23. Preimplantation genetic testing

19. Karyotyping

90 Genetic Diversity and Disease Susceptibility

20. FISH

21. Microarray

plasia) or spinal muscular atrophy.

specific suspicion it is better to do karyotyping.

failure following in vitro fertilization and embryo transfer.

22. Exom and genome sequencing

expensive with last resorts.

Karyotyping detects the number of chromosomes (Aneuploidies or the gross structure of chromosomes). A karyotype detects any chromosomal anomaly greater than 5 Million base pairs. This could be monosomy, trisomy, tri and tetra ploidy, deletions, duplications, etc. Karyotyping is done by culture that may take 3–4 weeks. An example is detection of Down's syndrome by karyotyping of fetal cells obtained after amniocentesis). Any abnormality lesser than 5000 million base pairs cannot be detected by karyotyping. For example a small micro deletion or a mutation responsible for a genetic disease like CAH (Congenital genital hyper-

FISH will detect a specific chromosomal anomaly that has been previously suspected. In prenatal diagnosis typically abnormality of chromosome 21, 13, 18, X, Y. It gives results between 72 h. For example if the triple markers in maternal serum and first trimester are suggesting Down's syndrome, a specific FISH for chromosome 21 can be performed. But if there is no

A FISH may detect micro deletions also but it has to be planned which micro deletion we are looking for. For example 22q micro deletions responsible for Di George Syndrome in a cardiac anomaly like tetralogy of Fallot. Laboratories usually have a panel for common micro deletions. 22q micro deletion is the second most common chromosomal anomaly after Trisomy 21.

Almost all mutations are very small with only 100–1000 pairs. For these we have to depend on cytogenetic tests like Mutation testing (Sanger Sequence), microarray, Exome sequencing and genome sequencing. If we do not know what to test for than we have to order a microarray. It will scan the entire genome if some few hundreds of base pairs are missing or not. For example, microarray gene testing of uterine endometrium in cases of recurrent implantation

These are the most precise tests telling us abnormalities in the minute base pairs. These are

Preimplantation genetic testing is defined as a procedure to remove one or more nuclei from oocytes (polar bodies) or embryos for genetic testing before transfer [15, 16]. Preimplantation genetic diagnosis PGD is a term used to determine whether a certain mutation or an unbalanced chromosomal complement has been transmitted to the oocyte or embryo when one or both genetic parents carry a genetic mutation or a balanced chromosomal rearrangement. Preimplantation genetic diagnosis is done to avoid transfer of embryos with mutation and identify healthy embryos for transfer [17].

Preimplantation genetic screening is term used when both genetic parents are chromosomally normal and their embryos are screened for any genetic defects before implantation to improve the success rate of embryo transfer [18, 19].

In preimplantation genetic screening and 24 chromosome copy number analysis (CGH, array comparative genomic hybridization, real-time quantitative PCR, SNP microarray) the aim is

Figure 10. (a) Diagrammatic illustration of the method for polar body biopsy. (b) Blastomere biopsy. (c) Multiple cell biopsies from blastocyst.

only to improve the IVF rates. So the requirements of accuracy are less strict and high false positive results may be acceptable. The test should more importantly be less costly, rapid and non-invasive.

polar bodies 10% will be due to nondisjunction and the remaining 45% from half of the

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Blastomere biopsy can be obtained at day 3 embryos. Blastocyst biopsy can be done on Day 5

Blastomere biopsy is done by zona drilling with acid Tyrode solution or mechanical aspiration. Alternatively laser denudation can be done. If removal of two cells is considered it should be done only after an embryo has six or more cells. Blastomere biopsy is associated with low

Blastocyst biopsy on Day 5 or Day 6 embryos is done with noncontact infrared lasers. Laser is used to create an opening in the zona pellucida and the herniating trophectoderm is excised. Around 10 trophectoderm cells are removed and studied. Blastocyst biopsy of trophectoderm does not lead to reduced implantation and delivery rates. It should be remembered that the RCTs that demonstrated a beneficial effect of embryo biopsy were only done on blastocyst

27. Blastocyst comprehensive screening and single-embryo transfer

benefit and risks of multiple embryo transfer will be minimized.

diagram when clinical history is obtained (Figures 11 and 12).

28.1. Clinical indications for prenatal diagnosis

After a comprehensive chromosome testing a single best embryo can be transferred in women. The initial data collected has revealed that frozen embryo transfer after comprehensive gene testing leads to a better on going pregnancy rates as compared to a fresh blastocyst transfer that was screened only morphologically [25, 26]. With the development of newer modalities like comprehensive chromosome screening and single embryo transfer older women will

Direct testing on the fetus is offered when there is an ultrasound marker of anomaly; serum screening (triple markers, quadruple markers) is positive or when one of the parents or siblings is a carrier of chromosomal abnormality. Individuals and relationships are described as a

Diagnostic interventions in obstetrics are mainly directed at some form of fetal tissue sampling for genetic, biochemical, hematological and histological processing. Samples of fetal tissues like amniotic fluid, chorionic villi and blood can be obtained by a variety of tissue sampling

premature separation of sister chromatids.

26. Embryo biopsy

implantation rates [24].

28. Direct testing

stage with trophectoderm biopsy.

or Day 6 embryos.

In preimplantation genetic diagnosis diagnostic accuracy is most important. The test should be highly sensitive and specific with very low false negative results. There are three procedures polar body biopsy, blastomere biopsy and blastocyst biopsy [20] (Figure 10).
