Since I began to learn about genetics in biology last year, I have become very interested in how they work and the science behind them. Furthermore, this was encouraged by one of my first posts on owl classification when I talked about the differences in tytonidae and strigidae owl families. During the time when I researched that, I began to look into the genetic differences, and although finding none in these two species, I was eager to learn more. In terms of chromosomes, all I know is that humans have 23 pairs in every cell of their body except the gametes (sex cells) which only have one set of 23 meaning that when the sperm and egg fuse a complete set of 46 will be made. I also know that different animals have different numbers of chromosomes, which determines if they can reproduce with one another or not. However, I am interested to know how many chromosome pairs different animals have and the limitations between breeding because of this.
This wikipedia link lists a cross-section of different animal, plant and protist (eukaryotic (organism whose cells contain complex structures within a membrane) microorganisms) groups. The highest count of diploid ( 2 complete sets of haploid (number of complete chromosomes in a gamete)) is 1440, found in a fern called Adders-tongue.
Whilst the organism with least diploid chromosomes is the Jack Jumper Ant in which the female has 2 and the male is haploid so only has 1!
From the sites I am researching, I feel out of my depth because of the vocabulary being used and need to clear up the different types of ‘ploidy’. Ploidy is the number of sets of chromosomes in an individual cell. The haploid number is referred to as n and this is the number of chromosome sets in a gamete. But monoploid is the number of unique chromosomes within a single complete set. A pair of chromosomes is called a homologous pair. As the sets of chromosomes increase so does the name given to them, triploid (three sets), tetraploidy (four sets – common in plants), hexaploid (six sets) etc. Species, such as the Jack Jumper Ant, where one sex is haploid whilst the other is diploid are called haplodiploid. (http://en.wikipedia.org/wiki/Ploidy#Homoploid)
Although I am now overwhelmed with new vocabulary, I am beginning to understand the differences between different species and the number of chromosomes, as well as the number of sets of chromosomes, they have. But what determines these numbers?
But as no one really knows, this is a hard question to ask. However, related species generally have similar chromosome numbers. So how similar do they have to be to able to reproduce?
I decided to look at the example of the mule. In the list of animal chromosome numbers, horses have 64 and donkeys have 62, so a mule has 63. Therefore, the numbers of chromosomes have to be similar, although this does not mean that a dolphin could reproduce with a badger because they both have 44 chromosomes. The other thing that makes a difference is the similarities between the strands of DNA themselves. The genes have to be similar in lengths. A horse and donkey genes are similar enough to fuse, however they are not similar enough for mules to go through meiosis in order to make gametes. In this process, chromosomes have to match up and with different genes this cannot happen so mules cannot reproduce. Read more about mules: http://genetics.thetech.org/ask/ask225
In conclusion, the number of chromosomes, which range from 1 – 1,400, the number of sets and the length of the chromosomes determine the relationships animals can have with one another. However, when genetic mistakes occur, problems can arise. For example Down’s Syndrome is caused in humans by the presence of an extra chromosome number 21.