Biology B1, B2, B3 OCR 21st century

These are detailed revision notes which i have created for B1, B2, B3 OCR 21st century.

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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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When looking at the possibilities of inheriting and allele, we use a Punnett square diagram. This
shows all the possible pairings of alleles from sperm and egg at fertilisation.

For example if a male with a dominant A allele and recessive a allele was to mate with 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
(sex-determining 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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PGD has risks including inaccuracy in results-healthy embryo not being implanted and it may
also decrease the chance of the embryo surviving once it has been implanted.


Amniocentesis testing Chorionic Villus Sampling

However testing adults and foetuses for alleles that cause genetic disorders has implications that…

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For example: governments may have the ability to impose genetic tests on individuals by
implementing genetic screening programmes, but should they be allowed to do so? There is the
potential for genetic testing to be used to produce detailed genetic profiles. These could contain
information on everything from ethnicity to…

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Animals ­ clones in animals can occur naturally and artificially:

Clones of animals occur naturally when, during the earliest stages after fertilisation, the developing
embryo splits into two, they have the same genes. As the genes came from both parents they are
not clones of either parent, but they are…

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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) or they…

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The form of growth is known as EXPONENTIAL GROWTH. It follows the formula:

X(t) = a x b t/

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:…



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