Chromosomes can also be thought of as similar to a ball of wool - if you were to unravel the chromosome; like you would unravel a ball of wool; you would find long lengths of DNA – this is like the ‘thread’ of the ball of wool. Sections of DNA make up genes and genes are basically recipes for creating every function of the body; including looks; and as some researchers have found; personality and behaviour.
Because each person has 2 sets of chromosomes; that means they have 2 recipes for every gene. Usually; one version is stronger or more ‘dominant’ than the other and so that copy is seen or ‘expressed’ in the person.
When an egg or a sperm is formed in a parent; the chromosomes are shuffled around. Just like the red and black suits of cards getting mixed up. Not only that; the pairs of chromosomes actually ‘cross’ each other. This is like taking the red King and the black King; cutting them in exactly the same place through the centre and sticking the opposite parts together. Therefore; on one chromosome; the top section has the genes that were on the red set; the bottom section has the genes that were on the black set and vice versa. This is why children show characteristics of both sides of their family. After all of this shuffling about and crossing over takes place; the cell splits down the middle; taking only one complete set of shuffled up chromosomes.
When all this shuffling and crossing and dividing takes place; effectively; you have a random assortment of red and black recipes in each sex cell. That’s why you can have siblings that look wildly different and ones that look startlingly alike – it all depends on how the genes and chromosomes are shuffled and what versions end up in each egg or sperm cell. In the case of fraternal twins; 2 eggs have been released by the Mum and each fertilised by 2 different sperm from the Dad – there’s always the chance that those eggs from the Mum had a very similar set of shuffled up chromosomes and the same with the sperm – meaning that the fraternal twins could actually look very similar. The opposite is also true – the eggs from the Mum and the sperm form the Dad could; just randomly; have a very different shuffled up sets of chromosomes; meaning that the fraternal twins could look hardly anything like each other.