The Death of a Gene

When a gene is duplicated, the new copy is often dead-on arrival (lacks the regulatory elements). 

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One is unequal crossing over, in which recombination happens between different positions on chromosomes that are misaligned during meiosis. 

Another is replication slippage, in which the DNA polymerase loses its place and copies a segment of chromosome twice.

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A third way that gene duplicates arise is by retrotransposition.

Here the messenger RNA from a gene is reverse-transcribed to DNA, which is then integrated into the genome.

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As a chromosome with a new gene duplicate spreads through a population, there is polymorphism in the number of copies of the gene that individuals carry. This situation, called copy number variation.

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Mixtures of exons duplicated from genes with different functions can generate new genes with new functions, a process that is called exon shuffling.

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Gene duplication happens on a massive scale when whole genome duplication occur. whole genome duplication are two type, 

1.Autopolyploid

2.Allopolyploid