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When Meiosis goes wrong… 1.10.12. What can go wrong during meiosis?

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Presentation on theme: "When Meiosis goes wrong… 1.10.12. What can go wrong during meiosis?"— Presentation transcript:

1 When Meiosis goes wrong… 1.10.12

2 What can go wrong during meiosis?

3 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 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 301 in 952

12 Down’s Syndrome (Trisomy 21) Occurrence directly related to maternal age – Under 301 in 952 – Under 35

13 Down’s Syndrome (Trisomy 21) Occurrence directly related to maternal age – Under 301 in 952 – Under 351 in 378

14 Down’s Syndrome (Trisomy 21) Occurrence directly related to maternal age – Under 301 in 952 – Under 351 in 378 – Under 40

15 Down’s Syndrome (Trisomy 21) Occurrence directly related to maternal age – Under 301 in 952 – Under 351 in 378 – Under 401 in 106

16 Down’s Syndrome (Trisomy 21) Occurrence directly related to maternal age – Under 301 in 952 – Under 351 in 378 – Under 401 in 106 – Under 45

17 Down’s Syndrome (Trisomy 21) Occurrence directly related to maternal age – Under 301 in 952 – Under 351 in 378 – Under 401 in 106 – Under 451 in 30

18 Down’s Syndrome (Trisomy 21) Occurrence directly related to maternal age – Under 301 in 952 – Under 351 in 378 – Under 401 in 106 – Under 451 in 30 – Under 50

19 Down’s Syndrome (Trisomy 21) Occurrence directly related to maternal age – Under 301 in 952 – Under 351 in 378 – Under 401 in 106 – Under 451 in 30 – Under 501 in 11

20 Down’s Syndrome (Trisomy 21) Occurrence directly related to maternal age – Under 301 in 952 – Under 351 in 378 – Under 401 in 106 – Under 451 in 30 – Under 501 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 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


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