Screening for Genetic Disease

Many tests are now available to detect genetic diseases.

• sickle cell disease
• cystic fibrosis
• phenylketonuria (PKU)

• the fetus and even from a
• pre-implantation embryo.

Amniocentesis

During its development, the fetus sheds cells into the amniotic fluid. After 14–22 weeks of pregnancy, a small volume of this fluid can be removed (using a needle inserted through the abdominal wall).

• chromosome abnormalities (e.g., the three number 21 chromosomes of Down syndrome);
• certain enzymatic defects (e.g., an inability to metabolize galactose, hence milk);
• the sex of the fetus.

Chorionic Villus Sampling (CVS)

This is an alternate method of prenatal diagnosis.

A small amount of placental tissue is sucked out by a tube inserted through the abdominal wall or through the vagina (the latter avoiding the need for an incision).

For some tests the fetal cells can be examined immediately without the need to culture them.

Another advantage of CVS is that it can be performed earlier in pregnancy (after only 10–12 weeks) than amniocentesis. If an abortion is to be performed, it is a simpler process early in pregnancy.

Noninvasive Prenatal Diagnosis

• some of the blood cells of the fetus do manage to get into the mother's circulation where they may represent 1 in a million of her white blood cells (so only some 2–6 cells per ml of blood).
• fragments of fetal DNA (~ 300 bp long) also are found in the mother's plasma as early as 5 weeks after implantation.

This raises the possibility of using genetic tests (e.g., PCR) to identify mutations or chromosomal abnormalities in the fetus. However, only genes contributed by the father (e.g., SRY) can be detected because there is as yet no way to separate the mother's DNA from the fetal DNA. And the tests are not yet as sensitive as amniocentesis and CVS.

Two home blood test kits for determining the sex of the fetus are already on the market. The collected drops of blood are sent to a laboratory to determine whether any Y-chromosome-specific DNA (e.g., SRY) is present.

Preimplantation Genetic Diagnosis (PGD)

One of the remarkable facts about mammalian development is that all the cells in the early (e.g., 8-cell) embryo are not needed to produce a healthy fetus (which is why a single fertilized egg can on occasions produce identical twins, triplets, etc.).

So couples using in vitro fertilization (IVF) also can take advantage of genetic screening. While the embryo is in culture, a cell or two can safely be removed and tested for its genotype. For example:

• The sex of the embryo can be determined with a probe for Y-specific DNA. This permits prospective mothers carrying a severe X-linked trait like hemophilia A to choose a female rather than a male embryo for attempted implantation.
• Fluorescent probes specific for the DNA of particular chromosomes can detect (by FISH) if there is an abnormal number (aneuploidy) such as the three #21 chromosomes of Down syndrome.
• In fact the entire karyotype of the embryo can be determined. Random fragments of DNA prepared by the polymerase chain reaction (PCR) of all the DNA of a cell from the embryo can be
  • given a fluorescent label
  • applied to the metaphase chromosomes of a standard reference cell that has a normal karyotype along with
  • DNA fragments from the reference cell labelled with a different color.
  Comparing the intensity of the two colors from each chromosome shows whether the embryo has the normal amount of DNA for that chromosome or is aneuploid containing either:
  • too much (e.g. 3 copies of #21 — trisomy) or
  • too little (only a single copy of #14 — monosomy)

Genetic Diagnosis Before in vitro Fertilization (IVF)

Thanks to the polymerase chain reaction (PCR), the genotype of an egg can be determined before it is fertilized.

As meiosis I is completed, one set of chromosomes is extruded into the first polar body.

If the mother is heterozygous for a trait, the DNA of the polar body can be amplified by PCR and tested for both alleles.

• positive for the mutant allele AND
• negative for the healthy allele,

For simplicity, the figure shows only the pair of homologues carrying the locus of concern.

So it is not sufficient to test only for the presence of the mutant gene in the first polar body. You must also demonstrate that the healthy gene is absent. For if crossing over had occurred, the first polar body would contain one mutant and one healthy allele. In that case, a 50:50 chance exists that the other copy of the mutant allele will end up in the egg (instead of in the second polar body).