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Summary Grade 12 Life Sciences - Life Sciences (Biology)
Summaries of Genetics and Genetic Engineering
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GeneticsInheritance
Genetics is the study of heredity and variation in living organisms.
Hereditary characteristics are transferred by gametes.
During fertilisation the male gamete from the father and the female gamete from the mother fuse
to form a zygote which develops into the new organism.
The complete set of genes of an organism is known as its genome, with the human genome
consisting of about 25 000 different genes.
It is a copy of all the essential DNA coding required to synthesise all the proteins needed by an
organism.
Each somatic cell possesses a complete set of genetic instructions (genes).
(Red blood cells do not have nuclei and thus no chromosomes/genes.)
Although all genes are found in somatic cells, only certain genes are activated/switched on in
specific cells. Thus, only the proteins that provide the structure and function of a particular cell,
are synthesised.
Genes are switched on by complicated mechanisms involving master/control genes.
99,9% of all human DNA is identical while the other 0,1% makes each human unique.
Gregor Mendel performed experiments of pea plants in his free time to study the inheritance of
characteristics from one generation to another.
Through his experiments, he formulated the basic laws of genetics.
The value of his work was only recognised 20 years after his death.
Mendel is regarded as the ‘Father of Genetics’.
,Mendel’s Hereditary Experiments
Mendel incidentally chose pea plants to do his hereditary experiments. Although he didn’t realise
it at the time, pea plants were a good choice as they show clear, observable, contrasting
characteristics.
He studied seven of these contrasting characteristics:
- tall and short plants
- smooth and wrinkled seeds
- yellow and green seeds
- inflated and constricted pods
- green and yellow pods
- purple and white flowers
- axial and terminal flowers
A further advantage was that pea plants are naturally self-pollinating and do not cross-pollinate.
Mendel cut off the anthers (containing pollen) and then performed cross-pollination himself,
using pollen of his choice.
He ‘painted’ ripe pollen onto the stigma with a small brush and could thus control the cross.
Initially, Mendel used parent plants that were true breeding for a specific characteristic.
These are plants that self-pollinate and all the offspring produced have the same characteristic
as the parent, Eg. a tall plant self-pollinates and only produces tall plants.
Conclusion from Mendel’s experiments
A hereditary characteristic (e.g. plant length in peas) is determined by two ‘factors’ (genes).
One gene of the gene pair comes from the individual’s mother and the other gene from the
fathers.
The gene pair is carried on 2 chromosomes of a homologous chromosome pair.
A particular gene occurs in two (sometimes more) different forms, that affect the same
characteristic in different ways.
The alternative forms of the same gene are known as alleles.
, Different alleles contain different information about the same characteristic/trait.
Eg. in pea plants the plant length trait is determined by a gene that results in tall plants and its
allele that results in short plants, while the seed colour trait is determined by a gene that
produces yellow seeds and its allele that produces green seeds.
The specific position each gene has on a chromosome is called the locus (plural: loci).
The alleles of a particular gene occur at the same locus on a specific homologous chromosome
pair.
One allele of a gene pair can mask another and this is known as the dominant allele.
The allele that is masked and not visibly expressed in the organism, is called the recessive allele.
Mendel’s Law of Dominance: If two alleles are different, only the dominant one will be expressed.
Genotype and phenotype
The genotype is the composition of the gene pair (2 alleles) for a specific trait.
An organism’s genotype is represented by two letters. Each letter represents one allele of the
gene that controls the trait.
A capital letter shows the dominant allele and a small letter the recessive allele.
The dominant allele (capital letter) is always written first.
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