OCR 21st Century - Biology B1 Full revision notes You and Your Genes

Full revision notes on the Biology OCR 21st Century GCSE - B1 topic

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Biology Revision Notes B1
B1 - You and your Genes
Genes, Chromosomes and DNA
Nearly every cell in your body has a nucleus and the nucleus contains your genetic materials.
The human cell nucleus contains 23 pairs of chromosomes, so that's 46 chromosomes in total! All of
these pairs are numbered and known for what they control. Eg: you have two no. 19 chromosomes
and two no. 14 chromosomes etc.
Chromosomes carry genes which control the development of certain characteristics such as hair
A gene is a short length of chromosome which is one molecule of DNA.
The DNA is coiled up to form the arms of the chromosome.
Genes can come in different versions. Each version gives a different characteristic such as blue eyes,
brown hair etc. The different versions of the same gene are called alleles.
Each gene codes for making a different protein. Proteins build cells. Having different versions of
proteins means that we are able to have separate characteristics.
Some proteins are called structural proteins. They're part of making up our skin, hair, blood and the
cytoplasm in our cells. Other proteins are enzymes. Enzymes control all of the chemical reactions in an
organism eg. Respiration. Enzymes also help with digestion by breaking down food molecules.
Genes and Variation
Chromosomes come in pairs because we have to have two parents to be able to come into
existence. One chromosome in every pair has come from each parent.
The sex cells (The sperm and the egg) are different to ordinary body cells because they only contain
23 single chromosomes rather than the usual 23 chromosome pairs.
When the egg is fertilised by the sperm, the 23 chromosomes in the egg combine with the 23
chromosomes in the sperm.
The fertilised egg then has 23 pairs of chromosomes, just like a normal body cell.
The two chromosomes in a pair always carry the same genes and each gene is always in the same
place on both chromosomes. Because the two chromosomes in a pair came from different parents,
they might have different alleles of these genes. Alleles are different versions of the same gene.

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Children get some of their alleles from each parent. This is why most children look a bit like both of
their parents. But they won't look identical to either one of their parents because they don't have all
of one of their parent's alleles, some came from the other parent. The child will actually have a brand
new, unique combination of alleles ­ that's why no two people in the world look exactly the same
(apart from identical twins).…read more

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b Bb Bb
b Bb Bb
They're all normal and boring because all of them are heterozygous, which causes the dominant gene
to overrule.
If two of these offspring now breed, they will produce new combinations of alleles in their kids...…read more

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A human genetic diagram is pretty much the same as any other one, but instead of showing alleles, it
shows sex chromosomes.…read more

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Health ­ Some people are more likely to get certain diseases such as cancer through genetics, but
lifestyle can also cause it, eg. if you smoke or eat a lot of junk food, you're more likely to get ill.
Intelligence ­ One theory is that although your maximum possible IQ might be determined by your
genes, whether you get it depends on your environment, eg. your upbringing, school life and diet.
Sporting ability ­ Genes determine your potential, but training is important too.…read more

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Defective alleles are responsible for genetic disorders. Most of these defective alleles are
recessive. Cystic fibrosis is a genetic disorder of the cell membranes. It results in the body producing
a lot of thick, sticky mucus in the air passages, gut and pancreas.
The allele which causes cystic fibrosis is a recessive allele `f', carried by about 1 person in 25. Because
it's recessive, people with only one copy of the allele won't have the disorder ­ they're known as
carriers.…read more

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The carrier parent will be a sufferer of course as
the allele is dominant, so you could say that there aren't any carriers for Huntington's disorder as
they are all sufferers if they carry the allele. The symptoms do not appear until the age of 40, by
which time the allele has been passed on to children and even grandchildren, hence the disorder
persists.…read more

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Gene therapy
Gene therapy could soon make it possible to cure or prevent genetic disorders. Eg. scientists can
insert a healthy copy of a gene to make up for a faulty one.
Example 1 ­ Treating cystic fibrosis
Cystic fibrosis (CF) affects about 1 in 2500 people in the UK. Scientists are trying to treat cystic
fibrosis with gene therapy. One method being tried out is to use a virus to insert a healthy copy of
the gene into cells in the airways.…read more

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To get cultures of one specific type of cell, researchers try to control the
differentiation of the stem cells by changing the environment they're growing in. So far, it's still a bit
hit and miss ­ lots more research is needed.…read more

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Another consideration is that some people think it's unfair for people to benefit from something
which is only possible because someone else has suffered or taken a risk. The trouble is, all new
medicines and medical procedures must be tested on human volunteers ­ and that always involves
some risk.
As well as individuals, the law is involved in regulating scientific research. Animal research is
regulated.…read more


Shreya Kochhar

Thank You So Much xxxx

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