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Mutations A mutation is a change in the structure or amount of an organisms genetic structure.

Change in chromosome number 1. Non-disjunction during meiosis: this

occurs when a spindle fibre fails to form and members of a homologous pair of chromosomes fail to become separated

e.g. Down’s syndrome If non-disjunction of chromosome pair 21 happens in a human egg cell, then an abnormal egg (n=24) is formed. If fertilised by a normal sperm (n=23) and abnormal zygote is formed (2n=47).

Nearly 80% of non-disjunction leading to Down’s syndrome are of maternal origin. Frequency of occurrence relates to maternal age:

Pregnant women over 35 are routinely offered a foetal chromosome analysis.

2. Non-disjunction of sex chromosomes: this results in the formation of unusual gametes. e.g.1: Turner’s syndrome – if a gamete forms possessing no sex chromosomes and fuses with a normal X gamete, the resulting zygote has2n=45, i.e. 44+XO where O represents the lack of a second sex chromosome.

Individuals with Turner’s syndrome are always female and short in stature. They are also infertile. e.g.2: Klinefelter’s syndrome – If an XX egg is fertilised by a normal Y sperm or a normal egg is fertilised by an XY sperm, the resulting zygote has the chromosome complement 2n=47 (i.e. 44+XXY).

Individuals with Klinefelter’s syndrome are always male but, again, are infertile.

3. Complete non-disjunction and polyploidy When all the spindle fibres in a gamete mother cell fail during meiosis and none of the homologous pairs separate, complete non-disjunction results. This is most common in plants. Diploid gametes are produced which, when fertilised, produce plants which possess extra complete sets of chromosomes. This is called polyploidy.

Economic significance of polyploidy Polyploid plants are normally larger than diploid individuals. They also produce increased seed yield and fruit size. Many crop plants such as wheat, coffee, apples, tomatoes and strawberries are polyploid. Polyploid plants are also sterile however this is useful to humans as they produce seedless fruits, e.g. bananas. Polyploid plants often show increased vigour and disease resistance

Change in structure of one chromosome (Chromosome mutations) 1. Deletion: The chromosome breaks in

two places and the segment in between is lost with the two ends joining up giving a shorter chromosome which lacks certain genes. Deletion normally has drastic effects on the organism involved e.g. cri du chat in humans

2. Duplication: When a chromosome’s

homologous partner becomes attached at the end or inserted within it somewhere along its length. Sometimes the duplicated genes may introduce a new characteristic to the organism.

3. Inversion: A chromosome breaks in two places with the segment in between becoming reversed. If inversion has occurred in a chromosome it forms a complicated pairing with its homologous partner at meiosis making it difficult for the chromatids to separate leading to non-viable gametes.

4. Translocation If a section of one chromosome breaks off and joins to another chromosome (not its homologous partner) this is called translocation. This causes problems with pairing of homologous chromosomes during meiosis forming non-viable gametes

Top Tip and Handy Hint DICTD Duplication Inversion (Chromosome mutations) Translocation Deletion

NB Take care to read questions on mutations carefully – are you being asked to write about chromosome mutations (DICTD) or gene mutations (DIGSI)?

16a Chromosome mutations