Bio Ch. 26 - BIO 105 Ch26 Genetic Counseling Mar 10 `08...

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BIO 105 Ch26 Genetic Counseling Mar 10, ‘08 26.1 Counseling for chromosomal disorders Genetic counseling Determines risk of chromosomal or genetic mutation in a family Allows couples to understand mode of inheritance, medical consequences, and possible decisions Counseling for chromosomal disorders Disorders result in syndromes-groups of symptoms Karyotyping- visual display of chromosomes Amniocentesis-14-17 th week of pregnancy Samples amniotic fluid for baby’s cells Chorionic villi sampling- 5 th week of pregnancy The karyotype Cells stimulated to divide in culture medium Chemical stops division in metaphase when chromosomes are most dense Cells are killed, stained, and viewed under a microscope Computer can be used to organize pairs according to size, shape, and banding pattern There should be 23 pairs of chromosomes 22 homologous pairs of autosomes 1 pair of sex chromosomes Human karyotype preparation Changes in chromosome number Abnormalities in chromosome number may be due to nondysjunction Egg with 24 chromosomes fertilized by sperm with 23- trisomy Egg with 22 chromosomes fertilized by sperm with 23 chromosomes- monosomy Normal development depends on the presence of exactly 2 of each kind of chromosome -Trisomy Three copies of one kind of chromosome The only one compatible with a reasonable chance of survival is trisomy 21 Down Syndrome Monosomy One copy of one kind of chromosome Most are incompatible with life Chances of survival are greatest if monosomy or trisomy involves the sex chromosomes Down Syndrome Most common trisomy in humans Short stature, eyelid fold, flat face, stubby fingers, a wide gap between the first and second toes, large fissured tongue, round head, palm crease, simian line, mental retardation 3 copies of chromosome 21 75% of cases- egg has 2 copies, sperm has 1 Can be detected by a karyotype Abnormal chromosomal number Changes in sex chromosome number Turner syndrome
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