THE USE OF GENETICS IN BREEDING CHAMPIONS: PART 2
In part 1 we discussed the development and compositions of genes. Most of you probably already knew about this and, like me when I had to work through it, found it rather boring and old hat. Sadly one must first lay a foundation before you can build a house. As we go along you will notice that we gradually move from the very academic, though low level, genetics, to the practical breeding of racing pigeons.
In this part we need to cover some more background information. Bear with me – this is important if we are to see the whole picture.
CELL DIVISION AND REPRODUCTION
As we already discussed, cells divide to produce new cells within all organisms, each of which requires the genetic information found on DNA. Yet simply splitting the DNA of a dividing cell between two new cells would lead to disaster – the two new cells would have different instructions and each subsequent generation of cells would have less and less genetic information to work with. Imagine how chaotic it would be to rip an annual racing programme in two, give different halves to different club members and expect them to enter pigeons for all scheduled races. Just as each fancier would require a full programme to enter pigeons for all races, each new cell needs a complete copy of an organism’s genetic information to function properly.
Organisms use two types of cell division to ensure that DNA is passed down from cell to cell during reproduction. Simple one-celled organisms and other organisms that reproduce asexually – that is, without joining of cells from two different organisms – reproduce by a process of mitosis. During mitosis a cell doubles its DNA before dividing into two cells and distributing the DNA evenly to each resulting cell. In life forms that reproduce asexually, all offspring share the exact same genes and are identical to their parents. The genetic transmission that occurs in organisms that reproduce sexually is far more complex. An individual that forms by the union of two gametes inherits its chromosomes from two distinct parents. Consequently, sexual reproduction guarantees that offspring with new combinations of genes will continually arise.
We will not discuss one-celled organisms further, seeing that pigeons belong to the second group, namely organisms that reproduce sexually.
Organisms that reproduce sexually use a type of cell division that is different from organisms that does not reproduce sexually. These organisms produce special cells called gametes, or egg and sperm. In the cell division known as meiosis, the chromosomes in a gamete cell are reduced by half. (Pigeons and other higher organisms also undergo mitosis for cell division that is not involved in gamete production. For example, the cell division that occurs during the moult is achieved via mitosis. Fortunately we don’t need to worry too much about this – it does not really impact on our objective, namely to breed top class racers.) During sexual reproduction, an egg and sperm unite to form a zygote, in which the full number of chromosomes is restored, i.e. half of the chromosomes are inherited from each parent.
The reduction process that creates cells containing half the number of chromosomes that a normal cell contains is called meiosis. During meiosis, two-cell-division occur to produce four daughter cells from the original parent cell. Each resulting cell has half the chromosomal DNA of the parent cell. A half set of chromosomes in an organism is known as the haploid number. In the first cell division of meiosis the chromosomes of a gamete cell duplicate and join in pairs. The paired chromosomes align at the equator of the cell, and then separate and move to opposite poles in the cell. The cell then splits to form two daughter cells. As meiosis proceeds, the two daughter cells undergo another cell division to form four cells, each of which bears half of the number of chromosomes found in the other cells of the organism.
Meiosis ensures that reproduction will produce a zygote that has received one set of chromosomes from the male parent and one set of chromosomes from the female parent to form a full set of chromosomes. The entire set of chromosomes in an organism is known as the diploid number. Once formed, the zygote continues to divide and grow through the process of mitosis. This can be illustrated as follows:
Figure 3: The process of meiosis.
PATTERNS OF INHERITANCE
Certain patterns of inheritance were evident long before scientists discovered the molecular structure of DNA and chromosomes. Throughout history, people have recognized that certain traits, whether in humans, animals, or agricultural crops, could be passed from generation to generation. Yet for centuries, people were unable to reconcile many confusing observations about the mechanisms of inheritance.
The first person to make sense of this complex subject was the Austrian monk Gregor Mendel, who constructed a series of experiments on pea plants beginning in the 1850s. Mendel observed the results of crossbreeding plants with different traits, such as height, flower colour, and seed shape. His conclusions from these experiments led him to develop explanations for how traits are transmitted from generation to generation. We will discuss the Mendelian Laws next time.
q The pigeon consists of cells.
q The cells contain cell-cores.
q Each cell-core contains 40 pairs of chromosomes. One chromosome per pair is inherited from the cock and the other from the hen. A pair of such chromosomes is called an allele pair.
q Each chromosome contains a number of genes. Each pair of genes (on the two similar chromosomes) is called an allele pair. Genes determine the performance capabilities of our pigeons.
Dr Jaap Nel. Tel no (012) 653 2119. Fax no: (012) 653 7030. Email: firstname.lastname@example.org.