Mutations Chromosomal Abnormalities Chapter 13 Mutations Change in
Mutations & Chromosomal Abnormalities Chapter 13
Mutations Change in structure or amount of genetic material – results in a mutant Caused by nucleotide alterations in DNA ‘Point’ mutations: Substitution or inversion Rarely critical, unless a critical amino acid is changed E. g Haemoglobin S for sickle cell ‘Frameshift’ mutations: Deletion or insertion Quite critical – large section of DNA altered
Mutation examples Phenylketonuria (PKU): Dietary phenylalanine normally broken down by an enzyme - phenylalanine hydroxylase PKU – allele of gene to make the enzyme is faulty Alternative metabolites build up Brain cell metabolism affected Effects minimised by a strict diet
Other examples Albinism: – failure to synthesise melanin - enzyme responsible (melanocyte tyrosinase) not synthesised Cystic Fibrosis: Mucus secretion controlled by gene on chromosome 7 Homozygous mutants = abnormally thick mucus produced 1 in 2500 with condition (1 in 25 carriers)
Frequency of mutation Rare, random, and spontaneous Mutation rates vary widely Most mutant genes are recessive Mutagenic Agents: Increase mutation rates e. g chemicals (mustard gas) & radiation (UV, X-rays, gamma)
Chromosomal Abnormalities During meiosis chromosomal compliment can become abnormal Non-disjunction – spindle fibres fail - homologous chromosomes fail to separate Some gametes have extra chromosome, some have less
Down’s Syndrome Non-disjunction at chromosome 21 Eggs/sperm can have 24 chromosomes If fertilised by sperm = diploid with 47 Features: mental retardation, physical changes 80% of mutations are of maternal origin More common in older women
Non-disjunction of sex chromosomes Turner’s Syndrome (1 in 2500): Gamete with no sex chromosome + a normal X gamete Zygote with 45 chromosomes – female (44+ XO) Features:
Non-disjunction of sex chromosomes Klinefelter’s Syndrome (1 in 1000): XX egg + normal Y sperm, or normal X egg + XY sperm Undeveloped sexual features – lack of testosterone Features:
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