Genetic Changes - Gene Pool And Mutations

Gene Pool

All of the alleles possessed by an entire population of organisms, which may potentially be passed onto the next generation

Allele Frequencies

When all genotypes in a population are known, the number of each kind of allele can be counted

• Often expressed as percentage or decimal

Changing Allele Frequencies

They can change due to:

• New alleles can arise from mutations

• Some alleles can disappear due to mutations

• Changing environmental pressures can favour some alleles over others and they may become more common

Mutations

changes in DNA that can occur randomly during DNA replication

• All genetic variation between species and individuals of the same species is due to this

• Can be neutral or beneficial

Neutral Effects Of Mutations

Have no effect on survival

• Eye colour, birth marks

Harmful Effects Of Mutations

Decrease likelihood of survival

• Sickle cell anemia, down syndrome

Beneficial Effects Of Mutations

Increase likelihood of survival

• Sickle Cell anemia providing ‘immunity’ to Malaria, immunity to HIV

Somatic Mutations

Occur in body cells and only affect that individual

Germline Mutations

Affect gametes and therefore heritable

• This allows new mutations to enter the gene pool

Point Mutations

Only affect only one triplet in the genetic code

• Single base mutations as they layer, add or remove a single nucleotide base from a sequence of DNA or RNA

Substitution Mutations

Replacement of one nucleotide by another in the DNA. They may be silence, missense or nonsense

Frameshift

Involve one or two nucleotides being added or removed, altering every codon from that point onwards. They may due to an insertion or deletion of a nucleotide base

Substitution And Silent Mutations

Result in a new codon that codes for the same amino acid. This will have no effect on the organism because even after the code has changed, the instruction remains the same.

• The reason DNA code is known as redundant

Missense Mutations

Result in an amino acid replacement

• A protein is still produced, although it may not function depending on the amino acid that was replaced

Nonsense Mutations

Result in the creation of a stop codon

• Changing of a single base will have a major effect as it changes instructions completely

Frameshift Mutation

Involve one or two nucleotides being added or removed, altering every codon from that point onwards

• Significant effects on polypeptide because every codon is altered so too is every amino acid they code for after that is mutation

Block Mutations

Involve large sections of chromosomes and typically multiple genes. Come in five main forms:

• Duplication

• Deletion

• Inversion

• Insertion

• Translocation

Block Mutations - Duplication

Replication of sections of chromosomes that results in multiple copies of the same genes on that chromosome

• Thousands of repeats

Block Mutations - Duplication

Sets of genes can be replicated such as sections of the X- chromosome in fragile X syndrome

• Can be over 700 repeats of a small section of chromosome

Block Mutations - Deletion

Remove sections of chromosome, sometimes whole genes

• Lead to disrupted or missing genes,, which can have serious effects om growth and development

Block Mutations - Inversion

Section of the chromosome breaks off, rotates 180° and reattached to the same chromosome

• Could happen to only two bases or several genes

Block Mutations - Insertion

When one section of chromosomes breaks off and attaches to a different chromosome

• Depending on how the chromosomes move during meiosis some gametes may end up with two copies of the inserted gene, or none at all

Chromosomal Abnormalities

When a mutation involves whole chromosomes or changes to the number of chromosomes. Two main forms of chromosomal abnormalities:

• Aneuploidy; an extra or missing chromosomes

• Polyploidy; Having more then two full sets of chromosomes

Aneuploidy

The addition or loss of chromosomes, usually caused by non-disjunction.

• Aneuploidy often results in miscarriages in humans.

• Aneuploidy occurring in plants results in sterility.

Polyploidy

When gametes are not haploid, and may involve the fusion of a haploid gamete with a diploid gamete. This would create a ‘triploid’

• Rare condition in animals but common in plants

• Occurs due to errors in mitosis