Polyploidy In Animals
Geneticist Hermann Muller argued that polyploidy is more rare in animals than plants because animals have a more complex development, with more organ systems that are fine-tuned to dosages of genes. Any given gene is represented three times in a triploid. If the amount (dosage) of gene product causes a heart, brain, or other vital organ not to form, the embryo will abort. When these developmental genes produce too much or too little of the products that induce organ formation, as they might if there are too many or too few copies of the genes, events occur too soon or too late to be coordinated. Muller raised the possibility that the sex chromosomes serve as a barrier to polyploidy in most animals. Plants, by contrast, do not usually have sex chromosomes, and thus this sexual reproductive barrier is not a problem for them.
Muller noted that most animals use a sex-chromosome mechanism for sex determination. In fruit flies and humans, diploid males have the sex chromosomes XY, whereas diploid females have XX. A triploid fly or human would have three chromosomes along with three sets of autosomes. In such a triploid, XXX will result in a female. However, a zygote having XXY XYY may not produce a male. Rather, it may result in an intersex organism, with abnormal mixed male and female reproductive organs.
While human triploids do not survive, this is not the case for fruit flies. The XXY or XYY is an intersex, sterile form, but the triploid female is fertile. If the 3N female is mated to a 2N XY male, however, only a relatively few offspring will emerge, because many of the eggs will have an incorrect number of chromosomes. This state of excesses or deficits of chromosomes in an otherwise diploid or triploid cell is called aneuploidy. Aneuploid embryos rarely survive in humans or other animals, although there are exceptions (such as infants born with Down syndrome).
Human triploid embryos are a major reason for first-trimester spontaneous abortions (popularly called miscarriages). Polyploid amphibians, on the other hand, have evolved an alternate means of sex determination that allows them to have fertile triploid or tetraploid (4N) forms. As with polyploid plants, these forms are generally larger in size than their diploid relatives. It is not yet known why stillborn or short-lived human triploids do not display this enlarged size.