Karyotypes, sex chromosomes and change of chromosome number

Chromosomes are arranged in order of size and the position of the centromer to give the karyotype or chromosome complement of the individual.

Similar chromosomes can be distinguished by the horizontal banding patterns formed by quinacrine staining or chromosome painting (involves tagging with fluorescent dyes).

The karyotype varies according to the sex. Humans have 46 chromosomes but males have two that are not a pair. The larger is the X chromosome and the shorter the Y chromosome.

At meiosis in a female the two X chromosomes form a bivalent and separate as an autosome.

At meiosis in the male the X and Y chromosome pair at a short pseudoautosomal region common to the tips of the chromosome. This alows the chromosomes to separate cleanly at anaphase I.

Maleness in humans is caused by the presence of the Y chromosome not by the lack of a second X chromosome. Obvious from studying sex chomosome abnormalities such as Klinefelter syndrome (causes XXY males) and Turner syndrome (causes X females).

Incidence of sex chromosome disorders is high. Caused by nondisjunction at anaphase I and II of meiosis. This causes some of the gametes to be unbalanced as both sex chromosomes will pass to one daughter nucleus.

The Y chromosome actively diverts the undifferentiated gonad in the developing embryo there is evidence of a testis-determining gene. As crossing over commonly occurs in the pseudoautosomal region, it indicates the gene is in the sub-terminal region of Y.

Other genes are present on the sex chromosomes than sex determinants. These can cause sex-linked disorders such as haemophilia. They show a criss-cross pattern through generations similar to the transmission of sex chromosomes.

Autosomes can also suffer nondisjuction at meiosis which gives rise to aneuploidy where the chromosome number is not an exact multiple of the haploid number.

Trisomy is the appearance of three copies of a particular chromosome and monosomy is when there is only one copy of a particular chromosome.

The physiological and developmental disorders caused are more severe than for sex chromosomes. Autosomal monosomy is generally lethal in utero but three autosomal trisomies are compatible with postnatal survival.

The most common is trisomy 21 or Down syndrome. This results from nondisjuction in maternal meiosis giving an egg with two 21s.

Normal development requires a balanced set of chromosomes but the X chromosome is present twice in females and only once in males.

A dosage compensation mechanism operates to inactivate one of the X chromosomes in female cells. This is thought to occur at about the 20 cell stage in development.

It remains in this state throughout the resulting cell line.

Polyploids are organism with more than one complete set of chromosomes e.g triploids (3n) and tetraploids (4n).

Polyploidy is generally lethal in higher animals but is common in plants where roughly 50% of angiosperms are of this type.

Triploids are generally sterile but tetraploids generally have normal fertility as they produce gametes of the n+n type.