributed to genetic factors in a study involving 110 pairs of adult twins [22]. A successive study on 64 monozygotic and 53 dizygotic pairs of adult twins disclosed the fact that genetic variance contributes to almost half of the variance in disease pattern in the population. From the results of the study, it was concluded that MZ twins were more alike than DZ twins for all clinical parameters [23].

Therefore, the difference in concordance between MZ and DZ twins for a specific phenotype could be used to evaluate the relative contribution of genes (heredity) and environmental factors to a disease and analysing disease presentation in twins is an essential first step in this process. Though twin studies overcame the drawbacks of segregation analysis, few such studies have been conducted because of the inadequacy of such twins.

#### **3.4 Population studies**

Environmental or behavioural risk factors for a disease are usually first detected in significant epidemiological or population-based studies. A genetic polymorphism is the long-time manifestation in a population of two or more genotypes that could not be maintained by frequent mutation.

The frequencies of polymorphisms of candidate genes can be compared between diseased individuals and controls [4]. It can be proved that the candidate gene determines the vulnerability to disease when there is a clear cut difference in the frequency of a specific polymorphism, between a case group and a control group. In this way, pathogenesis, causal heterogeneity of disease process and individuals most at risk for the disease can be interpreted well [24, 25].

In chronic periodontitis, no evidence of any simple pattern of genetic transmission that would support an etiologic role for a single gene mutation is demonstrated. In contrast to simple genetic diseases that may be caused by a single genetic

mutation, the additive effect of multiple genes is a determinant of disease susceptibility in complex diseases such as chronic periodontitis [14].

#### **3.5 Linkage studies**

Linkage analysis is a technique used to map the gene responsible for a trait to a specific position on a chromosome. These studies are based on the information that genes that are located closely on the same chromosome incline to have inherited together as a unit. Such genes are said to be linked and defy Mendel's law of independent assortment.

The distance between two allele at different loci will determine whether they will recombine. This is termed as recombination or crossover event. There is 50% chance that any two maternal or paternal alleles will recombine and be transmitted together to an offspring. However, alleles at nearby loci are linked and they tend to segregate together.

Linkage study necessitates use of very expensive DNA markers which was acceptable only after learning strong evidence of a genetic basis for a trait using segregation analysis or family aggregation analysis. By identifying the genetic markers that are associated with the disease causing alleles, the researchers can modify the location of a disease allele. Inheritance of a disease can be established if the distance between marker and disease allele is within 20–30 centimo grams (cM). In humans, 1 cM represents approximately 1 million nucleotide bases.

In this way, segregation of a trait in a manner consistent with linkage to a known genetic marker can be tested. Once the linkage is detected, the gene responsible for the trait can be placed in the vicinity of the linked genetic polymorphism since the exact chromosomal location of the genetic marker is known. Hence the genetic basis of disease is proved by linkage. Linkage is usually used as an initial step to identify the approximate location of a gene of interest, allowing the successive studies to determine the mutation responsible for a disease trait.

Linkage studies usually start by identifying markers (Single Nucleotide Polymorphisms) on a section of chromosome and then narrowing down the region until the gene of interest is found. DNA markers that are located proximal to a disease gene will be inclined to be inherited together with the disease gene. The closer a marker is to the disease gene, the closer the linkage and the more likely it is that they will be inherited together.

Linkage studies use sets of families, containing multiple affected individuals. Genotypes are determined for affected and unaffected family members, and complex statistical models are used to decide whether marker allele and disease co-segregate in the families under a given inheritance model.

Linkage is calculated using a LOD (Logarithm of odds) score. It is described as the ratio of probability that the disease and the marker loci are linked rather than unlinked. Supporting linkage gives a LOD score of +3 (1000:1) whereas, absence of linkage denotes a score of −2. Boughman et al. was first to assess the linkage between dentinogenesis imperfecta and aggressive periodontitis [26].

Marker linked to disease allele within a family may not be linked with disease in the population, which implies that same marker allele need not be transmitted with the disease allele in all affected families. However, Allelic associations (which is discussed below) occur when the same marker allele is linked to disease in multiple families.

#### *3.5.1 Drawbacks*

• Linkage studies have been successful only in identifying the genetic basis of simple Mendelian traits, where mutation of a single gene can cause a disease. Nevertheless, Linkage studies of complex diseases are not successful since complex diseases are due to the combined effect of multiple genes of minor effect and each gene contribute a small amount to the disease phenotype [27, 28].

• It has extremely low statistical power for diseases in which there is extensive heterogeneity among different families that have different combinations of vulnerable genes and environmental exposures.
