Genes and Variation

Sperm and egg cells have half the normal amount of DNA

 In body cells all cells except from sex cells, chromosomes come in pairs because of two parents. One chrmosome in every pair has come from each parent. 

1) The sex cells are different from ordinary body cells because they contain just 23 single chromosomes -  that's one chromosome from each pair. 
2) When the sperm fertilisers the egg during asexual reproduction, the 23 chromosome in the egg combine with the other 23 from the sperm.
3) The fertilised egg then has 23 pairs of chromosome.
4) The 2 chromosomes in a pair always carry the same genes in the same place. Alleles are different versions of the same gene.

Children resemble both parents, but are identical to neither

Half of a child's chromomes have come each parent. This means that

1) Children get some of their alleles from each of their parents
2) This is why most children look a bit like both their parents
3) They won't exactly be like either one of the parents because they haven't got all the same alleles as some come from the other parent.
4) Every child will have a new unique combination of alleles except twins

Genes are shufffled together randomy to make eggs and sperm 

When people produce eggs and sperm cells, their pairs of chromosomes separate and go into different cells 

 1) 2 chromosomes in a pair are never identical because they have different alleles.

 2) So, when they go into 2 different sex cells, eah of the 2 cells gets different alleles

 3) Each of the 23 chromosome pairs separates independantly

 4) So there are millions of different chromosome combination that can be produced from thr separation of 23 pairs. 

 5) This means that all sex cells produced by one individual will probably all be genetically different

 6) When a woman releases an egg it can be fertilised by any one of millions of different sperm released by her partner 

 7) All of this means that the chances of two siblings being identical are absolutely minuscle. Brothers and sisters tend to look a bit alike, but there are always differences.