1. Abnormal meiosis Sometimes mistakes occur during meiosis –Segments of chromosome (block of genes) can be lost or added – chromosome mutation / aberration –Cell may be missing a chromosome or have an extra chromosome (aneuploidy – incorrect number of chromosomes) – Plant cells have more than 2n sets of chromosomes in nucleus (3n, 4n – called polyploidy) Examples of aneuploidy –Down’s syndrome (3 x chromosome 21- trisomy) –Turner’s syndrome (female has 1 X chromosome – monosomy) –Klinefelter’s syndrome (3 x sex chromosome – XXY) 1Tommy Botha

2. How aneuploidy happens Aneuploidy – Trisomy – chromosome 21 Chromosomes of homologous pair 21 do not separate during meiosis One gamete (♀) may have two chromosomes for chromosome 21 and the other (♀) does not have a chromosome 21. Fertilisation: 2 x 21 (♀) + 1 x 21 (♂) = 3 x chromosomes 21 Extra chromosome 21 (trisomy) - baby This process is called non-disjunction (failure to separate) 2Tommy Botha

3. The diagram below shows an abnormality during a certain phase of meiosis with the number 21 chromosome pair (labelled A). 1. What is the specific purpose of this phase of the cell division? 2. What specific genetic condition does this abnormality (shown by A) cause? 3.Give TWO symptoms of this genetic disorder. 4.People with this condition are often sterile. Explain why you think this is so. 3Tommy Botha

4. How aneuploidy happens Aneuploidy – Sex cells - Trisomy and Monosomy Chromosomes of homologous pair 23 do not separate during meiosis One gamete (♀) may have two chromosomes for chromosome 23 and the other (♀) does not have a chromosome 23. Fertilisation: 2 chromosomes 23 (♀) + 1 chromosome (♂) = 3 chromosome 23 Extra chromosome 23 (trisomy - XXY) AND Fertilisation: 0 (♀) + 1 (♂) = 1 X chromosome 23 Only ONE X chromosome (monosomy - X0) 4Tommy Botha

5. POLYPLOIDY Polyploidy – Plant cells with more than the diploid set of Chromosomes Chromosomes of all the homologous pairs of one organism do not separate during meiosis One gamete (♀) may have two sets of chromosomes. Fertilisation: 2n (♀) + n (♂) = 3n (triploid) OR Self pollination and Fertilisation: 2n + 2n = 4n (tetraploid) 5Tommy Botha

6. Polyploidy Common in plants and rare in animals Can occurs spontaneously – natural polyploids forming – sympatric speciation – plant evolution Artificially - plant breeding for agriculture – ± 47 % of flowering plants are polyploids e.g. wheat, corn, bananas and apples – specially developed / breed Cells of polyploidy plants are bigger – results in bigger fruits, flowers and plants. Tetraploid apples can be twice the size of diploid apples 6Tommy Botha