10 Importance and Implications of Pedigree and Genealogy

Ms. Nivedita Laishram

Contents:

1. Introduction

2. Pedigree

2.1 Definition

2.2 Methods of Pedigree Analysis and its Importance

3. Genealogy

3.1 Definition

3.2 Genealogical Method and its Importance

4. Summary

Learning Objectives:

To discuss:

•  about pedigree and genealogy
•  how pedigree analysis helps in understanding inheritance of certain genetic traits
•  how genealogy helps in understanding one’s ancestors and also in understanding social organisation between members of a society
•  about the uses of genealogy in modern urban anthropology

1. Introduction

Family studies have been used since long back in anthropological field work and research. It is considered as one of the important method in ethnographic researches. Family studies helps in understanding the relationships between the members of the family as well as history of the family. Family studies also help in understanding the social structure and network of relationship among individuals in a society. Such study of family origins and history is known as genealogy. The word genealogy is derived from the Greek words meaning ‘to trace ancestry’. It is the science of studying family history.

In human genetics, family studies also provide genetic information of the family as well as members of the family which helps in tracing a particular genetic trait running in the family. Such human genetic traits can be discovered through analyzing the inheritance pattern observed from mating lines that is through pedigree analysis. The term ‘Pedigree’ comes from two Latin words ‘pes’ meaning ‘foot’ and ‘grus’ meaning ‘crane’ used to indicate lines of descent. Pedigrees are family trees which show the parents and offspring across generations, as well as who possessed particular traits.

In the present chapter, emphases have been made on understanding the pedigree analysis and methods of genealogical study. It also focuses on the importance and implications of pedigree and genealogical study.

2. Pedigree

2.1 Definition

A pedigree is a diagrammatic representation showing the nature of genetic inheritance of particular trait or traits for two or more generations of biologically related individuals. In another words, it is the representation of relationships of family members using symbols and lines of descent. It helps in visualizing relationships of individuals in a family, particularly in large extended families. It is often used to determine the mode of inheritance of various genetic diseases. To understand the modes of inheritance, tracing of family history is done through drawing family tree using standard symbols. In pedigree, male and female are represented by different symbols and relationships are shown with different patterns of lines. Moreover, affected individuals or carrier of a genetic trait are represented by different symbols. The symbols, patterns of lines and other characteristics used in pedigree are depicted in figure 1.

2.2 Methods of Pedigree analysis and its Importance

Analysis of pedigree helps in understanding the nature of inheritance of a particular trait. The inheritance pattern could be autosomal (recessive and dominant), X-linked (recessive and dominant) or Y-linked. Some of the inheritance patterns are discussed in the following sections.

a) Autosomal Recessive Inheritance

The autosomal recessive inheritance is characterised by presence of homozygous recessive alleles in autosomal chromosome. Individuals carrying the homozygous autosomal recessive allele showed the affected phenotype and the unaffected phenotype are represented by the corresponding autosomal dominant allele. Such affected phenotype appears in the progeny of unaffected parents and both male and female progeny are affected equally. To conceive a recessive homozygote individual, both the parents must have had the affected allele. The formation of an affected individual usually depends on the chance of union of heterozygote carriers. This chance of becoming affected increases when the mating occurs between the relatives. Mating between relatives produces higher risk of affected homozygous recessive phenotype than do mating between nonrelatives. Therefore, first cousin or close relative marriages are responsible for a large portion of recessive diseases in human populations. Albinism is an example of autosomal recessive disorder which is caused by a defect in enzyme that synthesizes melanin. The albinos, affected individual, possesses light colored hair, lack of pigmentation of the skin and pink eye pupils because of the unmasking of the red hemoglobin pigment in blood vessels in the retina. For example, albinism is determined by an allele let’s say ‘a’, and the normal condition by ‘A’. The affected person ‘albinos’ of the disease would have ‘aa’ genotype and unaffected individuals would be either AA or Aa genotype (Figure 2).

