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Genetic diversity and adaptation - A-Level Biology notes
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Genetic diversity and adaptation - A-Level Biology notes
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Chapter 9: Genetic diversity and adaptation9.1 Gene mutation
Mutation: any change to the quantity or the base sequence Chromosome mutations: a random change in the
of the DNA of an organism structure or number of whole chromosomes. It takes
- Gene mutation: any change to one/more nucleotide 2 forms:
bases or a change in the sequence of the bases 1. Changes in the whole sets where someone has 3
or more sets of chromosomes instead of 2. This is
Substitution of bases: called polyploidy and occurs mostly in plants.
- Gene mutation where a nucleotide in the DNA is replaced 2. Changes in the number of individual
by another nucleotide. The significance of this change will chromosomes. Sometimes homologous pairs fail
depend on the precise function of the amino acid. There to separate during meiosis (non-disjunction) and
is little significance of the triplet of bases still codes for results in a gamete having either more or fewer
the same amino acid as before. chromosomes than one. This will result in all the
Deletion of bases: body cells having more or fewer chromosomes
- Gene mutation can occur due to removal of a nucleotide than usual.
from the DNA sequence which will cause a frameshift to a. Ex. down syndrome has an additional
the left. The polypeptide is unlikely to function properly. chromosome 21
9.2 Meiosis and genetic variation Process of meiosis: (2 nuclear divisions)
1. Meiosis 1: homologous pairs lineup and crossover
Cell division: where the chromatids twist around one another
1. Mitosis – 2 identical daughter cells with the same which create tension and results in some pieces
number of chromosomes at the parents cell falling off. These pieces then attach to the opposite
a. The importance of meiosis is to maintain a constant chromatid creating a new genetic combination
number of chromosomes in adults by halving the 2. Meiosis 2: chromatids randomly move apart by
number of chromosomes at a stage during meiosis independent assortment which is when homologous
2. Meiosis – 4 different daughter cells with each have the chromosomes are halved to form haploids
number of chromosomes as the parent cell
o Number of possible combinations of chromosomes for each daughter cell ¿ 2n
o Number of different combinations of chromosomes in the offspring produced ¿( 2¿ ¿ n)² ¿
(where n is the number of homologous pairs)
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