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Genetics Index

Chromosomes
Meiosis
Meiosis: Crossing over
Mitosis and Meiosis
Introduction to Mendelian Genetics
Test Cross
Codominance
Multiple Alleles
Pedigree Charts
The dihybrid cross
Dihybrid Cross : Test Cross
Autosomal Linkage
The Genetic Diagram for Linked Genes
Calculating the cross over value using a test cross
Sex determination and sex linkage
Sex linkage
Genetic diagram for sex linked genes
Blood Clotting and Haemophilia
The Retina and Daltonism
Genetic Modification
Cloning Animals
Cloning Plants

Topic Chapters Index

 

The limitations of twin studies

Twin studies often try to use twins that have been separated from birth so they grow up in different environments.

  • Monozygous and dizygous twins are difficult to distinguish at birth - finger prints and blood proteins are used.

  • Twins are not representative members of the population:

    • They have a slower physical development.
    • They have a slower development of their IQ.
    • They have a high degree of lefthandedness (18% higher than the rest of the population).

  • Twins may have experienced different environments before birth (e.g. unequal blood supply).

  • The upbringing of twins in the same household may influence their development (parental influence).

  • One of a pair of monozygous twins has a higher death rate than dizygous twins.

  • Twins separated at birth for used in twin studies are often from lower social groups.

  • The sampling of twins is often biased, most research bodies use advertising. The twins that respond do not represent the population this results in sampling error.

So twin studies have their limitations. Data from them have to be treated with care.

One famous study which tried to determine the influence of heredity on intelligence was accused of fraud. It seems that the researcher invented some data because of a shortage of a suitable data.

 

 

GENETICS

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Twin Studies

Twins: nature's little experiment

It is unethical to perform breeding experiments to study human genetics. However, there are natural "experiments" that have helped geneticists to study the influence of heredity and environment on the phenotypes of humans.

 

Dizygotic and monozygotic twins

Dizygotic twins are non-identical twins.

They share 50% of their genes like any other children from the same couple of parents.

Dizygotic twins are the product of two sperms fertilising two eggs. Many other animals produce multiple births like this (e.g. cats produce litters of 4 or 5 kittens). Humans normally produce single births because the parents invest a lot of care and attention on their offspring.

Monozygotic twins are identical twins.

They share 100% of their genes.

Monozygotic twins are the product of one egg fertilised by one sperm. Early on in the development of the embryo (usually during the first two weeks) the cells of the embryo split into two groups and develop into two embryos.

Monozygotic twins are a clone. They may provide an opportunity to determine whether a characteristic is controlled by inherited genes or the environment.

 

Concordance and discordance

The degree of concordance is the degree of similarity between two individuals. The data below shows the degree of concordance and discordance of twins for a selection of characters.

monozygotic twins

  • What does a high degree of concordance between the twins indicate?

  • What does a high degree of discordance between the twins indicate?

  • What do the differences between the data for monozygotic and dizygotic twins suggest?

  • Is it possible to distinguish between environmental and genetic effects this way?

 

Phenocopy

It is not always easy to distinguish between the effect of the environment and the effect of genes on a character.

Some characters can easily be caused by either.

Examples include:

Rickets, the growth of weak bones, is often caused be a deficiency of Vitamin D (calciferol) in the diet but it can also be caused be a mutation of a gene.

Albinism, a lack of pigment melanin in the skin, it can also be caused by a deficiency of protein in the diet or by a mutant of a gene.

Cretinism is a form of mental retardation. It can be caused by a deficiency of iodine in the diet or by a mutation of a gene.

 

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