In this article we will discuss about the aneuploidy in animals.
Aneuploidy of sex chromosomes and autosomes have been reported in several animals. Nondisjunction is major source of aneuploid gametes Sturtevant reported that trisomic for chromosome 4 (triplo-IV) in Drosophila melanogaster ABCDEFGH was fertile in the female sex.
In mouse, trisomics were found to be sterile or semi-sterile, but their phenotype was normal. Kilgour and Bruere in 1970 reported that sheep with XXY chromosome constitution showed testicular hypoplasia. In chimpanzee, McClure and associates reported trisomy for a small acrocentric chromosome (autosome). It showed the characteristics similar to Down’s syndrome.
In Drosophila, an individual with XX chromosome is a normal female, while the individual with XY chromosomes is a normal male. However, the sex is governed by a balance between the number of X chromosomes and the set of autosomes (A). If the X/A ratio is 1.0 the individual develops into a normal female, while the ratio of 0.5 results in a normal male.
The X/A ratio between 1.0 and 0.5 produces an inter sex. In this fly, the monosomic for sex chromosome (XO) develops into a male but it is sterile. Sometimes, XO/XX mosaics (chimeras) are also observed. They develop when one X chromosome is lost or eliminated during cell division in female embryo (XX→XO).
If this event happens early enough in the development, one half of the body will be XO (with male phenotype) and remaining part will be XX (female phenotype). Such sexual mosaics are called gynandromorph orgynander. By use of genetic markers on the X chromosome, it can be known which chromosome – paternal or maternal was lost to produce XO cells.
In Drosophila, frequency of gynandromorph can be increased by using females possessing a ring-X chromosome (formation) of ring chromosome is shown in Fig. (16.24). During the embryonic cell division, the abnormal ring-X chromosome is eliminated and male and female tissues are produced side by side.
In some strains of Drosophila and Bombyxmori,bi-nucleate eggs containing one egg nucleus and one polar body have been reported. They originate due to meiotic irregularity. Such a bi-nucleate egg when fertilized with two genetically different sperms (double fertilization) may develop into gynandromorph or mosaic.
Mosaics or chimeras can be produced artificially by mixing the cells from two genetically different fertilized eggs. The membranes of two different eggs (fertilized) are digested by pronase solution and mixed. They are cultured in vitro.
At blastocyst stage, they are implanted into the uterous of a female. The progeny produced may show mosaic phenotype. By this method, Mintz in 1967 produced chimeras in mice which showed transverse bands of alternating pigmentation. This technique was used by Fehilly and associates in 1984 in sheep (Ovisaries; 2n = 54) and goat (Capra hircus ; 2n = 60). They produced a chimera which is partly sheep and partly goat.