Grassroots Genetics 2
Genes, Chromosomes and Transmission of Characteristics
Part of a series of articles by Dr Mike Tempest
Last time I set the context for this series of articles by acknowledging that the dog we see is not all due to its genetic make-up because the environment in which it lives (and by this we mean for example nutrition, exercise, grooming, tail-docking etc) may restrict the appearance of the dog below the potential endowed upon it by its genes. At the same time, I explained that we could not push a dog’s real appearance – it’s true conformation - beyond that determined by its genes, and I hope you all concluded that this is why genetics is important, and therefore it is what we want to pay more attention to in these articles. So, the next step is to deal with how characteristics are passed from parent to offspring, and here we have to look at genes and chromosomes.
Genes – the units of inheritance
The unit of inheritance is known as a gene. Basically this is a chemical plan that controls every characteristic of the dog – its shape, its growth, its form – in other words from the union of one egg cell with one sperm cell the differentiation into bone cells, muscle cells, brain cells right down to every minutiae of say retina cells which we shall deal with in a later article. The chemical that the genes are made of is a complex molecule called DNA, which is short for deoxyribonucleic acid. DNA consists of two strands wrapped around each other to form a helix. The strands are made up of four different chemical structures called bases that have the initials A, C, G and T. Each gene has a unique sequence of the four bases, for example A-G-T-T and T-C-A-A, and it is the sequence in which these bases are joined up that determines which bit of the master plan each gene will control. I felt it necessary to explain this bit of molecular chemistry, but I need to point out that the chemistry is extremely complex, so to help get a mental picture and because the DNA is like a long thread and the genes are arranged along the thread, I want you to envisage that the genes are like beads on a necklace, and in genetic-speak we call the necklace the chromosome.
The chromosome is the structure that carries the “thread” of DNA, and thus the “beads” of genes, in the nucleus of all the cells in the body. Some experts have estimated that if the DNA thread of each cell was teased out and laid in a straight line it would be about two metres long, yet a cell has a diameter of only about 0.0001 of a centimetre. So, the DNA thread has to go through an immense amount of folding to be able to fit in to the cell’s nucleus, and in essence it is the folded DNA that forms the chromosome.
Each animal species has a set number of chromosomes. In the dog this is 78, in humans it is 46, in cats 38 and in horses 64. Although the chromosomes come in different shapes and sizes, the dog’s chromosomes are not 78 different ones, but there is two of every kind, so we really have 39 pairs. The photographs in Figure 1 are taken with very high-powered electron microscopes, and are of two cross sections through the nucleus at different levels, with some parts of some chromosomes identified with fluorescent markers. Some pairs of chromosomes can thus be seen.
Transmission of Characteristics
We have now identified that the characteristics of a dog are controlled by the genes, and the genes are located on the chromosomes, but what is the mechanism by which the genetic material is passed from one generation to the next?
It can be seen in Figure 1 that pairs of chromosomes do not necessarily hang about together in the cell nucleus, but at a certain time they do come together. This is when the cell divides to make sperms in males and eggs in females. After pairing-up the two chromosome of each pair then move to opposite ends of the nucleus. The cell then divides and each sperm and egg carries only 39 chromosomes. This is called segregation. When the sperm fertilises the egg and the sperm’s 39 chromosomes recombine with the egg’s 39 different chromosomes, but matching in pairs with the sperm’s, so that the resulting embryo acquires the full number of 78 chromosomes in 39 matching pairs. If the number of chromosomes had not halved when the sperm and egg were formed, the embryo would have ended up with 78 + 78 or 156 chromosomes, and the number of chromosomes would not have stayed constant at the correct number for the species.
So, this is the reason for the chromosomes existing in pairs - one of each pair is derived from each parent, and several combinations of each pair are passed to the many offspring when an animal is bred. But the next key question is: when the pairs of chromosomes came together before moving to opposite ends of the nucleus, which one of the chromosome pair went to which end of the nucleus, and which one of all the other pairs went with them. If we call the chromosomes in each pair A and B, did A from chromosome 1 go with B from chromosome 2 and with A from chromosome 3 and B from chromosome 4 and so on, or was it A from chromosome 1, A from chromosome 2, B from chromosome 3 and B from chromosome 4. With 39 pairs of chromosomes it is obvious that the number of possible different combinations is enormous – in fact it is 39 multiplied by itself 39 times. So, each sperm is likely to have a different combination of the chromosomes, and each egg likewise, because segregation and recombination will have occurred entirely at random. Thus, the genes carried by each sperm and egg will depend on the chance allocation of the chromosomes.
The final element of chance is which sperm fertilises which egg! Although a tremendous amount of variation is possible, and there is no known way by which dog breeders can control the random allocation and recombination of chromosomes, to those who are serious about dog breeding, it is this element of randomness and chance that makes breeding so fascinating and a never-ending challenge.
The challenge of dog breeding
In future parts we shall move gradually onto what breeders can do to reduce chance variation and to increase the prospects of getting what they want by careful selection of breeding stock, planning of matings, some knowledge of genetics and a lot of knowledge about the breed being bred. Next time we will move onto simple modes of inheritance.
This article was originally published in Show Dogs Ireland and is reproduced here with the kind permission of its editor Caroline Reynolds and the author Mike Tempest#