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Types of gene mutation?

Base addition - extra bases added to the sequence
Base deletion - Removal of nucleotides
Base substitution - Nucleotide is replaced by a nucleotide with a different base


Chromosome mutation?

During cell division, chromosomes are not distributed evenly



- Cell starts with diploid number
- DNA replication occurs, forming two identical chromatids attached at the centromere
- Homologous chromosomes pair up and lie side by side forming a bivalent (Point of chiasmata)
- First division - homologous chromosomes separated; two cells with a bivalent in each and 23 chromosome in each; one chromosome from each homologous pair goes to opposite ends
- Second division; chromatids are pulled apart
- 4 cells with a unique set of 23 chromosomes that later develop into gametes


Homologous chromosomes

Genetically identical chromosomes, paired together, 1 from mother and one from father


Haploid and diploid

Haploid - half the norm number of chromosomes
Diploid - The normal number of chromosomes


Source of variation - Independent assortment

Chromosomes from mother and father can be re-arranged during Metaphase 1 and Anaphase
To calculate do 2 to the power of the number of pairs


Source of variation - Crossing over

During prophase 1 when the bivalent has formed the two chromatids can cross over, break and rejoin in different positions. The position at which the non-sister chromatids appear joined to each other are called chiasmata. When the chromosomes separate in anaphase 1, parts of the chromatids are swapped. When the cells are replicated it will be a mixture of the fathers and mothers chromosomes.


Source of variation - Random fertilisation

Any female gamete can join with any male gamete, all containing different combinations of chromosomes and alleles.


Directional selection

Example - melanic peppered moth during the industrial revolution or increase in the population of anti-biotic resistant bacteria
Change in the environment changed the selection pressure


Stabilising selection

Example - human birth weights
Polygenic characteristic - affected by many different genes
Selection pressure on babies with low or high birth weights
Babies with normal birth weight allele are more likely to survive


Why is it more useful to calculate the index of diversity than to record the number of species present?

- shows number of species present and number of individuals in each species
- therefore shows if there are a small number of each species in one


How to compare genetic diversity?

Compare the base sequences of DNA
Compare base sequences of RNA
Check for mutations