Why do twins look the same?

Ever wondered why the twins have such a striking resemblance with each other that they look identical? Well, the reason behind these similarities is the inheritance of the same genes from their parents. 

So, what are genes and why do their inheritance create such striking features?

A gene is the basic unit of heredity which is transferred from parents to offsprings. A common example of heredity is that the offsprings tend to have the same eye colour and blood type as that of their parents. In most cases, hair colour, dimples, curly hairs, and even baldness are passed down from parents to their young ones – so it is pretty clear that genes play a very crucial role in determining how a person appears. 

Genes are essentially composed of DNA or deoxyribonucleic acid. The genetic code in all living organisms is made up of DNA and genes are nothing but a specific sequence composed of DNA. DNA molecules themselves are made up of three major components:

1. a ribose sugar

2. a nitrogenous base

3. a phosphate group

The nitrogenous bases that are found in DNA molecules are of four types: adenine, thymine, guanine and cytosine. A DNA molecule is helical in structure and is made up of two complementary chains of equal length. The nitrogenous bases of each chain pair with their complementary base in the corresponding chain to form base pairs. A gene varies considerably in size ranging from a few hundred base pairs to as long as 2 million base pairs!

Every individual has two copies of each gene, one that is inherited from the biological father and the other inherited from the biological mother. That’s why we have the characteristics of both our father and mother. 

Now let’s explore why most twins look alike

During mating, the sperm cell of a man and the egg cell of a woman combine to form a single zygote. Normally a single zygote develops and grows within the female to produce a baby. But when this zygote cell splits into two individual cells and each cell develops into individual babies, they become identical twins. Since these two cells have come from a single sperm and single egg, they carry the same genes. Having the same genes enables them to have the same physical appearance and resemblance with one other. Such twins are known as identical twins.

Some identical twins may have different heights and weights. This is because height and weight depend not only on genetic factors but on external factors as well such diet. 

Some of the famous identical twins around the world are the Olsen twins, Disney stars Dylan Sprouse and Cole Sprouse, and many more.

Why do all twins don’t have a similar physical appearance?

There are often siblings who look very different from each other but are in fact twins. They are called non-identical or fraternal twins. Sometimes, there are two egg cells released, and both get fertilized by separate sperm cells. This case is very very rare and results in non-identical twins, and the offsprings may have different appearances and different traits. Unlike identical twins that share nearly 100 per cent of their DNA, fraternal twins share only about 50 per cent of DNA composition.

Fun Facts

1. Even though identical twins have a near-identical appearance, they have different fingerprints.
2. Twins start interacting in the womb at 14 weeks.
3. Nigeria has the highest rate of twins birth. In Nigeria, the chance of having twins is 1 in 22.

True/False

1. All identical twins have the same physical appearance and resemblance with one other    (T/F)
2. Gene is the basic unit of heredity.                                                                                                     (T/F)
3. An individual has only one copy of a gene.                                                                                      (T/F)
4. Fraternal twins share 100 per cent DNA composition.                                                                  (T/F)
5. DNA molecule is composed of adenine, guanine, cytosine and thymine.                                 (T/F)

Objective Questions

1. Which physical traits children inherit from their parents.
   (a) fingerprints
   (b) eye colour
   (c)  tattoos
   (d) none of the above
2. What is the percentage of shared DNA between identical twins?
   (a) 50 %
   (b) 70%
   (c) 100%
   (d) 20%
3. Which of these countries is among those with the highest rate of twin births?
   (a) Japan

   (b) Nigeria

   (c) Brazil

   (d) India
4. What are genes made up of?
   (a) DNA
   (b) RNA
   (c) plant cell
   (d) none of the above