Biology B1, B2, B3 OCR 21st century

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  • Created on: 28-09-14 17:39
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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 codes for a specific protein by
specifying the order in which AMINO ACIDS must be joined together.
These proteins can be: STRUCTURAL PROTEIN: Gives the body structure, rigidity and strength
JJJJJJJJJ E.g. Skin, Hair, Muscles etc
FUNCTIONAL PROTEIN: Enables the body to function E.g. Enzymes,
ffffff Antibodies etc.
The differences between individuals of the same species are described as VARIATIONS.
Variations may be due to:
GENOTYPE ­ The genetic makeup of an organism. The different characteristics that an individual
inherits, E.g. whether you have dimples or not.
PHENOTYPE ­ The observable characteristics the organism has. How the environment changes an
individual, E.g. cutting the skin may cause a scar.
IDENTICAL TWINS have the same set of genotype however any differences between them is
because of environment.
CONTINOUS VARIATION shows when some characteristics are controlled by several genes working
together e.g. eye colours and height. For instance it was originally believed that eye colour was due
to a single gene. It is now known that there are a number of genes coding for different pigments in
the iris, mainly on chromosome 15 in humans. This means that there is enormous variation in eye
B1.2 Why can people look like their parents, brothers and sisters but not identical to them?

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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 of chromosome, or different (HETROZYGOUS) ­ For example to give
blue eyes or brown eyes.
Sex cells only contain one chromosome from each pair.…read more

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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
alleles, the following diagram could be drawn:
A a
a Aa aa
A Punnett square diagram can also be used to represent how sex is determined.…read more

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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 gene is present i.e. both
an X and a Y chromosome are present, then testes will begin to develop.
Six weeks after fertilisation, the undifferentiated gonads start producing a hormone called
ADROGEN.…read more

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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 need to inherit both copies. You can inherit the disorder if one or both of your parents
carry the faulty allele, because it is DOMINANT.…read more

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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.…read more

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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 whether they are prone to certain conditions (e.g.
obesity) or diseases (e.g. cancer).…read more

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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 natural clones of each other.
It is now possible to make clones artificially by taking the nucleus from an adult body cell and
transferring it into an empty, unfertilised egg cell.…read more

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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 can cause us harm (e.g. bacteria that cause food
PATHOGENS are microorganisms that cause infectious diseases. BACTERIA and VIRUSES are the
main PATHOGENS.…read more

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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:
Chemicals in tears
Chemicals in sweat
Stomach acid
The body's first line of defence is called PASSIVE IMMUNITY, which means preventing the
PATHOGEN from entering in the first place.…read more



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Sum Ting

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