1. Abnormal Chromosome Number

2. A Change in Chromosome Number If the spindle fails at meiosis, this causes an incorrect number of chromosomes in the gametes. This is called Non-disjunction.

4. Aneuploidy After non-disjunction, one cell may have 2 copies of a homologous chromosome and the other cell may have none. If the 2 cells are fertilised by a normal gamete, you can get –A zygote with 3 copies of the homologue, which is called Trisomy. –A zygote with only 1 homologue – a condition called Monosomy.

5. Tisomy

6. Monosomy

7. Anueploidy This can occur with the autosomes or the sex chromosomes. Most autosome non-disjunction leads to death of the foetus.

8. Autosome Aneuploids Formed By Non-disjunction Trisomy 21- Down’s Syndrome (47 Chromosomes) 3 copies of chromosome 21. Produces a child with a sunny nature, characteristic facial features which include heavy skin folds above the eye, an enlarged tongue, short stature, and various internal disorders such as heart defects and susceptibility to respiratory infections.

9. Trisomy 21- Down’s Syndrome These children are usually mentally retarded, (severity varies). Most are sterile. They are prone to develop leukaemia and Alzheimer’s disease (these are known to be on chromosome 21) The incidence of the syndrome can be related to the mother’s age at conception.

10. Trisomy 21- Down’s Syndrome It is suggested when the mother is over 35 years she should have an amniocentesis test for Down’s Syndrome. The extra chromosome forms by non- disjunction during gamete production in one of the parents.

12. Trisomy 18- Edward’s Syndrome These babies have 3 copies of chromosome 18. It occurs once in every 10 000 live births, and affects almost every body organ. Babies rarely survive for more than a year, most die within a few weeks of birth. The mother’s age affects that occurrence as with Down’s Syndrome.

14. Trisomy 13- Patau Syndrome Babies have 3 of chromosome 13. Occurs 1 in every 5000 live births. Babies have serious eye, brain and circulatory defects as well as a cleft palate. Babies rarely survive for more than a year, most die within a few weeks of birth. The mother’s age affects that occurrence as with Down’s Syndrome.

17. Trisomys The trisomys of most other autosome chromosome aneuploids result in spontaneous abortion or miscarriage. Only Down’s Syndrome and the sex chromosome aneuploids are exceptions.

18. Sex Chromosome Aneuploids formed by Non-disjunction This results in live babies and abnormal adults.

19. Turner’s Syndrome XO called monosomy X; 45 chromosomes Produces sterile females who are abnormally short, have webbed neck and do not develop mature sexual organs. They usually have normal mental function.

21. Trisomy X XXX, 47 chromosomes Produces tall females who are frequently sterile and may be mentally retarded.

22. Klinefelter’s Syndrome XXY; 47 chromosomes Produces males who are tall and thin, sterile and underdeveloped sexually. Most have breast development resembling that of a mature woman. Often suffer from thyroid dysfunction, chronic pulmonary distress and diabetes. They often have a high-pitched voice and may be mentally retarded.

23. XYY Syndrome 47 chromosomes Produces males who are normal except for their extreme height and a tendency to severe acne. They are genetically sterile and have reduced IQ.

24. Extreme Cases XXYY; XXXY; XXXYY; XXXX; XXXXX; even XXXXYY Each syndrome has its own distinguishing characteristics, however there are 4 generalisations we can make about sex chromosome abnormalities.

25. Generalisations –A human must have at least 1 X chromosome to live. –The presence of a Y causes the individual to develop as a male, if Y is absent the individual is female. –The reason why these syndromes are not fatal is probably that all but 1 of the Xs will form a barr body –The more sex chromosomes a person has the taller they will be.

26. Polyploidy Sometimes all the chromosome pairs undergo non-disjunction simultaneously. This results in gametes that have a double set of chromosomes, ie they are 2n. If these are fertilised by an n gamete, the zygote will be Triploid or 3n. If 2 diploid zygotes fuse, a tetraploid or 4n.

27. Polyploidy In animals, a polyploid cross between 2 species is usually infertile (if they survive at all). In plants, polyploidy is relatively common, however these plants often fail to set seed as their homologous chromosomes do not match, although they can grow vegetatively.

28. Polyploidy in Gamete Formation Autopolyploidy Remember Auto means self. This is an individual with more than 2 chromosome sets, all delivered from a single species. Infertile offspring when the result is a 3n zygote. The triploid offspring can form if the non-disjunction occurs in only 1 gamete.

29. Autopolyploidy They are infertile because there are no matching pairs of homologous chromosomes to line up at meiosis, so they can’t make gametes. Fertile offspring form when diploid gametes formed by individuals of the same species fuse to form tetraploid offspring.

30. Autopolyploidy They are fertile because they have matching pairs of homologous chromosomes to line up at meiosis, so can form gametes. Ref pg 146 txtbk

31. Allopolyploidy This occurs when individuals of different but related species are accidentally hybridised.

32. Advantages of Polyploids They usually have larger cells which result in larger and more resistant plants. –E.g. modern cotton and wheat Polyploids with uneven numbers are sterile, so result in fruit with no seeds. –E.g. seedless watermelons –The banana is an infertile triploid- although it forms fruits, the seeds inside are underdeveloped, and the plant must be propagated by asexual means.

34. Seedless Watermelon and Bananas

35. Advantages of Polyploids The greatest advantage is the fact that it extends the range of adaptations of the plant, as a result of the combination of characteristics from 2 species. Ref pg 147 txtbk