The karyotype

May 24 2011

All our genetic information is encoded in DNA. DNA is found in the nucleus of every cell of our body as part of the structures called chromosomes. The analysis of these chromosomes is called a karyotype.

A karyotype is performed on cells called lymphocytes obtained from a simple blood sample, but it can also be performed on amniotic fluid cells from amniocentesis or chorionic villi.

Each chromosome can be distinguished from the rest by its shape, size and banding pattern which can be observed along the entire chromosome after specific staining completed in the laboratory.

The alterations detected in a karyotype can be classified into two categories:

Numerical alterations and structural changes.

1. Numerical alterations: The number of chromosomes is characteristic of each species. Human beings have 46 chromosomes (23 pairs) in the nucleus of every cell in our bodies. These 23 pairs are arranged in 22 pairs, called autosomes, and a pair of sex chromosomes (X and Y) which differentiates the sexes. Women have two X chromosomes, whereas men have one X and one Y chromosome.

The most widely-known numerical alteration is Down’s syndrome. In the karyotypes of these patients there are three chromosome 21 instead of the usual two, causing varying degrees of learning disabilities.

There are other numerical alterations that generate reproductive problems. For example, men with Klinefelter syndrome have two X chromosomes and one Y. These patients suffer testicular failure resulting in fertility problems.

On the other hand, women who suffer from Turner syndrome have only one X chromosome karyotype. The absence of one X chromosome causes underdeveloped sexual characteristics and consequently leads to infertility.

2. Structural alterations: Certain chromosomal regions are found to be duplicated or even absent. This gain or loss of genetic material has extremely varied consequences depending on the genes involved.

Other structural abnormalities are translocations and inversions. In the first case there is an exchange of material between two chromosomes and in the second, a chromosomal region is inverted with respect to its normal position.

Structural alterations may have different clinical consequences such as low semen quality in men or repeat miscarriage in women.

If a chromosomal abnormality is detected in one member of the couple, the solution to ensure healthy biological descent is Preimplantation Genetic Diagnosis (PGD). PGD is a technique for detecting chromosomal alterations in an embryo prior to its transfer to the mother’s uterus.

In addition to the alterations already mentioned, there are chromosomal variants called polymorphisms. These polymorphisms are relatively common in the general population and show no apparent phenotypic consequences, but recent scientific studies have linked them to infertility.

A karyotype is an important diagnostic test in patients who require assisted reproduction techniques because there is a higher incidence of abnormal karyotype in couples with fertility problems. This technique is indicated mainly in cases of seminal alteration, ovarian failure and in couples with repeat miscarriage or implantation failures.

Dr José Antonio Ortiz, Molecular Biologist at Instituto Bernabeu.

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2 responses so far

  • Bal Shrestha says:

    Hello There,

    Both my and my wife’s Karyotype test is normal and my wife has got three miscarriages (pregnancy naturally) ; its been tested that she has high Natural Killer Cells (more than 18.3), is it worth doing Array PGD (Advanced Embryo Selection and Chromosomal Problem) during IVF?

    Thanks for your help in advance

  • Dr. José A. Ortiz says:

    Dear Mr. Bal Shrestha,

    Thank you very much for your question.

    To be able to give you an answer I would require some additional data such as the cytogenetics study of abortion and sperm fluorescence in situ hybridization (FISH) study. It would also be necessary to make a complete study of possible thrombophilia in your wife as well as to rule out any uterine problem to explain recurrent miscarriages.
    If it concludes that there is a suspicion of a chromosomal problem, than it would be indicated to perform an array CGH.

    I hope I have resolved your doubts and remember our ability to assess your fertility problem and offer you a customized solution from our multidisciplinar medical team. We would be delighted to welcome you to our clinic.

    Regards,

    Dr. José A. Ortiz

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