Mutation Chapter 16

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

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

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

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

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

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

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

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