Chromosomal Genetic Diseases

In biology, we are currently studying genetics, and I had the opportunity to some independent research into genetic diseases and in particular what causes a few of the many hundreds which exist.

The chromosomes in the genetic make up of an organism can be altered in a number of ways causing problems to occur within the growth, development and abilities of the organism.

  • Deletions are when a portion of the chromosome is missing or deleted.
  • Duplications happen when a portion of the chromosome is duplicated, resulting in extra genetic material.
  • Inversions are when a portion of the chromosome has broken off, turned upside down and reattached, therefore the genetic material is inverted.
  • Rings formed when a portion of a chromosome has broken off and formed a circle or ring. This can happen with or without loss of genetic material.
  • Translocations are when a portion of one chromosome is transferred to another chromosome. There are two main types of translocations. In a reciprocal translocation, segments from two different chromosomes have been exchanged. In a Robertsonian translocation, an entire chromosome has attached to another at the centromere. Only chromosomes 13, 14, 15, 21 and 22 in humans are acrocentric and therefore, only they can take part in Robertsonian translocation.

When considering this, it is needed to be known that all chromosomes are metacentric, submetacentric or acrocentric, depending upon where the pair of chromosomes are joined.

Only five chromosomes in humans are acrocentric as well as chromosome Y.

However, in dogs all but the sex chromosomes are acrocentric.

In cats none of the genes are acrocentric but are all metacentric.

A horse has 18 pairs of acrocentric chromosomes and 13 which are metacentric.

 

In humans, the five acrocentric chromosomes sometimes take part in Robertsonian translocation. When this happens, as shown in the picture, most people will be left with only 45 chromosomes. It often does not have any consequences, however, if an extra chromosome 13 is left this causes trisomy 13, Padua’s Syndrome. This is the same in chromosome 21 which results in Down Syndrome.

Robertsonian translocation allows mitosis to take place normally, however, it can affect meiosis, meaning that the offspring can have impairments.

 

These acrocentric chromosomes can also result in other genetic diseases:

Chromosome 13 – there are 13 diseases which are known to result from chromosome 13, including retinoblastoma which is a cancer in the cells on the retina.

Chromosome 14 – this can be linked to about 14 diseases including Alzheimer’s.

Chromosome 15  – this can account for about 11 diseases including Breast Cancer.

Chromosome 21 – about 12 diseases result from this, many which are cancers as a result of translocations of this chromosome.

Chromosome 22 – about 13 diseases can be associated with this chromosome, including schizophrenia. Many of these diseases result from deletions.

This is just a short summary of chromosomal diseases which can occur in humans, however, every chromosome in every organism can undertake a chromosomal change which can cause a very large number of diseases. But a lot is unknown and there is always more to discover about genetics and the diseases associated with them. I would like to be able to do more research into this, especially the affects upon different species, and have been enlightened by the opportunities I gained through school.

Otters’ Gestation Period – Delayed Implantation

The book The River People by Philip Wayre is about his lifelong involvement with otters. From Norfolk to Malaysia, he outlines the life of an otter. From recently reading this book, I picked up on the intriguing gestation period of a female North American otter.

The gestation period of the North American otter can vary from nine to twelve months. This astonishing variety is caused by delayed implantation of the foetus. After mating, the fertilised egg remains in the uterus in the form of a blastocyst. A blastocyst is a structure containing a mass of stem cells and surrounded by an outer layer of cells which will later form the placenta. Blastocysts are used in the process of IVF in humans to determine a more successful embryo.

However, when a blastocyst is formed in this species of otter, it does not anchor itself into the wall of the womb for some time causing its development to be halted. It could be several months before finally implanted, triggered by the production of certain hormones. The trigger of this is the amount of daylight, suggesting the advantageous gains of going through this process because of the benefits of raising young in a season rich in food and ideal conditions.

On further research, I found that about 100 mammals go through delayed implantation, or embryonic diapause, including rodents, bears, marsupials and mustelids, such as the otter. There are actually two types of delayed implantation. The one I described as occuring in North American otters is called Obligate Diapause and is controlled by seasonal changes.
In contrast to this, Facultative Diapause is controlled by the period of a mother’s lactation. This is a brilliant strategy in order to prevent two litters needing milk at once.

I think that this is an amazing evolutionary step towards preserving one’s young.

References

http://en.wikipedia.org/wiki/Embryonic_diapause

Wild Mammals of North America: Biology, Management, and Conservation by George A. Feldhamer, Bruce C. Thompson, Joseph A. Chapman

The River People by Philip Wayre