Twin Study | Anthropology Optional for UPSC PDF Download

Introduction

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.

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.

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
  

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

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.

Conclusion

Twins can be classified into two types: monozygotic or identical, and dizygotic or fraternal. Monozygotic twins develop from a single zygote that splits into two embryos, while dizygotic twins develop from two separate eggs fertilized by two separate sperm cells. By studying the similarities and differences between these types of twins, researchers can better understand the roles of genetics and environment in determining human traits. Various methods, such as placental examination, similarity testing, DNA fingerprinting, dermatoglyphics, genetic markers, and skin grafting, can be used to determine whether twins are monozygotic or dizygotic. Concordance and discordance measurements can further help in assessing the influence of genetics and environment on specific traits. Ultimately, twin studies provide valuable insights into how human traits are shaped by both genetic and environmental factors.

Frequently Asked Questions (FAQs) for Twin Study

What is the difference between monozygotic and dizygotic twins?

Monozygotic, or identical, twins develop from a single zygote that splits into two embryos, resulting in genetically identical individuals. Dizygotic, or fraternal, twins develop from two separate eggs fertilized by two separate sperm cells, resulting in genetically different individuals that may not be of the same sex.

How can we determine whether twins are monozygotic or dizygotic?

There are several methods to determine the zygosity of twins, including placental method, similarity method, DNA fingerprinting, dermatoglyphics, genetic markers, and skin graft. DNA fingerprinting is the most reliable method, as the DNA patterns of monozygotic twins are very similar.

What is concordance and discordance in twin studies?

Concordance and discordance are ways to measure phenotypic similarities and differences for a particular character in twins. If both twins possess or are free from a character, the pair is concordant (phenotypically similar). If only one member of the pair possesses the character, the pair is discordant. By measuring the degree of concordance between identical or fraternal twins, we can assess the role of environment and heredity in determining traits.

How do twin studies help in understanding the influence of genetics and environment on human traits?

By comparing the phenotypic similarities and differences between monozygotic and dizygotic twins, researchers can determine whether genetic factors or environmental factors play a more significant role in determining a specific trait. Traits with a large genetic component will show a higher concordance rate for monozygotic twins than for dizygotic twins. If the concordance and discordance ratios are more equal between the identical and fraternal groups, it signifies less emphasis on heredity and more emphasis on the environment in determining the trait.