May 19, 2015
Complications with Cell Division: Mitosis: if an error occurs during division, there is usually zero harm to the body. (non sex cells are called somatic cells) Meiosis: if an error occurs in the creation of sperm or egg, and that gamete (sex cell) becomes part of a new baby, every cell in the baby is affected.
during anaphase I of meiosis, homologous pairs are supposed to split and move to opposite poles. sometimes the pairs get stuck to each other, resulting in daughter cells with the wrong number of chromosomes monosomy: having one too few chromosomes (22 in sperm/egg) trisomy: having one too many chromosomes (24 in sperm/egg)
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May 19, 2015
abnormal meiosis = nondisjunction ("Failure to separate" in anaphase I)
Examples of Genetic Disorders Caused by Nondisjunction during Meiosis *1. Trisomy 21 (Down Syndrome) 1/600 live births (if the mother is 35 or older, the odds increase to 1/40) full, round face, enlarged/creased tongue. short height, mental delay, heart defects 2. Trisomy 18 (Edwards Syndrome) 1/3000 live births (chances increase as the mother ages) heart abnormalities, kidney malformations, intestines forming on the outside of the body, developmental and mental delays, droopy eyelids, clenched hands, webbed toes, undescended testicles
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May 19, 2015
3. Trisomy 13 (Patau Syndrome) 1/25 000 live births (increased chance as mother ages) cleft lip and palette, clenched hands, closeset eyes, decreased muscle tone, extra digits, hernias, severe mental delay, seizures, scalp defects (missing skin), small head, jaw and eyes
4. Monosomy 7 ("preleukemia") causes problems with blood (low white blood cells, low platelets, anemia, can cause leukemia)
5. Monosomy 5 (Le Jeune's Syndrome) malformed larynx causing a high pitched voice, other developmental disabilities
*6. Klinefelter's Syndrome (XXY) male with an extra X chromosome appears male at birth, but at puberty, this individual produces high levels of female sex hormones because of the extra x chromosome because both male and female sex hormones are produced, either testosterone or estrogen therapies (drugs) will be needed to keep them strictly male or female sterile (incapable of producing children) 1/500 male births
*7. Turner Syndrome (X0) female with only 1 X chromosome appears female at birth but this person never develops sexually at puberty short stature, thick neck 1/3000 female births (most babies with this disorder are miscarried by 20 weeks of pregnancy) 3
May 19, 2015
Sample Karyotype
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May 19, 2015 Diagnosing Nondisjunction confirmed by preparing a karyotype (WBC in a solution that stimulates mitosis stopped at metaphase) Prenatal Testing recommended in women over age of 35 (increased risk) Chorionic Villus Sampling: 8 weeks remove cells from outer membrane (chorion) surrounding embryo miscarriage risk: 1/100 procedures doesn’t measure severity of disorders 98% accuracy false positives
Amniocentesis: 1114 weeks remove cells from amniotic fluid miscarriage risk: 1/200 1/400 procedures (infection or water breaks) 98% accuracy false positives Multiple Marker Screening: 1520 weeks test hormone levels (if some produced by placenta are high may indicate birth defect) alpha fetal protein released in presence of neural tube defect
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