How are identical twins formed?
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Twins - identical and fraternal
** Twins - identical and fraternal **
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Twins - identical and fraternal
Twins account for over 90 per cent of multiple births. Identical
(monozygotic) twins form when a single fertilised egg (ovum) splits in two.
Fraternal (dizygotic) twins develop from two eggs fertilised by two sperm,
and are no more alike than individual brothers or sisters (siblings) born
at different times.
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Multiple births are more common than they were in the past, due to the
advancing average age of mothers and the associated rise in assisted
reproductive techniques, in particular the use of fertility drugs. Twins
account for over 90 per cent of multiple births. There are two types of
twins – identical (monozygotic) and fraternal (dizygotic).
To form identical twins, one fertilised egg (ovum) splits and develops two
babies with exactly the same genetic
Identical twins - Simple English Wikipedia, the free encyclopedia
** Identical twins **
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*Identical twins* are genetically identical: they have the same genes. They
are formed by a fertilised egg dividing into two separate individuals, and
are always of the same sex. They may be called *monozygotic* or MZ twins
(mono = one; zygote = fertilised egg). They contrast with /fraternal
twins/, who are formed by two separate eggs fertilised by two separate
sperms, and who are not always the same sex (DZ = dizygotic). Both types of
twin are carried in the same uterus at the same time, so their birth
environment is the same.
Research shows that the frequency of monozygotic twinning is one in 240
births. Fraternal twins are twice as common.^
Identical twins are natural clones. Because they carry the same genes (and
this can be proved), they may be used to investigate how much heredity
contributes to individual people. This is the nature vs nurture question.
Studies with twins have been quite interesting. If we make a list of
characteristic traits, we find that they vary in how much they owe to
heredity. For example:
· Eye colour: entirely inherited.
· Weight, height: partly inherited, partly environmental.
· Which language you speak: entirely environmental.
The way the studies are done is like this.^ Take a group of identical
twins and a group of fraternal twins, and a group of siblings from the
population. Measure them for various traits. Do a statistical analysis
(such as analysis of variance). This tells you to what extent the trait is
inherited. You will find that all those traits which are partly inherited
will be significantly more similar in identical twins.
Studies like this may be
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