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I.9.1Methods for study of genetic principles in man-family study- pedigree analysis

Pedigree analysis is the classic method of human genetics. Pedigree is a family history of hereditary condition or diagram of a family history indicating the family members, their relationships to proband, and their status with respect to a particular hereditary condition.

Following symbols are used for study-

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Methodology

  1. A pedigree includes two generations of ascent from the consultant (the person requesting medical or genetic information) or proband (the affected individual that brings family to medical attention), and two generations of descent.
  2. Information is collected on first-degree relatives (children, siblings and parents), second-degree relatives (half siblings, aunts, uncles, nieces, nephews, grandparents and grand children), and sometimes third-degree relatives (e.g. first cousins)
  3. The first step in pedigree analysis is to observe the number and relationships of all individuals who express the same or similar clinical features.
  4. Second step is to evaluate the status of family members in the pedigree
  5. The third step is to calculate the risk
    • For those traits exhibiting dominant gene action:
      • affected individuals have at least one affected parent
      • the phenotype generally appears every generation
      • two unaffected parents only have unaffected offspringDominant pedigree
    • And for those traits exhibiting recessive gene action:
        • unaffected parents can have affected offspring
        • affected progeny are both male and femaleRecessive pedigree

Analysis

  1. An autosomal disease can usually be distinguished by seeing male-to-male transmission of mutation, but since males pass only the Y chromosome to their sons, there should never be father to son transmission of an X-linked gene.
  2. Males will be most commonly affected in an X-linked disease, whereas males and females should be equally affected in autosomal disorders.
  3. A dominant disease will be seen in approximately half of the individuals in each generation, but recessives occur very rarely.
  4. If the mutation is in the mitochondrial genome, affected mothers will pass the trait to all of their children, but none of the offsprings of an affected male should have the disease.
  5. By carefully observing the position of affected individuals, mutation carriers may be identified. From this data, the risk of carrier status for other family members or the chance that a couple may have an affected child can be estimated.

Use

  1. pedigree is an important method of establishing patient rapport,
  2. serves as a visual demonstration for providing patient education (e.g., variation in disease expression in a family),
  3. identifying order or relatives at risk for disease or who can be informative for inclusion in genetic research.
  4. to identify inherited traits and infer patterns of transmission.
  5. method used in genetic diagnosis and research.
  6. helps the researchers to determine if a trait is Mendelian.
  7. helps establish the mode of inheritance.
  8. it can be used in segregation analysis, heritability, calculation of in- breeding coefficients and in studies of linkage.
  9. to determine if the disorder is dominant or recessive, autosomal or X-linked

Conclusion

Its usefulness will continue to develop with the rising knowledge of molecular, bioinformatics and proteomic tools.

Example-

Autosomal Recessive Disorders

phenylketonuria (PKU) is a autosomal recessive -the affected progeny include both males and females equally.

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An albino- phenotype is caused by homo-zygosity for a recessive allele in the chemical synthesis of the dark pigment melanin in the cells of skin, hair, and eye retinas.

Autosomal Dominant Disorders-  the normal allele is recessive and the abnormal allele is dominant.

  1. Achondroplasia- a type of dwarfism , the phenotype tends to appear in every generation of the pedigree and that affected fathers and mothers transmit the phenotype to both sons and daughters.
  2. Tay-Sachs disease
  3. Most human populations are dimorphic for the ability to taste the chemical phenylthiocarbamide (PTC): people can either detect it as a foul, bitter taste or—to the great surprise and disbelief of tasters—cannot taste it at all. Two tasters sometimes produce nontaster children. The allele for ability to taste is dominant and that the allele for non tasting is recessive.

X-Linked Recessive Disorders

Hemophilia, the most common type of hemophilia is caused by the absence or malfunction of one of these proteins, called factor VIII.

Duchenne muscular dystrophy

Testicular feminization syndrome,

X-Linked Dominant Disorders

1.Affected males pass the condition on to all their daughters but to none of their sons
2. Females married to unaffected males pass the condition on to half their sons and daughters.

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