23 Methods of Studying Human Genetics

Dr. Sanjenbam Meitei

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Contents of this unit

• Introduction

• Pedigree analysis

• Twin studies

• Karypotype analysis

• Adoption Studies

• Biochemical methods

• Epidemiological Studies

• Association studies

• Population genetics Studies

• Epigenetic studies

• Summary

 

Learning Objective

  • What are the different methods for studying human genetics?
  • Why such methods are different from other lower organisms?
  •  What is the concept of ‘nature nurture interaction’ in human genetics?
  • How can nature nurture interaction be assessed in human genetics?
  • What is the emerging field in human genetics, if there is any?
  • What are the scopes for undergoing research in human genetics?

Introduction

As we already know that human genetics concerns the scientific study of inherited human variation. Genetic information is passed on from parental generation to the younger generation and it is expressed in the form of phenotypic characteristics. Though genetic is basic approach to understand the inheritable variation in all organisms, the methods of study the same in human is quite different and unique from those of other organisms. It is due to the fact that human’s life cycle is inconveniently long and family sizes are too small to undergo such studies. Moreover, it is difficult or illegal to procure desired matings as can be done in lower organisms. In this module, some of the important methods for studying human genetics will be discussed.

 

1. Pedigree analysis

Pedigree analysis is the method to identify the mode of inheritance of any trait based on Mendelian inheritance. It can be defined as the diagrammatical representation showing the inheritance pattern of Mendelian traits. Certain symbols are used in the pedigree studies which are given in figure 1. The study of the characters is started from proband (sometimes it is also known as propositus and propositia in males and females respectively), and further traced out those who bear common characters among his or her relatives in both preceding and following generations, as many as possible. In the chart, the earliest generation is put at the top followed by the later generations. Different inheritance patterns such as autosomal and sex linked: both dominant and recessive inheritance can be determined by analyzing the pedigree. This has been used to understand the inheritance pattern of any genetic disease or traits.

 

Figure 1: Symbols used in human pedigree analysis. (Source: Bodmer and Cavalli-Sforza, 1976)

 

Figure 2: A hypothetical pedigree indicating autosomal recessive inheritance

 

There are certain characters that are unique to each inheritance and hence it can be used to assess the inheritance. As an example, skip of generation indicates the recessive inheritance and female being carrier in X linked recessive inheritance.

 

2. Twin studies

Twin studies have been extensively used in human genetics to estimate the heritability of a particular trait wherein relative contributions of heredity and environment may results into varied phenotypic expression. The method analyzes genetic contribution of a particular trait as against the environmental factors. Perhaps the first, a systematized study on twins was reported in 1924 where geneticist Hermann Siemens compared school transcripts of identical versus fraternal twins to understand the genetic influence on traits (Lewis, 2009. Twins can be monozygotic twin (MZ, identical) or dizygotic twin (DZ, non identical). MZ twin results from the fusion of one sperm and one egg resulting into a single zygote which further divides into two at the embryo stage. While DZ twin results from two different zygotes each formed by the fusion of a sperm and an egg separately. MZ twins are of same sex while DZ twins may be of same sex or different sex. Since MZ twin is developed from a single zygote, it is regarded as having common genetic constitution and DZ as having different genetic constitution like that of siblings.

 

Methodologically, twin studies is a longitudinal studies where the researchers compare traits of interest in DZ twins rearing together in the same environment or in MZ twins rearing apart in different environment or vice versa. It is based on the assumption that the differential traits developed in MZ twins reared apart would be affect of the environmental factors. Similarly, the alike traits developed in DZ twins reared together would be because of common environmental exposure. One of the important measurements to find out the relative contributions of heredity and environment for quantitative characters is Concordance scores of a trait. It is defined as the percentage of pairs in which both twins express the trait among pairs of twins in whom at least one has the trait (Lewis, 2009) Twins sharing a common phenotype are said to be concordant and as well twins sharing different phenotype are said discordant. Twin studies have been considered as important method to assess nature (genetic) and nurture (environment) approach. Twins both MZ and DZ reared apart shows that the influence of genetic and environmental factors in phenotypic expression. Concordant traits developed in twins reared apart indicate the role of genetic factors.

