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B1 ­ You and your genes

B1.1 What are genes and how do they affect the way that organisms develop?

A GENE is a short section of DNA. Genes carry instructions that control how you develop and function ­ they are
long molecules of a molecule called DNA. Each gene…

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Parents pass on their genes to their offspring in their sex cells.

A pair of chromosomes carries the same genes in the same place, on each chromosome within the pair. However,
there are different versions of a gene called ALLELES. These alleles may be the same (HOMOZYGOUS) on each pair…

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A a
a Aa aa

A Punnett square diagram can also be used to represent how sex is determined. This is because one of the 23
pairs of chromosomes in a human cell is the sex chromosome. In females the sex chromosomes are the same ­

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Sex Determination:

The sex of an embryo is determined by a gene on the Y chromosome called the SRY (sexdetermining region Y)
gene. If the gene is not present i.e. if there are two X chromosomes present, the embryo will develop into a
female and ovaries will grow. If the…

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HUNTINGTON'S DISEASE is another genetic that affects the central nervous system. However is caused by a
dominant allele ­ the presence of just one dominant allele can cause the disease. You only need to inherit one
copy of the faulty allele to have Huntington's disorder, unlike cystic fibrosis, where you…

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However testing adults and foetuses for alleles that cause genetic disorders has implications that need to be
considered, including:

Risk of miscarriage as a result of cell sampling for the genetic test
Using results that may not be accurate , including false positives and false negatives

Outcome Test Result Reality…

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Plants asexual reproduction in plants can take a number of forms:

Some plants such as strawberries produce shoots called RUNNERS. These eventually break off and become new
strawberry plants, clones of the original.…

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EMBRYONIC STEM CELLS are unspecialised cells that can develop into ANY type of cell, including more embryonic
stem cells.

As a result of being unspecialised, stem cells from embryos and adults offer the potential to treat some

For example ­ skin can grow as a treatment for serious burns…

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B2 ­ Keeping Healthy

B2.1 How do our bodies resist infection?

MICROORGANISMS are organisms that are too small to see with the naked eye. They include BACTERIA , VIRUSES
and FUNGI . They can be beneficial to us (e.g. the bacteria that live in our intestines can produce certain vitamins)…

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When microorganisms enter the body, they release toxins. The toxins damage cells to cause the symptoms of the
disease. The body's first line of defence is its NATURAL BARRIERS which include:

Chemicals in tears
Chemicals in sweat
Stomach acid

The body's first line of defence is called PASSIVE IMMUNITY…


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