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8.4: Cytogenetics –Introduction

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  • The genetic material of humans is contained on 23 pairs of chromosomes in the nucleus of each somatic cell. 22 pairs are autosomes and 1 pair is the sex chromosomes. Human females inherit 2 X sex chromosomes while males inherit 1 X and 1 Y chromosome.

    Cytogeneticists examine chromosome structure, seek out causes of chromosomal abnormalities, and study how chromosome structure relates to an individual's phenotype (appearance and biochemical traits). Abnormalities in chromosomes can result in early embryonic death, congenital defects, development of cancer, and infertility or sterility.

    Studies of chromosomes begin with the extraction of chromosomes from cells. Chromosomes are then placed on a glass slide, stained with dye, and examined under a microscope. Each chromosome pair is assigned a number (from 1 to 22, then X and Y) that is based on staining pattern and length. A karyotype is a representation of a person's chromosomes. The chromosomes are shown in pairs and arranged in order of decreasing size. Individual genes cannot be seen on karyotypes.

    What to look for in a human karyotype

    • Are there 46 chromosomes?
    • Is there one pair of each autosome and 1 pair of sex chromosomes?
    • Are there any deletions, rearrangements, or other abnormalities in the chromosomes?

    Chromosomal notation: Karyotypes are presented in a standard form. First, the total number of chromosomes is given, followed by a comma, and the sex chromosome constitution.

    Normal human female is designated as 46, XX

    Human male with an extra chromosome 15 is designated as 47, XY, 15+

    Human female with an extra X chromosome is designated as 47, XXX

    There are many disorders that can be diagnosed by examining chromosomes. In Down syndrome, an extra chromosome 21 is present. In Turner syndrome, a sex chromosome is missing so that the individual has 45 total chromosomes. Fragile X syndrome, the most common inherited cause of mental retardation, takes its name from the appearance of the X chromosome.