 

 

Table 1: Concordance values in MZ and DZ twins (Source: Cummings, 2010)

Concordance Values
Trait MZ Twins DZ Twins
Blood Types 100 66
Eye Color 99 28
Mental Retardation 97 37
Hair Color 89 22
Down Syndrome 89 7
Handedness (Left or Right) 79 77
Epilepsy 72 15
Diabetes 65 18
Tuberculosis 56 22
Cleft Lip 42 5

 

3. Karypotype analysis

Karypotype analysis is another important method for studying human genetics. Karyotype is a descriptive chart showing the set of chromosomes, including the diploid number, chromosome lengths, and distinguishing morphological features of some of the chromosomes (Pasternak, 2005). In this method, samples such as blood sample, amniotic fluid, chorionic villi or any other biological samples containing cells are collected from an individual or patients through different techniques. Further the sample is processed through a number of laboratory procedures such as culturing the cells (chorionic villi cells do not need cultured as it contains enough dividing cells for analysis), cell harvesting, separating and staining the chromosomes, scanning microscopic slides for observation etc. Cell images are photographed and manipulated with better resolution through digital computer software. This photographed chromosome image is kept under examination for any sort of chromosomal analysis (National Genetics and Genomics Education Centre: http://www.geneticseducation.nhs.uk/laboratory-process-and-testing-techniques/karyotyping). The chromosomes are captured while the cell is undergoing cell division, particular at the metaphase stage where the chromosomes become condensed and visible. For better analysis of the chromosome, the chromosomes are stained with certain dyes that show patterns of light and dark bands. Such technique is commonly known as banding technique and G banding, which uses a dye called Giemsa’s reagent, being the common one. This method has been employed to analyze the chromosomal abnormalities – both structural and numerical. Numerical abnormalities may be trisomy and any other excess or less of diploid chromosomes, while structural abnormalities include deletion and insertion of segments, translocation etc. Such detection of chromosomal abnormalities would be helpful in understanding the prognosis of genetic disorders.

Figure 3: A normal human male Karypotype: a) A metaphase array of chromosome seen under microscope, b) Arranged

chromosome in a karyotype sheet (Source: Cummings, 2010)

 

4.Adoption Studies

Adoption Studies are also other methods to investigate the nature nurture approach. It is well established fact that the adopted children do not share the genetic constituents with the adoptive parents. Comparison of adopted children with biological and non biological parents can provide an understanding on the heritability of the traits. Concordant characters between the adopted children and the non biological parents would be assessed as the influence of environmental factors. This method has been extensively used in analyzing the heritability of behavioral traits. Adoption studies have shown that the children born out of schizophrenic parents and adopted by non schizophrenic parents have higher chance of developing the disorder than the children out of normal biological parents and adopted by the same (Pasternak, 2005). This method has some limitations too as such studies require access to good adoption records and stable populations over generations, so there are a limited number of situations in which they can be used (Stanford et al., 2013).

 

5. Biochemical methods

Biochemical genetics is also an important scope for understanding human genetics and its association with certain inherited disorders. Biochemical analysis of proteins, enzymes, hormones etc. are very much important for understanding inter or intra population variation too. Population affinities and diversity can be estimated from the frequency of the variant of such biochemical markers. Beyond this, biochemical analysis also reveals the fate of inherited genetic disorders. Well established scientific techniques are very importance for the detection of such biomolecules. Quantification of such molecules also provides another facet of biochemical analysis. Data so obtained could be compared with the standard level, and excess and short of the same would be considered as threat to inherited health problems, depending on the metabolic or physiological pathway that involves the analyzed biomolecules.

