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10.

The deleterious effect of aneuploidy is most pronounced when
the absolute chromosome number is low, and it grows weaker with
increased polyploidy.

In species with high chromosome numbers, there is often a certain
resilience around that high number without any distinct effect. In plant
species with high basic numbers, the sensitivity to aneuploidy is also less pronounced.
This is probably caused by that the high basic numbers (13: Pogonias,
17: Pauridaceae) are actually polyploid, so the plants are secondary polyploids
(Delgigny & Meggitt 1928). Examples other Drosophila species, L=28 x=14; for e.g. B. megastigma 2
Agricoleplidy: Lepidoptera, Coleoptera, insects.

B-chromosomes:

Everything that has been said so far about chromosomes has demonstrated
that these bodies are indispensable for living organisms. Strangely enough, there are
also chromosomes that have been found to be indifferent or even deleterious
in their effects.

In papers published in 1927 and 1928, A.E. Langley & H.F. Randolph
reported the occurrence of what they called B-chromosomes in corn.
This designation was used for a kind of small chromosomes that occurred
in varying numbers in some individuals of corn in addition to the ordinary
chromosome complements. All normal corn plants have 20 so-called A-chromosomes,
and alterations in the number or structure of these chromosomes have just as
acute effects as in Drosophila and Datura. The B-chromosomes, on the other
hand, are only present in some corn plants, and occur in varying frequencies
in different strains. Plants without B-chromosomes have quite normal properties,
and there is no evidence that the plants containing B-chromosomes are in any way
superior to plants without B-chromosomes. On the contrary, it is clear that
plants with many B-chromosomes are weaker and less fertile than other plants.
This effect, however, will not be obvious when the number of B-chromosomes is fairly large.
This number may range from one to more than 20. An interesting property
of the B-chromosomes in corn is that they are heterochromatic, i.e., they have
rather sticky surfaces and stain somewhat differently from A-chromosomes.

Before the discovery of B-chromosomes in corn, rye plants in different
countries were observed to have 16 instead of 14 chromosomes. The latter
number is the normal one, and at the outset it was assumed that the increase
from 14 to 16 was due to a transverse division one of the ordinary rye chromosomes.
However, from investigations by Jorgensen & Rasmussen, it became gradually clear
that the 16 chromosome type carries two supernumerary chromosomes - quite
specific type, and also that these supernumerary chromosomes behave in a deviating
way during the first mitosis of the pollen grains.