To conceive a recessive homozygote individual, both the parents must have had the affected allele. The formation of an affected individual usually depends on the chance of union of heterozygote carriers. This chance of becoming affected increases when the mating occurs between the relatives. Mating between relatives produces higher risk

b.Autosomal Dominant inheritance

The autosomal dominant inheritance is characterized by the presence of an autosomal allele which is dominant in nature. In other words, the normal allele is recessive and the abnormal allele is dominant. In autosomal dominant inheritance, the affected individual tends to appear in every generation since the abnormal allele carried by the affected individuals has come from one of the affected parent. It means every affected individual has at least one affected parent. Only one affected parent either mother or father can affect all the siblings. The affected parents transmit the trait to both sons and daughters equally. Some of the common examples of autosomal dominant disorders in human are Achondroplasia, Huntington’s disease, Phenylthiocarbamide (PTC) taste etc. A typical pedigree for a dominant disorder of PTC sensitivity is shown in Figure 3. In the given pedigree, individuals who can taste bitter is denoted by either ‘TT’ or ‘Tt’, whereas, individuals who could not taste the bitterness are represented by the presence of ‘tt’ genotype.

c) X linked Recessive Inheritance

In X-linked recessive inheritance, affected phenotypes showed sex differentiation. Typically, more males are affected than the females. This is because a female showing the phenotype can result only from a mating in which both the mother and the father bear the allele, whereas a male with the phenotype can be produced when only the mother carries the allele. For example, if the father is normal and the mother is affected then all the sons will be affected while half of the daughters will be carrier of the affected trait. In case of normal father and carrier mother, they will give birth to sons, half of whom are affected and the other half normal. On the other hand, when the father is affected and mother is normal then none of the offspring are affected. All the sons will be normal but all the daughters must be heterozygous “carriers” because females received one of their X chromosomes from their fathers. Some of the common examples of X-linked recessive disorders in human are haemophilia, duchenne muscular dystrophy (DMD), testicular feminization syndrome etc. Perhaps the best known example of X-linked recessive disorder is hemophilia, a difficulty in which a person’s blood fails to clot due to the absence of protein called factor VIII. The most famous cases of hemophilia are found in the pedigree of the royal families of Europe.

For example, haemophilia is determined by an allele Xa, and the normal condition by XA. The affected haemophilic person is represented by XaXa and XaY in female and male respectively. Only one copy of the affected allele is required to express the phenotype in males. Female who carries one copy of the affected allele that is XAXa do not show the disease phenotype. Such a person is known as carrier of the disorder and the normal unaffected person would have ‘XAXA’ or XAY genotypes in female and male respectively (Figure 4).

d) X linked Dominant Inheritance

In X-linked dominant inheritance, only one affected allele is sufficient to cause the disorder. This type of disorder is equally affected in both males and females. It is a rare inheritance disorder in humans. The affected males pass the trait on to all their daughters but to none of their sons. When the affected females married to unaffected males, the disease trait is passed on to half of their sons and daughters. Some of the common of examples of X-linked dominant disorders in human are hypophosphatemia, a type of vitamin D – resistant rickets, Rett syndrome, fragile X syndrome etc. For example, hypophosphatemia is determined by the presence of XD allele. Individual affected with hypophosphatemia is represented by XDXd, XDXD and XDY. Only one allele is sufficient to express the disease phenotype. Both males and females are equally affected. The normal genotype is represented by XdXd and XdY for females and males respectively (Figure 5).

e) Y-linked Inheritance

The Y-linked inheritance is sex specific. Inheritance of such Y-linked genetic trait is observed only among the males because Y chromosome is found in normal males only. Such traits are not observed in females. The Y chromosome is received from the father by all sons, and therefore, if the father is affected all his sons will be affected. This chromosome will not be transmitted to any of the daughters. The phenotype expression of such genetic trait is limited to the male sex only. One of the common examples of Y-linked disorder is Y-chromosome infertility. For example, Y chromosome infertility is determined by presence of ‘I’ allele that is ‘XYI’. Affected males pass the disorder to all his sons but to none of their daughters (Figure 6).

Analysis of family pedigree helps in tracing out the origin of disease phenotype that is running in the family members. It also helps in predicting the probability of certain events that might occur in future. Pedigree analyses are used in genetic testing of new born babies to identified inheritance of genetic diseases. It is also used for testing of heterozygotes (carrier) as well as screening before symptoms occur. Pedigrees of individual families are used by genetic counsellors, to aid them in providing information to families who may be at risk for various genetic conditions.

3. Genealogy

   3.1 Definition

Genealogy is the study of one’s ancestors – parents, grandparent great grandparent and so on. The genealogical method was originally developed by W.H.R. Rivers during the Torres Straits expedition of 1898-99. It was used for the first time in his field research work on the Todas. It is one of the standard procedures adopted in ethnographic researches in Social Anthropology. The primary aim of genealogy is the analysis of social organization that is the interpersonal relations and living arrangements between members of a society. It required extensive interviewing of individuals in order to record their descent, succession and inheritance. Genealogy is very much helpful in studying kinship, and thereby in understanding the social structure or network of relationship among individuals. It is achieved by collecting demographic and social data and mapping residence details. Therefore, genealogy is often used along with census and observation method. It is also used in studying the pattern of migration, to trace out early migrants, discovery of heritage, creating story of family and even creating of medical history. Therefore, genealogy is more than just tracing a family tree.

