Chromosomes are the carriers of genes. Each chromosome acts as an individual, faithfully passing its structure on to its offspring each time a cell divides. But ‘faithfulness’ has its limits – occasionally a ‘mutant’ chromosome is produced that will then be replicated. The number of chromosomes within a cell can also be faithfully replicated down the generations, but occasionally they too can change.
When we talk about chromosomal mutations, then, we need to differentiate between changes in chromosome number and structure. Today, we will talk about number.
1. Change in the number of sets of chromosomes
By ‘set’ Dobzhansky is referring to the number of chromosome types. Humans have 46 chromosomes, but there are only 23 types. Therefore a human ‘set’ contains one representative of each of these 23 types. Changes to a set could involve either:
a. Complete loss of a set (known as haploidy), such as when an egg is fertilized by a chromosome-less sperm. Without the father’s chromosome set, a human foetus would have only 23 chromosomes. In humans, haploidy is lethal.
b. Complete gain of a set (known as polyploidy). I showed you an example of this in a human foetus in my last post. If 23 is the haploid number in humans, then 46 is the diploid number in humans (23 x 2). A triploid human would have 69 chromosomes (23 x 3); a tetraploid would have 92 (23 x 4), etc. Any multiple of the haploid number greater than two is considered a polyploid. There are two types of polyploids
i. Autopolyploids – this occurs when members of the same species mate, yet their offspring have a multiple of the parental chromosome set. It is caused by errors during gamete production, such that the gamete keeps the diploid (or more) number of chromosomes. If a diploid gamete joins a haploid gamete, the result is a triploid; if two diploid gametes meet, the result is a tetraploid, etc.
ii. Allopolyploids – these are formed when two different species mate. The resulting hybrids have chromosomal imbalances. In normal sexual organisms, each chromosome has its match, but in these hybrids, some or none of the chromosomes have a partner. This can lead to numerous problems during egg or pollen or sperm formation, and can leave the hybrids largely sterile. But occasionally the hybrid may produce offspring with a doubled chromosome set, restoring the balance. Now each chromosome has a partner, and these polyploids can be very fertile.
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| Example of allopolyploidy. Radish (Raphanus) when crossed with cabbage (Brassica) produces a hybrid that is largely sterile. Occasionally the hybrid produces fully fertile offspring, but these are always polyploids in which all of the chromosomes from either parent have doubled. This restores an imbalance caused by the failure of the chromosomes to find a partner during meiosis. This new species (Raphanobrassica) is largely inedible, having the head of a radish and the roots of a cabbage. |
2. Change in the number of separate chromosomes within a set
This category includes any gain or loss of a single chromosome, and can be divided as such:
a. The loss of one chromosome (monosomic) – a human missing one chromosome will be diploid for 22 chromosome pairs, but will be haploid for one. If two humans, each haploid for the same chromosome, were to mate, some of their offspring would be missing an entire chromosome type (say, both copies of chromosome I). Humans can often survive being haploid for one chromosome, but to lose both copies is (I think always) lethal in humans.
b. The gain of one chromosome (polysomic) – A human with one extra chromosome will be diploid for 22 chromosomes and triploid for one chromosome.
These categories are not discreet, of course. An individual could be polysomic for one chromosome and monosomic for the other; a polyploid individual could gain or lose one of the multiplied chromosomes, etc.
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| This is a karyogram of a polysomic individual who has three sex genes: two X (arrow) and one Y. This is known as Klinefelter's Syndrome, and some estimates suggest that one in 500 American males have such a karyogram, although not all exhibit symptoms of the syndrome. |
What causes changes in chromosome number?
The answer is not too terribly complicated, but requires that you go back to grade 11 biology and remember meiosis, the process that forms our sperm and egg cells (it also applies to plant pollen and the gametes of other organisms).
The basic process is as such:
2. The chromosomes migrate to the centre of the nucleus, and here they do something really interesting and really important: they find their partner. Chromosome I from your mom finds chromosome I from your dad. They latch on to one another, and there they wait, in the middle of the nucleus, in a loving embrace…
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| The basics of meiosis, showing two different possibilities for a two-chromosome set. |
3. …Only to be ripped apart. Evil spindle fibres launch out from either pole of the cell. The spindle fibre from one end grabs the chromosome from your mom; the one from the other end grabs the chromosome from your dad; and they pull. The chromosomes migrate to opposite poles of the cell, and then the cell divides. The result is two haploid cells, each of which contains one full set of chromosomes. It is an elegant, amazing display. It is important to note that which chromosome goes to which end is perfectly random, such that each gamete will have a different mix of maternal and paternal chromosomes.
4. But the process isn’t quite done. The chromosomes still have twice as much DNA as they need. So they move to the center of the nucleus again, but this time there is no partner for the chromosomes to find. They sit there, alone, deriving solace from their sister chromatids…
5. …Until those spindle fibres launch out from either end of the cell once again. This time they attach to the centromere, ripping the chromosome in half, moving each chromatid to opposite poles. The cell then splits.
6. So, we began with one diploid body cell with twice as much DNA as it needed, and ended with four haploid gamete cells that each contained a normal amount of DNA. Thus sperm and eggs are ‘born’.
Meiosis is a delicate dance, and requires everything to be functioning precisely in order to work. It is easy to see how errors during meiosis could lead to changes in chromosome number.
For example, consider an error in which one spindle fibre fails to attach to one chromosome pair prior to the first split. They fail to get pulled apart; the result is that, when the cell divides, one cell gets both members of the pair, and the other cell gets neither member of the pair. When an egg with both partners meets a sperm with one partner, the result is a polysomic individual. When an egg with neither partner meets a sperm with one partner, the result is a monosomic individual.
Or imagine a scenario in which all of the spindle fibres from one pole fail. All of the chromosomes then migrate to the same pole. The result is one diploid gamete and one chromosome-less gamete. When the diploid gamete meets a normal haploid gamete, the result is triploidy. When the chromosome-less gamete meets a haploid gamete, the result is haploidy.
In this way, and in ways comparable to it, chromosomal mutants can be formed.
This can clearly then drive diversity within members of a population. But the question is, are there important evolutionary consequences to changes in chromosome set? Can it drive the formation of new species? Can it account for the diversity we see between species?
Check back later this week as we continue to explore chromosomal mutations.
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