Non-disjunction, Down’s syndrome, and karyotypes IB Topic 4.2

Non-disjunction Meiosis is sometimes subject to errors One example is when homologous chromosomes fail to separate at anaphase Termed: non-disjunction Result: gamete has an extra chromosome or is deficient in a chromosome Human fertilization: 45 or 47 chromosome

Non-disjunction ic%20Nondisjunction%20Meiosis%20I I.htm

Down syndrome and Turner’s syndrome Down syndrome Trisomy 21 Person has 3 chromosome 21 instead of 2 Hearing loss, heart and vision disorders, physical deformity (range), and mental/growth retardation Klinefelter’s syndrome Boys have extra X chromosome(s) Turner’s syndrome Girls are missing one of their X chromosomes Intellectually normal, but physical/sexual maturity problems

Testing for Down’s Syndrome Blood test performed on expectant mother Look for unusual levels of AFP & HCG AFP = alpha-fetoprotein HCG = human chorionic gonadotropin Trisomy 21 Abnormally low (25% lower) levels of AFP Abnormally high (2x higher) levels of HCG If blood test raises concern, then parents are advised to have a karyotype produced

Karyotype An organized image of metaphase fetal chromosomes Technicians stain chromosomes How do we obtain fetal chromosomes? Amniocentesis Needle through mother’s abdominal wall, using ultrasound to guide the needle Amniotic fluid is drawn (cells of the fetus are used) Chorionic villus sampling (CVS) Cells are taken from the placenta, specifically the chorion, rather than the amniotic fluid Can be done earlier than amniocentesis Sampling tool entered through vagina

Then what? Cells are stained and prepared on a glass slide to see their chromosomes under a light microscope Images are obtained during mitotic metaphase Images are cut Pairs of chromosomes are placed in order by size and position of centromere

Look at the karyotype on page pairs of autosomes 1 pair of sex chromosomes Male = XY Female = XX X chromosomes are longer than the Y Is this a karyotype of a female or male? How do you know? What’s up with the 21 st pair of autosomes?

Pair and share With the person next to you (or a group of 3 or 4), discuss the questions in the pink box at the top of page 99, and the teal box in the upper right hand corner Meet in 3 rd floor computer lab tomorrow Homework (due Thursday): #5, 6, 7, 9 page 99