1. Mutation Chapter 16

2. Mutation A change in structure or amount of genetic material If phenotype is altered – affected organism is called a mutant Mutation can be from a tiny change in DNA structure to altering entire chromosomes

3. Chromosome mutations – change in no. Non-disjunction When spindle fibres fail during meiosis Homologous chromosomes fail to separate Gametes can gain or lose chromosomes Down’s Syndrome - non-disjunction of chromosome 21 in gamete mother cell - gamete receives 24 chromosomes – if fertilised, zygote has 47 - condition characterised by mental retardation/physical features - 80% of non-disjunctions of maternal origin

4. Chromosome mutations – non- disjunction of sex chromosomes Turner’s Syndrome Gamete with no sex chromosome fusing with a normal gamete Zygote has 45 chromosomes Results in females that are short, & infertile Klinefelters Syndrome Zygote with 47 chromosomes (XXY) Results in males that are sterile, infertile & short in stature

5. Chromosome mutations – complete non-disjunction When all spindle fibres fail Gametes are diploid (2 full chromosome sets) Produces zygotes with a full extra set This is also known as polyploidy These plants are often larger – economic benefit They are also often sterile – seedless Many have increased vigour & disease resistance E.g. normal & spelt wheat

6. Change in structure of 1 chromosome Change in number or sequence of genes Deletion - chromosome breaks in 2, and a segment is detached - certain genes can be lost - normally drastic effects E.g. cri du chat Duplication - a piece of a chromosome added into it’s homologous partner - set of genes is repeated

7. Change in structure of 1 chromosome Translocation - a section of 1 chromosome breaking off and re- attaching to a non-homologous chromosome - usually forms non-viable gametes Inversion - a section breaks out and re-attaches the opposite way round - when homologous partner pairs up, a loop is formed - crossing over causes chromatids to join permenantly - often leads to non-viable gametes E.g. turnicate locus in corn

8. Gene Mutations Change in 1 or more nucleotides in a DNA strand Codons are altered, so amino acids produced are altered Substitution & Inversion - ‘point’ mutations - bring only minor changes - rarely harmful (unless a critical amino acid is affected) Insertion & deletion - ‘frameshift’ mutations - more severe – large section of DNA mis-read - e.g. critical enzymes not produced

9. Frequency of mutations Mutations arise spontaneously (at random) Mutation rates very from gene to gene Most mutant alleles are recessive Mutagenic agents can raise mutation rates: - chemicals e.g. mustard gas - radiation e.g. gamma, X-rays, UV Mutation is the only source of new variation Sometimes mutant alleles can be an advantage - raw materials of evolution