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Mistakes
in
Meiosis
What are Chromosomes?
What are chromosomes?
•Human cell = >6 billion nucleotide base pairs (~2
meters)
•Wrapped around protein = chromatin
•DNA/protein = chromosome
3
How many chromosomes do humans have?
• Humans are diploid (2n)
• Two of each chromosome, one from each parent.
• n = 23 unique chromosomes (haploid Curly
#) hair Strait hair
allele
allele
• 2(n) = 46 total chromosomes
• The two copies of each chromosome
in human cells are homologous
• Different versions - same genes in same
locations but different DNA sequence.
• Different versions (alleles) of a gene may
promote different traits (e.g. hair type).
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How do cells get
the right number
of chromosomes?
DNA
DNA
DNA
DNA
DNA
DNA
DNA
Cell Division
1. Duplicate cell components
• Organelles
• Cytoplasm
• Chromosomes
2. Separate the material into
two daughter cells
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Correct Cycle
Meiosis
Cells divide 2x
Mitosis
Cells divide 1x
2n
2n
2n
2n
n
n
Diploid Cells
n
n
n
n
Haploid Cells
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Nondisjunction
• The failure of homologous chromosomes to
separate properly during meiosis.
What should happen
Nondisjunction
The results
• If the abnormal gametes is fertilized the
results
Polyploidy
• the condition where cells have multiple sets of
chromosomes
• usually 3-4 sets
• Most often found in plants, rare in animals
Examples of Polyploidy
Diploid & tetraploid
many plants
Octoploidy
strawberries
Triploid
seedless watermelon
Trisomy
• The zygote has an extra chromosome
• Organisms with an extra chromosome
sometimes survive
Examples of trisomy in humans
• Trisomy 21: Downs Syndrome
• Trisomy 18: Edwards syndrome
• Trisomy 13: Patau syndrome
• XXY: Klinefelter's syndrome
• Trisomy 1: rare
• Trisomy 2: associated
with loss of pregnancy
• Trisomy 3: lethal
• Trisomy 4: lethal
• Trisomy 5: lethal
• Trisomy 6: rare, no heath
concerns in know cases
(six total cases)
• Trisomy 7: never reported
in live birth childern
• Trisomy 8:
• Trisomy 9:
• Trisomy 10: lethal
• Trisomy 11:
• Trisomy 12:majority have
normal outcomes
• Trisomy 13: Patau
syndrome 0.03
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Trisomy 14:
Trisomy 15: lethal
Trisomy 16: lethal
Trisomy 17: not reported
Trisomy 18: Edwards
syndrome .2%
Trisomy 19:
Trisomy 20: not viable
Trisomy 21: Downs
Syndrome
Trisomy 22:
Trisomy 23:
Trisomy x:
XXY: normal
development, slightly
taller, decreased sperm
quality
Trisomy X: super female
syndrome
Monosomy
• The zygote is lacking a chromosome
• Organisms lacking one or more
chromosomes rarely survive
Examples of monosomy in humans
Monosomy X: Turners Syndrome
Human Karyotype