Methods for study of chromosomes include use of chemicals that disrupt microtubules or stabilize microtubules, differential staining of chromosomes, and chromosome painting.
Chromosome numbers of various species have been artificially altered to study the effects.
In general, the study is called cytogenetics.
A simple method for viewing chromosomes involves putting cells in culture, treating them with a chemical compound that disables the spindle apparatus. See Figure 6.1.
In this way, cells are arrested in metaphase, a period when chromosomes are condensed.
Humans have 46 chromosomes.
Often fluorescence is used to assist in making each chromosome distinct. See Figure 6.4.
Human Karyotype
Chromosomes are computer imaged, and the image is manipulated to order the chromosomes.
The short arm of a chromosome is called its p arm.
The long arm is called the q arm.
The designation, 5 p, would be the p of the chromosome number 5. See Figure 6.5.
Each chromosome in Figure 6.5 is composed of two sister chromatids, what would have been two homologous chromosomes destine for each of the two daughter cells had the cell not been sacrificed for karyotyping.
Ploidy
Ploidy refers to the number of sets of chromosomes.
Humans normally have 2 sets of 23 chromosomes (46 total).
Humans can only have 2 sets, one or three or more and the embryo dies early in development.
Plants and some animals can have more than one set of chromosomes.
If an organism's cells have an even number of sets, the organism is often fertile and also tends to be larger than the normal diploid.
They also then to produce larger fruits, seeds, eggs and sperm.
Odd number of sets usually result in sterility. See Figure 6.8.
Sterile Polyploids
Three sets of chromosomes make distribution of complete sets of chromosomes difficult during meiosis.
Genetic dosage balance requires complete sets.
No serious problem occurs during mitosis, obviously, since chromosome pairing and reduction divisions do not occur during mitosis.
Therefore, organisms tend to be sterile because many different combinations of chromosome imbalances occur. In fact, many triploids often fail to generate fully developed seeds.
Bananas are an example of the benefit of sterility. The bananas we eat are from triploid trees and have tiny seeds. They are propagated asexually using plant-cloning techniques.
Fertile Polyploids
When large seeds and fertility is desirable, and you want to generate a new species, a fertile allopolyploid might make an interesting solution.
Cross two plants from different species.
If you succeed to get hybrid plants, then treat plant growing regions with a substance that causes chromosome doubling, such as colchicine.
The plant would then be an allopolyploid. See Figure 6.9.
Sometimes two different species cross in nature, and chromosome doubling occurs naturally just by chance. See Figure 6.10.
If the plant does become diploidized, it becomes the founder of a whole new species. See Figure 6.12.
Rarely, a plant gets diploidized without having been the product of inter-species hybridization and becomes tetraploid. They are also called autopolyploid. See Figure 6.11.
Such a tetraploid would be genetically isolated from its parent species, and could breed only with itself and its progeny. It would therefore be considered a different species.
If you wish to make a sterile plant like a seedless raspberry, you first make a tetraploid by "diploidizing" a diploid.
You would cross this tetraploid to a normal diploid and the progeny will be triploid (and likely sterile).
Did you read the one about the Russian cytologist who tried to create an allopolyploid plant with roots of a radish and leaves of a cabbage? He got a plant with the roots of a cabbage and leaves of a radish.
Tissue Specific Polyploidy
A process called endomitosis can yield cells with greater number of chromosomes.
This can occur in certain cells of some organisms.
Drosophila larvae salivary glands are an example of such cells. (How did a scientist discover THIS?) See Figure 6.14.
In these cells, about 500 copies of the chromosomes form.
They align together forming "polytene" chromosomes.
It is interesting that the chromosomes are aligned during this non-meiotic stage.
Aneuploidy
At times there may be an extra or missing chromosome, just one, not the whole set.
This can be caused by non-disjunction during meiosis, or even during mitosis.
If this occurs during meiosis, and the gamete is used to generate a progeny, the progeny often has gross abnormalities.
One interesting exception is found in a species of plant, Datura Stamonium,whichactually has strains with various aneuploidy as a normal status.
Most trisomies are not observed because they are lethal, and development fails prior to birth.
Some, like Edward Syndrome (trisomy 18) die shortly after birth.
Extra X chromosomes do not cause too great a problem as would be expected from our knowledge on X-chromosome inactivation.
Monosomy for X is also not serious when compared to non-sex chromosomes. This is called Turner Syndrome.
