AS Biology Chapter 2.2 The basics of genetics

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Chapter 2.2 ­ The basics of genetics
Phenotype ­ Physical and chemical characteristics that make up the appearance of an organism. The
phenotype is partly the result of the genetic information (genotype) passed from parents to
offspring and partly the effect of the environment in which the organism lives.
Humans have 46 chromosomes. Half are from the mother and half from the father. The two sets can
be arranged as matching homologous pairs in a karyotype.
Along each chromosome are thousands of genes, each gene being a different segment of DNA
coding for a particular protein or polypeptide. The chromosomes in a homologous pair carry the
same genes (except for sex chromosomes). The gene for a particular characteristic is found in the
same position or locus, so we usually have two genes for each characteristic.
Different versions of a gene are called alleles.
If both alleles coding for a particular characteristic are identical then the organism is homozygous for
that characteristic ­ it is a homozygote.
If both alleles coding for a particular characteristic are different then the organism is heterozygous
for that characteristic ­ it is a heterozygote.
Easy and cheap to raise, to Have a short life cycle so that the
maximise the chance of results of crosses and/or mutations
successful breeding and can be seen quickly.
minimise the cost of the
Suitable organisms
experiment ­ for valid results,
for a genetic
genetic investigations need lots
experiment have
of organisms.
the following
Produce large numbers of offspring so
that the results of any crosses are
Have clear, easily
statistically relevant.
distinguished features.
Polygenic ­ human traits controlled by several interacting genes.
Thalassaemia affects the polypeptide chains of the haemoglobin molecule, which carries oxygen in
the blood.
Haemoglobin is made up of 574 amino acids arranged in four polypeptide chains held together by
sulphur bridges.
There are two types of polypeptide chains in haemoglobin, the alpha and beta chains.

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Each chain is arranged around an iron-containing haemoglobin group.
Depending on the type of the disease, thalassaemia prevents the formation of either the alpha or
beta haemoglobin chains.
Alpha thalassaemia ­ It is inherited from genes in two loci on chromosome 16. So there are a total
of four genes, two on each chromosome (polygenic inheritance). The cause of alpha thalassaemia is
the deletion of one or more of the genes.…read more


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