1. When Meiosis goes wrong…

2. What can go wrong during meiosis?

3. What can go wrong during meiosis?
Nondisjunction – The failure of homologous chromosomes or non-sister chromatids to separate during anaphase I or anaphase II

4. What can go wrong during meiosis?
Nondisjunction – The failure of homologous chromosomes or non-sister chromatids to separate during anaphase I or anaphase II
One gamete ends up with two copies of a chromosome, the other with zero
Mitosis then spreads this chromosomal error throughout the organism

5. Alteration of Chromosome #
Trisomy = 3 copies of a particular chromosome
Monosomy = 1 copy

6. Alteration of Chromosome #
Trisomy = 3 copies of a particular chromosome
Monosomy = 1 copy
Polyploidy = double/triple/etc. of entire genome (whole set of chromosomes)
Kills animals (prevents development), but makes plants bigger and stronger
Can occur from 2 sperm fertilizing 1 egg or by a diploid gamete

7. Alteration of Chromosome #
It is common for humans to have an irregular # of chromosomes, but usually disastrous
1/3 of all pregnancies end in spontaneous abortions (natural miscarriage), and ½ of these are due to nondisjunction

8. Down’s Syndrome (Trisomy 21)

9. Down’s Syndrome (Trisomy 21)
3 copies of chromosome #21
1 in 500 children in U.S.
Flattened face, short stature, mental retardation, shortened lifespan, heart defects, sterile

10. Down’s Syndrome (Trisomy 21)
Occurrence directly related to maternal age
Under 30

11. Down’s Syndrome (Trisomy 21)
Occurrence directly related to maternal age
Under in 952

12. Down’s Syndrome (Trisomy 21)
Occurrence directly related to maternal age
Under in 952
Under 35

13. Down’s Syndrome (Trisomy 21)
Occurrence directly related to maternal age
Under in 952
Under in 378

14. Down’s Syndrome (Trisomy 21)
Occurrence directly related to maternal age
Under in 952
Under in 378
Under 40

15. Down’s Syndrome (Trisomy 21)
Occurrence directly related to maternal age
Under in 952
Under in 378
Under in 106

16. Down’s Syndrome (Trisomy 21)
Occurrence directly related to maternal age
Under in 952
Under in 378
Under in 106
Under 45

17. Down’s Syndrome (Trisomy 21)
Occurrence directly related to maternal age
Under in 952
Under in 378
Under in 106
Under in 30

18. Down’s Syndrome (Trisomy 21)
Occurrence directly related to maternal age
Under in 952
Under in 378
Under in 106
Under in 30
Under 50

19. Down’s Syndrome (Trisomy 21)
Occurrence directly related to maternal age
Under in 952
Under in 378
Under in 106
Under in 30
Under in 11

20. Down’s Syndrome (Trisomy 21)
Occurrence directly related to maternal age
Under in 952
Under in 378
Under in 106
Under in 30
Under in 11
Trisomy 13, 15, and 18 also occur, but die shortly after birth. All other human trisomies fail to develop

21. Nondisjunction of Sex Chromosomes
Far less serious
Why?
Y chromosome carries very few genes
X chromosome will mostly shrivel up into an inactive state if an extra is present (Barr body)

22. Klinefelter’s Syndrome (XXY)
1 in 2,000 births
Underdeveloped male sex organs
Feminine secondary characteristics
Sterile

23. XYY
1 in 1,000
Normal looking male
Tall with relatively severe acne

24. XXX 1 in 1,000 Tall female with irregular menstrual cycle
Otherwise normal, needs karyotype (picture of chromosomes) for diagnosis

25. Turner’s Syndrome (XO)

26. Turner’s Syndrome (XO)
1 in 2,000 female births
Only viable monosomy (the only chromosome that humans can survive with only one copy of)

27. Turner’s Syndrome (XO)
1 in 2,000 female births
Only viable monosomy (the only chromosome that humans can survive with only one copy of)
Normal until puberty
Short, sterile, no secondary sex characteristics, normal intellect, treatable with hormone therapy