 

6. Epidemiological Studies

Genetic epidemiological studies are again one of the important methods for studying human genetics. As Morton has well defined that genetic epidemiology deals with etiology, distribution, and control of disease in groups of relatives, as well as inherited causes of diseases in populations (Morton, 1982). Different study designs have been employed to investigate the genetic epidemiological studies. Screening of risk or mutant alleles that cause the genetic disease is one of the important aspects of epidemiological studies. This can be applied in the detection of haemoglobinopathies or any other genetic disorders and can assess the risk of such disorders in the population, as a whole or sex wise. Certain specific techniques such as electrophoresis are used in the detection of mutation. Sometimes, it is referred as descriptive epidemiology in the epidemiological science. It is obvious that collection of biological sample, generally blood samples, is very much needed for any genetic analysis.

 

Association studies are also important facet of human genetic studies. Such studies are carried out to examine the possible associations of certain mutations or any other genetic factors in the development of genetic disorders, generally in such polygenic inherited traits or disorders. The role of these genetic factors in the disease progression and manifestation may differ from one population to another. Association studies can be family based or population based association studies. Case control or case only studies are some of the models of association studies. In case control studies, the affected individuals or those at risk are considered as case and those who don’t possess the characters of being case are included as controls. Association of risk mutant alleles or other genetic factors are assessed through statistical approach. In population based association studies, both case and controls are collected from the age sex matched individuals from the same population as different populations have varied gene pools. But in the case of family based association studies, controls are the family members who are not affected by the trait under consideration.

 

7. Population genetics Studies

Population genetics is an important branch of genetics dealing with describing the genetic structure of a Mendelian population. It has been considered as a focal point for evolutionary biology where in change in the allele and genotypic frequencies would determine the evolutionary fate of any population. Moreover, the genetic relationship of inter and intra population in the form of genetic differentiation and similarities can also be accessed through statistical methods. Statistics plays a very important role in analyzing and interpreting inter or intra genetic relationship of the populations. Such studies are carried out through certain genetic markers. Genetic markers can be either serological, biochemical or genomic markers. The allelic and genotypic frequencies of these markers are calculated based on the observed frequencies of the traits or phenotypes, which have been collected from the population through sampling approach. Analysis of genomic markers need sophisticated techniques such as DNA extraction, PCR amplification, genotyping etc. Genotyping is requisite technique even in serological and biochemical markers to identify the variant of the traits. Certain statistical analyses are done from the allele frequencies calculated and the results describe the genetic structure of the population. Secondary information such as population and mythological histories etc, provide an important validation in the findings of the genetic results. Population genetic studies have become a basic substratum for population stratification which could be used in epidemiological studies.

 

8. Epigenetic studies

Epigenetic analysis has brought a new trend in molecular biological research. Epigenetics deals with inheritable changes in DNA or its associated protein without alteration DNA nucleotide sequence. It has been considered as dynamic way by which genetic variation interacts with environmental exposures across development to produce a phenotype (Jablonka and Lamb, 2002). It is often associated with heritable gene expression. This new approach reassures the so called concept of nature

nurture interaction. Environment may provide change in the expression of genetic trait without modification of genetic elements of the individual. There are different methods to undertake epigenetic analysis such as histone modification and DNA methylation. It is well established that abnormal patterns of DNA methylation are associated with various complex disorders. Lower-than-normal levels of methylation (demethylation or hypopmethylation) are commonly observed in some types of cancer as removal of methyl groups can activate genes involved with cell growth and can increase genomic instability (Cummings, 2010). Depth knowledge in the subject matter would be required to design such studies. It is true that the approach of epigenetics in Anthropological investigation is less, but it needs to be given emphasis in the present scenario.

 

In the present days especially after Human genome project, methods for studying human genetics have become wider with the advancement of technology. Earlier it was more of study of inheritance pattern or heritability. Such an approach has been shifted to technology based models of study wherein the geneticists try to analyze the genes and their products. The methods may be varied but the objective is very much defined: to understand the effect of genetic change in the phenotypic expression and also to examine the influence of environment on genetic expression, beyond tracing the patterns of inheritance. Such depth studies would bring a new dimension in the applied aspects too.

