Down Syndrome

 

2n + 1 (chromosome 21)

Note:

                                                              i.      Where the chromosomes are either

1.     haploid (23)

2.     Diploid (46)

3.     Triploid (69)

4.  Tetraploid

Q. Why couldn’t a tetraploid situation occur in humans?  (4N)

                                                              i.      Abnormal chromosome number (too few or too many)

1.     Monosomy (2N-1)

2.     Trisomy (2N+1)

 

                  2N + 1                                                         2N – 1

Most fetuses that have an abnormal chromosome number are aborted

 

3 Ways to get Down Syndrome

 

  1. Trisomy 21  (Sticky Chromosomes) (95%)

 

 

Q. What was one possible explanation as to why chromosomes become sticky?

 

 

  1. Chance events occurring at Mitosis (1%)

 

 

  1. Translocation of the chromosomes (3-4%)

 

 

 

 

What is amniocentesis?
Amniocentesis is a prenatal test that allows you and your practitioner to gather information about your baby's health and development from a sample of your amniotic fluid. (This is the fluid that surrounds the baby in your uterus.) The test is most commonly done when a woman is between 15 and 18 weeks pregnant to determine whether the baby has genetic or chromosomal abnormalities, such as Down syndrome. But not all women choose to have this test because it carries a small risk of miscarriage.

Other reasons that you may need to have amniocentesis include:

• To check on the well being of your baby if you have a blood sensitization, such as Rh sensitization. This is a complex condition that may occur if your blood is a different type than your baby's.

 

Chorionic Villus Sampling (CVS)

Illustration demonstrating a transcervical chorionic villus sampling

Chorionic villus sampling (CVS) is a prenatal test that involves taking a tiny tissue sample from outside the sac where the fetus develops. The tissue is tested to diagnose or rule out certain birth defects. The test generally is performed between 10 and 12 weeks after a woman’s last menstrual period.

CVS may be offered when there is an increased risk of chromosomal or genetic birth defects, and parents would like test results as early in pregnancy as possible. Another prenatal test called amniocentesis can diagnose the same birth defects, but is performed a little later in pregnancy, usually between 15 and 18 weeks after a woman’s last menstrual period.

 Who is offered CVS?
 CVS is not routinely offered to all pregnant women because the test carries a small risk of miscarriage, and possibly other complications.

Fetal Cell Sorting

NON-INVASIVE DIAGNOSIS USING FETAL CELLS FROM MATERNAL BLOOD
 

During the last 30 years, extensive research has aimed at developing a non-invasive method for prenatal diagnosis based on the isolation and examination of fetal cells found in the maternal circulation. Erythroblasts have attracted most attention because they are abundant in early fetal blood; they are extremely rare in normal adult blood and their half-life in adult blood is only about 30 days. Trophoblastic cells entering the maternal circulation are cleared by the maternal lungs and are therefore not useful candidates for prenatal diagnosis. Fetal white blood cells are present in maternal blood but their number is very low and they have a very long half-life (about 5 years), which may therefore lead to contamination from previous pregnancies.

About 1 in 103 nucleated cells in maternal blood are feta. The proportion of fetal cells can be enriched to about 1 in 10?00 by techniques such as magnetic cell sorting (MACS) or fluorescence activated cell sorting (FACS) after attachment of magnetically labelled or fluorescent antibodies on to specific fetal cell surface markers. The most commonly used antibody is anti-CD71.  Magnetic cell sorting is cheaper, quicker and requires less expertise to perform than FACS. The technique utilizes metallic beads labelled with an antibody specific for the target cell. The antibody is incubated with the sample and the cellntibodyead complex is isolated by placing on a magnet. Successful use of MACS involves prior separation of cells by triple density centrifugation.
 


 

 

Staining

Certain chemical treatments of mammalian chromosomes yield differentially stained regions on chromosomes. The patterns obtained depend on the treatment used.