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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.
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.
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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