Genetics

GCSE biology

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DNA carries the genetic code which determines the characteristics of a living thing.
Genes are sections of DNA which code for a specific protein by specifying the order of the
amino acids are joined together.
Chromosomes contain DNA
Alleles are different forms of the same genes e.g. you can have a gene for brown eyes and
blue eyes but only the more dominant allele will appear in the offspring.
We have 23 pairs of chromosomes and 46 single chromosomes.
Our genes are in pairs ­ we have one gene for e.g. eye colour from mam and one gene from
dad. If mam has blue eyes and dad has brown eyes these are different forms of the same
gene so are called alleles. However as the allele for brown eyes is dominant this is the colour
that will appear in the offspring.
Genetic cross for male/female
The 23rd pair of chromosomes are labels either XX or XY they decide whether the offspring is
male or female. The Y chromosomes cause male characteristics to develop whereas the XX
cause female characteristics to grow.
X X
X XX XX
Y XY XY
The genetic diagram shows there is a 50% chance of the offspring being female and a 50%
chance of the offspring being male.
Mitosis produces an exact replica of a cell before it dies. This is also to replace damaged
cells too. This is an example of Asexual reproduction and as a result produces little variation.

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Process:
­ Firstly the cell replicates its DNA to form X shaped chromosomes.…read more

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Chest infections
Coughing and wheezing
The allele for the disease is recessive and so both parents need to have the disease/be
carriers of the disease for it to have a chance of showing up in the offspring. Parents can be
carriers of cystic fibrosis and not actually have it as it is caused by a genetic disorder.…read more

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However there are only a limited number of adult
stem cells in the body so e.g. if you have an accident and injure your brain severely there
aren't enough stem cells in the brain to turn into brain cells to replace cells lost.…read more

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However in Mendel's time his work was not accepted as no one knew anything of genes/DNA
etc.…read more

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