 

 

Summary

  • Genetic information is passed from one generation to the next generation in the form of phenotypes.
  • Methods for studying genetics in human are quite different in human in earlier days. But it becomes more or less same in the genomic era.
  • Human mating cannot be controlled as desired.
  • Pedigree is the diagrammatical representation showing the inheritance pattern of Mendelian traits.
  • Different inheritance patterns such as autosomal and sex linked: both dominant and recessive inheritance can be determined through pedigree analysis.
  • Twin method analyzes genetic contribution of a particular trait as against the environmental factors differential traits developed in MZ twins reared apart would be affect of the environmental factors. Similarly, the alike traits developed in DZ twins reared together would be because of common environmental exposure.
  • Concordant score can be defined as the percentage of pairs in which both twins express the trait among pairs of twins in whom at least one has the trait.
  • Karyotype is a descriptive chart showing the set of chromosomes, including the diploid number, chromosome lengths, and distinguishing morphological features of some of the chromosomes.
  • Karyotping analysis is used to detect chromosomal abnormalities – both structural and numerical.
  • Concordant characters between the adopted children and the non biological parents would be assessed as the influence of environmental factors.
  • Biochemical analysis of proteins, enzymes, hormones etc. are very much important for understanding inter or intra population variation too.
  • Biochemical analysis of enzymes, proteins etc. also reveal the fate of inherited genetic disorders.
  • Screening of risk or mutant alleles that cause the genetic disease is one of the important aspects of epidemiological studies.
  • Association studies are carried out to examine the possible associations of certain mutations or any other genetic factors in the development of genetic disorders, generally in such polygenic inherited traits or disorders.
  • Association studies can be family based or population based association studies.
  • The genetic relationship of inter and intra population in the form of genetic differentiation and similarities can be accessed through statistical methods in population genetics methods.
  • Genetic markers can be either serological, biochemical or genomic markers.
  • Secondary information such as population and mythological histories etc, provide an important validation in the findings of the genetic relationship of inter population.
  • Epigenetics deals with inheritable changes in DNA or its associated protein without alteration DNA nucleotide sequence.
  • Environment may provide change in the expression of genetic trait without modification of genetic elements of the individual.
you can view video on Methods of Studying Human Genetics

 

Reference

  • Bodmer W.F. and Cavalli-Sforza, L.L. 1976: Genetics, Evolution, and Man. San Francisco: W.H. Freeman and Company.
  • Cummings, M.R. 2010: Human Heredity, CA: Cengage Learning.
  • Jablonka, E. and Lamb, M.J. 2002: The Changing Concept of Epigenetics. Ann. N.Y. Acad. Sci.
  • 981: 82–96.
  • Lewis, R. 2009: Human Genetics – Concepts and Application, New York: McGraw Hill.
  • Morton, N.E. 1982: Outline of Genetic Epidemiology. New York: Karger Publication.
  • National Genetics and Genomics Education Centre: http://www.geneticseducation.nhs.uk/laboratory-process-and-testing-techniques/karyotyping
  • Pasternak, J.J. 2005: Human Molecular Genetics, NJ: Willey-Liss,
  • Stanford, C., Allen, J.S. and Anton, S.C. 2013: Biological Anthropology, NJ: Pearson Education, Inc.
  • Speicher, M.R., Antonarakis, S.E. and Motulsky, A.G. 2010: Vogel and Motulsky’s Human
  • Genetics. Heidelberg: Springer.
  • Thomas, D.C. 2004: Statistical Methods in Genetic Epidemiology. New York: Oxford University Press.
  • Cavalli-Sforza, L.L. and Feldman, M. 2003: The Application of Molecular Genetic Approaches to
  • the Study of Human Evolution. Nature genetics Supplement 33: 266-275.
  • http://www.ncbi.nlm.nih.gov/books/NBK21257/
  • http://www.dorak.info/epi/genetepi.html
  • http://www.ncbi.nlm.nih.gov/books/NBK20363/?report=reader