Post on 24-Feb-2016
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When Meiosis goes wrong…
1.10.12
What can go wrong during meiosis?
What can go wrong during meiosis?
• Nondisjunction – The failure of homologous chromosomes or non-sister chromatids to separate during anaphase I or anaphase II
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
Alteration of Chromosome #
• Trisomy = 3 copies of a particular chromosome• Monosomy = 1 copy
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
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
Down’s Syndrome (Trisomy 21)
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
Down’s Syndrome (Trisomy 21)
• Occurrence directly related to maternal age– Under 30
Down’s Syndrome (Trisomy 21)
• Occurrence directly related to maternal age– Under 30 1 in 952
Down’s Syndrome (Trisomy 21)
• Occurrence directly related to maternal age– Under 30 1 in 952– Under 35
Down’s Syndrome (Trisomy 21)
• Occurrence directly related to maternal age– Under 30 1 in 952– Under 35 1 in 378
Down’s Syndrome (Trisomy 21)
• Occurrence directly related to maternal age– Under 30 1 in 952– Under 35 1 in 378– Under 40
Down’s Syndrome (Trisomy 21)
• Occurrence directly related to maternal age– Under 30 1 in 952– Under 35 1 in 378– Under 40 1 in 106
Down’s Syndrome (Trisomy 21)
• Occurrence directly related to maternal age– Under 30 1 in 952– Under 35 1 in 378– Under 40 1 in 106– Under 45
Down’s Syndrome (Trisomy 21)
• Occurrence directly related to maternal age– Under 30 1 in 952– Under 35 1 in 378– Under 40 1 in 106– Under 45 1 in 30
Down’s Syndrome (Trisomy 21)
• Occurrence directly related to maternal age– Under 30 1 in 952– Under 35 1 in 378– Under 40 1 in 106– Under 45 1 in 30– Under 50
Down’s Syndrome (Trisomy 21)
• Occurrence directly related to maternal age– Under 30 1 in 952– Under 35 1 in 378– Under 40 1 in 106– Under 45 1 in 30– Under 50 1 in 11
Down’s Syndrome (Trisomy 21)
• Occurrence directly related to maternal age– Under 30 1 in 952– Under 35 1 in 378– Under 40 1 in 106– Under 45 1 in 30– Under 50 1 in 11
• Trisomy 13, 15, and 18 also occur, but die shortly after birth. All other human trisomies fail to develop
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)
Klinefelter’s Syndrome (XXY)
• 1 in 2,000 births• Underdeveloped male sex organs• Feminine secondary characteristics• Sterile
XYY
• 1 in 1,000• Normal looking male• Tall with relatively severe acne
XXX
• 1 in 1,000• Tall female with irregular menstrual cycle• Otherwise normal, needs karyotype (picture of
chromosomes) for diagnosis
Turner’s Syndrome (XO)
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)
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