Meiosis

Some of the chromosomes had different combinations of alleles than when they first started.-This is due to Crossing over of chromosomes. - During Meiosis 1, each chromosome lines up with its homologous pair, forming what is known as a bivalent. The chromatids twist around each other and create tensions. These tensions cause parts of the chromatid to break off at a point known as the chiasmata. This broken section rejoins with the chromatid of the homologous partner in a process known as recombination, resulting in an exchange of alleles. -This results in the combination of maternal and paternal alleles on one chromosome, so some chromosomes having a different combination of alleles then the chromosome in their parent cell.

Although all gametes had one of each 23 chromosomes, the combination of maternal and paternal chromosomes was unique in each one. -This is due to independent segregation. -During Meiosis 1, chromosomes line up with its homologous pair. When the homologous pairs arrange themselves, they do so randomly. The position that one chromosome takes in the pair (left or right) determines which one of the pair goes into which daughter cell. -Since the pairs are lined up randomly, so is the combination of maternal and paternal chromosomes that goes into the daughter cells. When these divided again, during Meiosis 2, the chromatids split and produce gametes. However if no recombination by crossing over of chromosomes occurs, only two unique variations of gametes would be produced from meiosis (ie. two pairs of genetically identical gametes) - Thus, recombination by crossing over must occur at least once in every meiosis case in order to produce genetically unique gametes.

The advantages of these two key stages in meiosis are that genetic variety is increased. This genetic variation allows species to adapt and so evolve, which is crucial to natural selection and thus survival. It also ensures that during random fertilisation, as the two gametes are genetically unique, and so to is the zygote formed. Without sufficient genetic variety, he gene pool would be significantly reduced and a similar affect to incest would be seen; offspring with higher chances of disability.