Structural Biochemistry/Genome Analysis/Karyotyping

A karyotype a complete set of chromosomes of a particular species. The number and appearance of chromosomes can very dramatically between different organisms. Human beings have 46 chromosomes, 22 pairs of autosomal chromosomes and one pair of sex chromosomes. The latter is what determines whether a developing embryo develops as a physiological male or female, with a male karyotype displaying the diminutive Y chromosome beside its larger X chromosome partner (women have two X chromosomes in their karyotype).

A karyotype is generally an image of a completed and arranged set of chromosomes as viewed through a light microscope. A chromosome set can be attained from nearly any type of tissue. Dividing cells are stained with a special dye, usually the Giemsa stain, and then cell division is halted during metaphase. An image of the dividing cells is taken when the chromosomes are all visible, and the individual chromosomes are cut out of the picture and rearranged on a separate medium based on size. Chromosome pairs are matched up also based on their size, banding pattern, and the position of their centromeres.

Once arranged and ordered, scientists can then study the number and appearance of chromosomes. This can lead to medical diagnoses, as is the case with Down’s Syndrome. Down’s Syndrome is a disease in human caused by an extra copy of chromosome 21 (the syndrome is frequently referred to as Trisomy 21 for this reason). Down’s syndrome is easily identified via a karyotype by the obvious extra chromosome present in the image. Another example of a chromosomal abnormality is Turner syndrome (the presence of only a single X chromosome in women instead of the usual two) Edwards syndrome is trisomy 18, and Patua syndrome is a result of trisomy 13. Both Edwards and Patua syndrome result in death while still in infancy.

Indeed, some people insist on doing karyotypes of their unborn babies early in their pregnancies to test for such disorders, the usual method of obtaining cells in amniocentesis. However, some scientific groups are now suggesting newer and more accurate tests to be performed in genetic tests, such as microarray studies.

How do some people end up with extra chromosomes? In humans, both sperm and eggs have one set of chromosomes, 23 in number. Eggs and sperm duplicate via meiosis, with 4 new gametes being produced in each cycle. If there is a mistake during meiosis, a chromosome pair might fail to properly separate and distribute into each forming cell, and a gamete might be left with two copies of a gene instead of one. When this gamete joins with its respective counterpart during fertilization, the number of chromosomes will be abnormal. If a sperm with two copies of a gene fertilizes and egg with one copy, the resulting embryo will have three copies, and suffer the problems listed above associated with trisomy. A sperm lacking a chromosome might also fertilize a normal egg, resulting in an embryo with only one copy of a gene that would also suffer genetic problems (this is called monosomy)