Biology B1

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Genes, DNA and chromosomes – the basics:
Most of the cells in the body have a nucleus, which contain specific
genetic material that is unique to each person. This is called DNA;
the arrangement of this makes people physically and internally
different. Long double helix strands of DNA are coiled up in
chromosomes. There are 23 pairs of chromosomes. Small lengths of these
chromosomes are called genes, which code for your different
characteristics. Alleles are different versions of genes. For example
the gene for eye colour could contain an allele for blue eyes or brown
The 23rd pair of chromosomes controls your gender. All men have an X
and Y chromosome. On the Y chromosome, there is an SRY(sex determining region Y). This produces the proteins to develop the male sexual organs (causes the testes to form instead of ovaries). All women have an X and X chromosome. In women, the protein is not produced so ovaries form.
Sex cells (eggs and sperm), which are called gametes, only contain 23
single chromosomes. This is because when they are fertilized, the
cells then contain 46 single chromosomes (23 pairs).
Genetic Disorders:
Cystic Fibrosis:
Cystic Fibrosis is a disorder caused by a recessive allele. Therefore


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