Chromosomally normal human somatic cells contain 46 chromosomes (22 pairs of autosomes and 1 pair of gonosomes) (Figure 1). Normal gamete cells contain half this number that is 23 chromosomes (22 autosomes and 1 gonosome).
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Figure 1: Normal male karyotype (46,XY) |
Cytogenetic methods are used for the numerical and structural analysis of chromosomes. Short and long culturing techniques can be used for karyotyping the cells.
Cytogenetic analysis can be applied to several kinds of samples:
Prenatal diagnosis, or prenatal screening, is testing for chromosomal abnormalities or diseases in a fetus before it is born. The aim is to detect chromosomal abnormalities such as Down Syndrome (Trisomy 21), Edwards Syndrome (Trisomy 18), and Patau Syndrome (Trisomy 13).
There are three methods of prenatal diagnosis: chorionic villus sampling (CVS), amniosynthesis and cordosynthesis (Figure 2).
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Figure 2: In prenatal and postnatal diagnosis, the aim is to achieve best quality (band resolution) metahaphase cells for karyotype analysis. |
If one or more of the following have been diagnosed during pregnancy:
If either partner
If the female partner
During CVS, samples of trophoblast cells and chorionic villi are withdrawn transabdominally or transcervically, depending upon placental and uterine position. Chorionic Villus Sampling (CVS) can be performed between weeks11 and14 weeks of gestation.
Under ultrasound guidance, a needle is inserted through the mother’s abdomen and approximately 20ml of amniotic fluid is withdrawn. Amniosynthesis can be performed from week 14 and usually up to about 20 weeks.
Cordosynthesis is performed after 18 gestational weeks. Under ultrasound guidance, a needle is inserted through the mother’s abdomen to obtain fetal blood from the umbilical vein.
Prenatal genetic diagnosis can also be performed for single gene disorders such as thalassemia, sickle cell anemia, cystic fibrosis, and fragile x syndrome. For these conditions, molecular analysis methods are used. In the case of an autosomal recessive disease, when both parents carry the same autosomal recessive disorder the fetus has a 25% risk of being affected. However, for autosomal dominant disorders, the risk of the fetus being affected is 50%. (Figure 2B)
Postnatal genetic diagnosis is the analysis of chromosomes usually in peripheral blood samples. It aims to identify whether there are any karyotipic abnormalities.
Postnatal genetic diagnosis is mainly carried out for couples with
Karyotype analysis can also be performed for children with dismorphic features or suspected conditions such as microdeletions or an unbalanced chromosomal rearrangement.
Couples with a family history of a genetic disorder, or chromosomal abnormalities are strongly advised to be screened for this condition before planning a pregnancy.