Karyotyping step by step

In a culture, as in the body, cells divide by mitosis.

Once the tissue culture for performing the karyotype is underway, mitosis is stopped in its tracks by adding a drug called colchicine to part of the culture. This freezes mitosis at a stage of cell division called metaphase, when the chromosomes are separate and can be photographed.

Till the end of the last century they were then literally cut out from the photograph with scissors and pasted up in order. During the 1990s the cutting and pasting came to be done on a computer screen -- with each chromosome pair displayed in sequence to check for completeness (see Figure 7.1). This is done for typically 20 or so metaphases -- an enormous amount of work, even for a computer.

Normal karyotypes are indicated this way: a normal female karyotype is 46,XX (to indicate 23 chromosome pairs, of which one pair comprises the sex chromosomes and the other 22 pairs are autosomes, or non-sex chromosomes); a normal male karyotype is 46,XY.

We have already encountered the term triploid, which means three times the haploid number of chromosomes (see Chapter 3), and usually comes about if two sperm fertilize an egg.

The general term for the gain or loss of a single chromosome is aneuploidy. The most common aneuploid chromosome complements are the trisomies, where there is an extra chromosome because of incorrect movement of the chromosomes during meiosis into the egg or the sperm -- for example, trisomy 21 (Down syndrome), in which an extra chromosome 21 gets into egg or sperm and therefore into the embryo; and trisomies of the sex chromosomes (Klinefelter syndrome = 47,XXY; triple-X syndrome = 47,XXX; and extra-Y-chromosome syndrome = 47,XYY), in which the embryo is endowed with an extra sex chromosome.

The loss of one of a chromosome pair, a monosomy, is not compatible with even early embryo development -- unless the chromosome lost is a sex chromosome, namely Turner syndrome (45,X), though even then a majority end as miscarriages, with only a small proportion surviving as babies.