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Twin Study Notes - UPSC

Document Description: Twin Study for UPSC 2022 is part of UPSC preparation. The notes and questions for Twin Study have been prepared according to the UPSC exam syllabus. Information about Twin Study covers topics like and Twin Study Example, for UPSC 2022 Exam. Find important definitions, questions, notes, meanings, examples, exercises and tests below for Twin Study.

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First lets understand about Twins

Twins are two offspring produced by the same pregnancy. Twins can be either monozygotic ('identical'), meaning that they develop from one zygote, which splits and forms two embryos, or dizygotic ('fraternal'), meaning that each twin develops from a separate egg and each egg is fertilized by its own sperm cell.
Twin Study Notes - UPSC

Types of Twins

Monozygotic/ldentical twin

  •  Arise from single ovum fertilized by a single sperm and become a zygote. Zygote splits into 2 @ an early stage & develop into genetically identical individuals.
  •  Are exactly (almost) alike & genetically identical.
  • Same sex, blood group.
  • Similar physical character
  • If any difference is noticed in their phenotype, it could be attributed to their Environment.
  • Therefore study of twins help in understanding the traits of human being which could be changed by providing better environment.
  • (Ex- IQ, Social behaviour)

Dizygotic twins/Fraternal twins

  •  When 2 different ovum are fertilized by 2 different sperms @ the same time it results in the formation of 2 different zygotes.
  • Not exactly identical.
  • May not be of same sex.
Analysis- How it is used to understand genetic principle

If phenotypic similarities for a particular character are greater among identical twins than among fraternal twins, we can ascribe this to genetic similarity of identical twin and genetic dissimilarity of Fraternal twins.
On the other hand, if phenotypic similarly difference for a particular character are same for both identical and fraternal twins, we can assume that genetic similarity or dissimilarity play less role and the phenotypic is influenced by environment.
Hence, it is important that we must diagnose a twin as Monozygotic or Dizygotic. SO how can we know whether a twin is Monozygotic or Dizygotic
Twin Study Notes - UPSC

Diagnosis of Twins
  1. Placental Method
    • Distribution of placenta & membrane can help to some extent in the diagnosis of zygosity.
    • In case of dizygotic twins, each twin has complete membrane.
    • In case of monozygotic twin:
      (i)    Diamniotic dichorionic separate.
      (ii)    Diamniotic dichorionic fused.
      (iii) Diamniotic mono chorionic
      (iv) Mono amniotic mono chorionic
  2. Similarity Method
  • Individual variations occurs in many genetic traits ex-blood group, serum protein, HLA system etc.
  • Monozygotic twins would have identical feature.
  • Morphological features like eye colour, nose form, ear form etc can also be used- but is less reliable.

3. DNA finger printing

  • DNA pattern of monozygotic twins is very similar.
  • Most reliable method.

4. Dermatoglyphics (study of Finger prints)

  • Pattern similar in monozygotic twins
  • Not fool proof.

5. Genetic Markers

  • Like blood groups, serum proteins & sex etc.
  • The more markers are same, the higher the probability that the twin are monozygotic.

6. Skin Graft

  • DZ twin: reject graft
  • MZ twin: accept graft from its co-twin.
Concordance and Discordance (Important)

It is one of the ways the phenotypic similarities & difference for a character can be measured in twins.

  • Note whether the character is present or absent in one or both member
  • If both possess the character or free from it, the pair is concordant (ie phenotypically similar.)
  • If only one member of the pair possess it, the pair is discordant.
  • By measuring the degree of concordance between identical or fraternal twins, we can measure/ assess the role of environment and heredity.
  • Traits with a large genetic component will show a higher concordance rate for MZ twins than for DZ twins. For 100% genetic trait, should be 1.0 in MZ twins and 0.5 for DZ of same sex. Concordance ratio: MZ/ DZ. the higher it is, the more genetic a trait is
  • More equal concordance & discordance ratio between the identical & fraternal group would signify less emphasis on heredity & more emphasis on environment in the determination of trait.

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