Genetics - Biology 
Meiosis
This is how gametes (sex cells) are produced
Gametes only have one copy of each chromosome - offspring get full amount of chromosomes as a result of this!
Steps of Meiosis
Before the cell divides it duplicates the genetic information which forms two armed chromosomes, one arm of each chromosome is and exact copy of the other arm
In the first division the chromosome pairs line up in the centre of the cell
The pairs of chromosomes are then pulled apart so that each new cell only has one copy of each chromosome. Some of father's chromosomes and some of mother's
The chromosomes line up in the centre of the cell in the second division
The arms of the chromosomes are pulled apart
DNA
A genome is a haploid (single) set of chromosomes in a gamete
Genes can be identified in a genome that are linked to different types of disease
Knowing what genes are linked to an inherited disease can help us understand them better
Genome
DNA is a chemical that all of the genetic material in a cell is made up from
Contains information which are used to make an organism what it is
DNA is made up of two strands which come together to create a double helix shape
Genes
A gene is a small section of DNA that is found on a chromosome
Structure of DNA
Each strand of DNA is made up of chemicals called bases
There are 4 different types of bases called Adenine, Thymine, Cytosine and Guanine (A,T,C,G for short)
A always pairs to T and C always pairs to G
A always pairs to T because they are paired by two hydrogen bonds whereas C always paris to G because they have three hydrogen bonds between them
Alleles
Different forms of the same gene are called alleles
Alleles can be dominant and recessive
If the allele is present in one or both of the chromosome pairs it is dominant
If the allele is present on both chromosomes in a pair then it is recessive
Click here for a bigger version of this diagram
Look at the diagram - the allele fro brown eyes is dominant. The allele for blue eyes is recessive.
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Someone who inherits one or two alleles for brown eyes will have brown eyes
Someone who inherits two copies of the allele for blue eyes will have blue eyes
Inherited Disorders
Some disorders are inherited from parents
An example is Cystic Fibrosis which is a genetic disorder of the cell membranes, it results in the body producing a lot of sticky mucus in air passages and in the pancreas
The allele which causes this disease is a recessive allele
About one in 25 people have this allele
People with only one copy of this allele will not have the disorder - known as a carrier
Both parents must be a carrier or have the disorder for a child to have the disorder
Genetic Diagrams
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Genetic diagrams show all of the possible alleles for certain characteristic