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 anaphaseTermed: non-disjunction Result: gamete has an extra chromosome

or is deficient in a chromosome Human fertilization: 45 or 47 chromosome

Non-disjunction

http://www.biostudio.com/d_%20Meiotic%20Nondisjunction%20Meiosis%20II.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-fetoproteinHCG = human chorionic gonadotropin

Trisomy 21Abnormally low (25% lower) levels of AFPAbnormally 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 98

22 pairs of autosomes 1 pair of sex chromosomes

Male = XYFemale = 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 21st 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 3rd floor computer lab tomorrow Homework (due Thursday): #5, 6, 7, 9

page 99