The condition of having XXY is called Klinefelter Syndrome. The male is subfertile with small testes, developed breast, long limbs and a feminine pitched voice.
Many XYY males are indistinguishable from the general population of males. See Table 1.
The vast majority of aneuploids in humans occur because of non-disjunction during the egg’s meiotic divisions.
Nondisjunction, particularly for chromosome 21, has been associated with the age of the female. Older women have children with Down Syndrome much more frequently.
Chromosomal Rearrangement
Chromosomal deletions are when there are missing segments of a chromosome.
Cri-du-chat Syndrome is an example in humans. This is due to the deletion in the p (short) arm of chromosome number 5.
These individuals are severely impaired, mentally and physically, and make cat-cry like sounds.
Although there is another copy of each missing gene on the homologous chromosome, the imbalance relative to normal individuals results in serious disease.
Duplications are extra segments, repeats of regions of chromosomes.
Some duplications are found repeated tandemly within a chromosome.
Some duplications are found attached to the end of the chromosomes they are normally found on, or even on other chromosomes, where they normally are not found. See Figure 6.22.
In humans, the duplication of a region of chromosome 21 causes a form of Down Syndrome called Familiar Down.
Inversions are caused by a flipping rearrangement of a segment of chromosome.
Inversions occur naturally, although rarely.
More frequently they occur after exposure to X-ray. The X-ray irradiation creates hydrogen peroxide, superoxide and oxygen singlets that can react with and break the phosphodiester backbone of the DNA of the chromosome.
Sometimes the pieces reattach correctly; but sometimes the pieces reattach in a reverse orientation. See Figure 6.23.
Pericentric inversions are inversions that include the centromere, meaning the centromere is somewhere inside the segment that flipped around.
Paracentric inversions do not include the centromere. See Figure 6.24.
Interesting is the fact that the paracentric inversion creates the most serious problems for heterozygotes in terms of fertility. Acentric (lacking a centromere) and dicentric (having 2 centromeres) result when crossovers occur within the inversion with another normal chromosome. The dicentric chromosome can be pulled apart during chromosome separation, whilst the acentric fragment is unable to be distributed at all.
View Figure 6.25 and follow the chromosomes to see the problems.
All inversions cause abnormalities during meiotic chromosome synapsing in those heterozygous for them, and affect the viability of resulting gametes, embryos and progeny. Paracentric inversion heterozygotes have half the normal fertility.
If a combination of genes within an inversion is particularly desirable, they can act as a "supergene" and can be selectively advantageous.
Gene expression is often influenced by the location of the genes on the chromosomes. This is another way that inversions and translocations can alter phenotypes. See Figure 6.31.
Between closely related species, inversions are frequently observed.
In small interbreeding populations, inversions and other chromosomal arrangements can be found. These help begin the process of specialization by reducing fertility of hybrids and eventually eliminating the fertility of hybrids.
Translocations are when a segment from one chromosome is detached and reattached to a different (nonhomologous) chromosome. See Figure 6.20.
Two pieces from nonhomologous chromosomes exchange when a reciprocal translocation occurs.
Translocations create problems during meiosis. A characteristic cruciform structure can be observed during meiosis Metaphase I. See Figure 6.26.
Chromosome segregation becomes more variable resulting in duplicate and missing copies of DNA in some cases. See Figure 6.27.
Compound chromosomes and Robertsonian Translation
Sometimes one chromosome fuses with another homolog.
The fused chromosome is called a compound chromosome.
If there is only one centromere, there is not a problem during mitotic divisions.
Reduced fertility occurs for heterozygotes for compound chromosomes.
Robertsonian Translocation
Robertsonian translocations are most interesting in regards to speciation.
Two nonhomologous acrocentric chromosomes fuse. See Figure 6.30.
Two acrocentric chromosomes fuse to form for example a new metacentric chromosome.
If inbreeding occurs, stable viable progeny can result.
Many different but related species are genetically isolated because of differences in chromosome number. The horse and donkey is an example. Bison and common cattle (Bos torus) is another. In fact, humans, gorillas and chimpanzees are also closely related but genetically isolated because of differences in chromosome numbers.
Once isolated by this simple mechanism, the now different species can adapt to the environment independently becoming more divergent as time progresses.
Phenotypic effects can be caused by the chromosome rearrangement.
Changes in the location of genes can sometimes alter their activity, and the amount of gene product. This is called position effect.
A change is sometimes positive, generating an adaptive advantage for the individual and thereby increasing the likelihood that a change will be established within a population.