Summary AQA Biology A Level Revision - Studying Inheritance - Unit 17
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Course
Unit 1 BIOL1 - Biology and disease (7402)
Institution
AQA
Book
AQA Biology A Level Student Book
These revision notes provide an in depth summary of this specific chapter of AQA Biology in the A Level Specification -
Very comprehensive notes and enabled me to achieve an A* in my A Level Biology exams.
- Genotype is the genetic make up of an organism
o Describes all the alleles that an organism has
- Genotype determines the limits within which the characteristics of an individual may
vary
- There are other factors that can affect the ultimate phenotype of the individual
- Phenotype is the observable characteristics of an organism
- Is the result of the interaction between the expression of the genotype and the
environment
Genes and alleles
- Gene is a length of DNA that is a sequence of nucleotide bases
- Codes for particular polypeptide – could be enzyme needed in biochemical pathway
that leads to the production of the characteristic
- Genes exist in two or more forms called alleles – position of a gene on a particular
DNA molecule is known as the locus
- An allele is one of the different forms of a gene
- Only one allele of a gene can occur at the locus of any one chromosome
- However, in diploid organisms the chromosomes occur in pairs called homologous
chromosomes
o Thus there are two loci that each carry one allele of a gene
If the allele on each of the chromosomes is the same, then the organism
is said to be homozygous for the character
If they are different, the organism is heterozygous
- In most cases where two different alleles are present in the genotype, only one of
them shows itself in the phenotype – we then have dominant and recessive alleles
- Can also have codominance where both alleles contribute to the phenotype
o Either a blend of both features or both features are represented
- A gene may also have more than two allelic forms
o In this case, the organism is said to have multiple alleles
Example is in the case of blood type in humans
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