3.2 Genealogical method and its importance

The genealogical method is a method used in ethnographic studies which is based on the study of an individual’s family history. It was initiated by early ethnographers to identify all important links of kinship determined by marriage and descent. The ancestral and descendants information of one or more families are collected through interview and expressed the findings graphically showing the connections between the members of the family. Genealogy commonly plays a crucial role in the structure of nonindustrial societies, determining both social relations and group relationship to the past. For example, the genealogical method was used, along with censuses and settlement plans, in field research for classical monographs by various scholars on the different population groups such as Todas by Rivers, Tallensi by Meyer Fortes, Tikopia by Raymond Firth, Ndembu by Victor Turner, Sinhalas by Edmund Leach are among others.

In genealogy, specific graphic symbols are used such as females are represented by circle symbol whereas males with triangle symbol. The Triangles and circles are connected by different types of lines showing the relationship that binds the subjects (Figure 7). The deployment of these lines relationships is known as genealogical tree, whether descent or descent to the subject on which it is building the set of relationships. Numbers of generations are written on the left side of each corresponding generation with Roman numeral. Each of the individuals in the genealogy is numbered by an Arabic numeral. Numbering of individual in genealogy is very useful as it helps in identifying a particular person very easily.

Genealogical method is used as a tool for data collection, representation and analysis of various social processes of a population. The genealogical method helps to determine the family history, historical events, family inheritance, family tradition, culture, and tracing of land ownership. It also helps in reconnecting members of relative families. It also helps in finding the true family history from the records. Generally, it is used during field work to collect and organize data for different applications. The primary purpose of genealogy is to improve the analysis for better understanding of interpersonal relations and social organization. The genealogical method required extensive interviewing in order to collect vital statistics of a population and to record information relating primarily to descent, succession, and inheritance.

In anthropological research work, the genealogical method is used in combination with ego centred network analysis. Set of symbols are used to draw kinship or genealogical chart to express relationship diagrammatically how any two persons are related by bonds of kinship. Such kinship relationships are traced from a reference individual who is known as ‘ego’. The importance of genealogical method extends beyond the specialized realm of kinship studies. It also provides the foundation of structural demography of the studied population highlighting the relationship between the functioning and durability of social structure and the actual size of population.

The genealogical method helps in assessing the risk of getting certain medical conditions that tend to run in families. Moreover, it further helps in determining the risk and effect of inheritance at individual level too. It also provides the clue to any prominent illnesses and unusual causes of death from the collected death records of the population.

With advanced in biotechnology nowadays, deoxyribosenucleic acid (DNA) uniquely different for each individual, is used as a source of genealogical information. The non-coding DNA sequences helps in determining whether and how closely individuals are related to other individuals. These non-coding DNA strands remain unchanged from generation to generation and if any changes in such sequences helps in distinguishing different lines of descent and determine how closely people may be related to each other from the closeness of their DNA matches.

Summary

Pedigree is a diagrammatic representation showing the inheritance pattern of a particular trait or traits. Pedigree analysis of man is done through analysing the results of mating that have already occurred. The nature of inheritance of autosomal recessive disorder, autosomal dominant disorder, X-linked recessive disorder, X-linked dominant disorder in man are commonly analyzed through pedigree.

An autosomal recessive disorder means two copies of an abnormal gene must be present in order for the disease or trait to develop. However, in autosomal dominant disorder presence of only one abnormal gene develop the disease and the disease phenotype tends to appear in every generation of the pedigree. X-linked recessive disorders are more frequently observed among males than females due to the presence of only one X chromosome. On the other hand, X-linked dominant disorder required only one copy of the allele to cause the disorder. Therefore, both males and females are equally affected when they inherited from their parent who has the disorder.

Genealogy, also known as family history, is the study of families and tracing their lineages and history. It used oral interviews, historical records, genetic analysis and other records to obtain information about a family and to demonstrate kinship relationship of its members. The results of genealogical studies are often displayed in charts or written as narratives. It is useful in different fields of studies such as anthropological field work, kinship relationships, tracing the missing link or members of a family, and assessment of risk getting certain disease in the family members. The DNA strands are also used in determining whether and how closely individuals are related to other individuals based on the closeness of their DNA matches.