Chapter 6- Chromosome mutations

1. Chapter 6- Chromosome mutations Sorry- we’re out of order on this one

2. Where we’re going: • Changes in chromosome number- both single changes and sets of chromosomes • Changes in chromosome structure or arrangement

3. Terms: Table 6.1 • in aneuploidy, you have only one or more chromosomes added or taken away: • Trisomy or monosomy. • In euploidy, you have whole sets of chromosomes added, in a variety of ways.

4. Adding or subtracting single chromosomes • Happens by nondisjunction:

6. Some relevant monosomies: • most monosomies/trisomies in humans die- they are lethal (a large % of miscarried babies have chromosomal defects). However, there’s one case where it results in viable individuals: • Turner syndrome- female with only one X chromosome. These individuals are sterile, but with certain traits- tend to be short, and have flaps on the neck. 1/2000 females- many die in utero.

7. Extra chromosomes: • Sex chromosomes • Klinefelter syndrome: These are XXY males- somewhat feminized males- no beard, sterile. 1/500 males • XXX (trisomy) females- sometimes normal, 1 in 1200 females. • XYY: 1/1000 in general population, much higher in prison and mental hospitals. However, there are lots of them that aren’t criminals- a classic nature vs nurture situation!

8. Autosomes: the only trisomy that lives a long time are Down syndrome trisomies. • trisomy 21- generally the result of a non-disjunction in the mother- 95% of the time. As you all know- likelihood increases w/ age- women’s eggs are all present at birth- and they sort of sit in prophase I, waiting to undergo further meiosis. So, the older a woman is, the more likely that a mistake occurs. • There’s also an inherited version-translocation-we’ll discuss later.

9. Changes in whole sets of chromosomes • N= haploid number, so we’re 2N, diploid. • Triploid, tetraploid, etc. possible. • Thought to occur by rare events- sex cells that have not undergone division. • Autopolyploidy and allopolyploidy:

10. Triploids are generally sterile, b/c you get strange combinations upon segregation, and these result in non-viable gametes.

11. Examples from plants: • triploids: bananas, seedless watermelons- triploids are usually sterile. When they separate, they would result in situations where the meiotic cells would have a mixture of some trisomy chromosomes and some normal ones. This results in a mess that usually isn’t viable. • tetraploids: coffee, peanuts, MacIntosh apples, strawberry is an octaploid! • Allopolypoidy- chromosomes from two related plants- American cotton, and some wheat types are tetraploid or hexaploid.

12. Hi everybody • Symposium Friday- no class  • 2 presentations, 2 pages, 5 points

13. So to review • Aneupoid or eupolid? • Down syndrome? • Bananas?

14. Variations in chromosome structure- these can occur due to things like X-rays, or as a result of unequal crossing over in cells, for some types. • Deletions and duplications • Inversions • translocations:

16. Inversions result in “loops” forming during meiosis; and a lowered level of fertility.

17. Crossovers within the inversion result in lethal duplications & deletions.

18. Translocations We’ve seen these before in cancer

19. X-ray goes ZAP! Half of the Meiosis I products will be defective!

20. Hereditary Down syndrome

23. What to know • Mostly terms- • Some stories- how we get mono-trisomies; • The fact that inversions/translocations lower fertility, but not the process.