Genetic Twins Information Resources
Genetic Twins
Twins are the result of a mother giving birth to two offspring who were
conceived at the same time, and developed in the womb together.
Non-identical (or fraternal) twins occur when two ovum are released from the
ovaries at the same time and are then fertilised by two different sperm.
These kinds of twins will not look identical, and can be of differing
genders.
Semi-identical twins occur when a single egg is released from the
ovary that splits before being fertilized by the sperm. The offspring will
be semi-identical because half of the genes they inherited (coming from the
mother) will be the same, whereas different genes will have been inherited
from the father. Identical or genetic twins, occur when one ovum is
fertilised by one sperm.
The egg then splits in two, creating two embryos
that share an identical genetic makeup. They will look identical when born,
although environmental, nutritional and other factors will shape the way
they look later in life meaning they could look quite dissimilar from one
another. Giving birth to genetic twins is considered to be a birthing
anomaly, accounting for around 0.3% of deliveries.
The birth of twins can cause various medical complications, and often
requires specialist supervision. Due to the finite size of the womb, twins
are born on average three weeks premature, which increases the chances of
contracting illnesses and disease during childhood. Due to being born
prematurely, twins will often be born with relatively low birth weights;
they are also likely to be lighter than singletons throughout their lives.
The predisposing factors to giving birth to monozygotic twins are as yet,
unknown.
The factors affecting fraternal twin births are clearer with the
chance of a woman giving birth to fraternal twins doubling once she reaches
the age of 35. Other predisposing factors include race (with those of west
African decent more likely to have twins), height and weight (with those
above average more likely to conceive twins) and if the mother has had
several pregnancies before, or is herself the offspring of a fraternal twin.
Here are some interesting statistics on the topic of twin inheritability. A
woman who has a fraternal twin is two and a half times more likely to give
birth to twins than someone who does not have such a twin.
Someone who has
given birth to fraternal twins is three to four times more likely to have
another set of fraternal twins. A woman who is an identical twin, however,
is no more likely to conceive twins than anyone else.
Since monozygotic (identical/genetic) twins share the same DNA makeup, any
differences they display in traits or behaviour can potentially be ascribed
to environmental factors. Because of this, ‘twin studies’ are of particular
interest to researchers and scientists alike, due to the unique insight such
studies can provide.
Traditionally such studies had focused upon looking at
identical twins who were separated at birth, because this allows for
environmental and genetic effects to be separated from one another. The goal
of this kind of research has been to give habitability estimates based on
the similarities and differences shown between the twins.
These kinds of
studies have become more difficult to conduct in recent times because of
policy changes by adoption agencies, which means that many more siblings are
homed together. |
