Meiosis, Germ Cells, and Fertilization
Meiosis, Germ Cells, and Fertilization
Meiosis, Germ Cells, and Fertilization
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<strong>Meiosis</strong> Frequently Goes Wrong<br />
• female meiosis, which arrests for years after diplotene<br />
• meiosis I is completed only at ovulation<br />
• meiosis II only after the egg is fertilized.<br />
• such chromosome segregation errors during egg development are the<br />
commonest cause of both spontaneous abortion (miscarriage) <strong>and</strong> mental<br />
retardation in humans.<br />
• Nondisjunction<br />
– the result is that some of the haploid gametes produced lack a particular<br />
chromosome, while others have more than one copy of it.<br />
• Aneuploid<br />
– <strong>Cells</strong> with an abnormal number of chromosomes<br />
• Down syndrome<br />
– an extra copy of chromosome 21<br />
• Segregation errors during meiosis I increase greatly with advancing<br />
maternal age.
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