Other genetic disorders
The
human genome consists of 23 pairs of chromo-somes carrying an estimated 30 000
genes. The pairs of matching chromosomes as seen at colchicine-arrested
metaphase are numbered in accordance with their size. A centromere divides each
chromosome into a shorter (p) and a longer (q) arm.
Any
individual’s chromosomal make-up (karyotype) can be expressed as their total
number of chromosomes plus their sex chromosome constitution. A normal male
therefore is 46XY. A shorthand notation exists for recording other
abnormalities such as chromosome translocations and deletions.
The
precise location of any gene can be given by naming the chromosome, the arm of
the chromosome (p or q), and the numbers of the band and subband of the
chromosome, as seen with Giemsa staining, on which it lies. One of the genes
important for atopy, for instance, lies on chromosome 11q13, i.e. on the long
arm of chromosome 11 at band 13.
Several
techniques can be used to identify the posi-tion of a gene.
1. A
clue maybe offered by finding that some affected individuals have chromosomal
deletions or unbalanced translocations, suggesting that the gene in question
lies on the abnormal segments.
2. Linkage
analysis. Genes are linked if they lie closetogether on the same
chromosome; they will then be inherited together. The closer together they are,
the less is the chance of their being separated by cross-overs, one to six of
which, depending on length, occur on each chromosome at meiosis. Each member of
an affected family has to be examined both for the pres-ence of the trait to be
mapped, and also for a marker, usually a DNA probe, which has already been
mapped. If linkage is established then the two loci will be close on the same
chromosome. The probability of the results of such a study representing true
linkage can be expressed as a logarithm of the odds (Lod) score. A score of
three or more suggests that the linkage is likely to be genuine.
3. Somatic cell hybridization. A
hybrid made by fus-ing a human cell with a mouse cell will at first have two
sets of chromosomes. Later human chromosomes are lost randomly until a stable
state is reached. Those cells that produce a particular human protein must
contain the relevant chromosome. A panel of such hybrid cells can be created
which differ in their con-tent of human chromosomes. By comparing these, the
chromosomal site of the relevant gene can be deduced.
4. In situhybridization. A cloned sequence of DNA,
ifmade single-stranded by heat, will anneal to its com-plementary sequence on a
chromosome. Radioactive or fluorescent labelling can be used to indicate its
position there.
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