B1: Genes and Variation

Sperm and egg cells have half the normal amount of DNA
1. The sex cells(sperm and egg) contain 23 single chromosomes each . 1 from each pair
2. When the sperm fertilises the egg the 23 chromosomes in each sex cell combine
3. The fertilised egg then had 23 pairs of chromosomes.
4. The 2 chromosomes in a pair carry the same gene, each gene is always found in the same place on the 2 chromosomes. They might have different alleles of these genes. - Alleles are different versions of the same gene
Half the childs chromosomes have come from each parent
1. Children get some of the alleles from each parent.
2. This is why most children look like both parents.
3. But they wont be exactly the same because, they havent got all the same alleles.
4. Each child has a new completely unique, combination of alleles- thats why no two people look exactly the same
When people produce sperm or egg cells, their pair of chromosomes separate and go into different cells
1. The 2 chromosomes in a pair are never identical, because they have different alleles
2. When they go into 2 different sex cells, each 2 of the cells gets different alleles
3. Each of the 23 chromosomes pairs separates independently
4. million of chromosome combinations.that can be produced form the separation of 23 pairs
5. This means that the sex cells produced by one individual will probably 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 looking identical are tiny but siblings tend to look a bit alike but there are always differences.

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B1: Genes and Variation

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