Inside every living cell, there is a nucleus (plural nuclei). This is the cells powerhouse, it controls the actions of the cell. Nestled in the cytoplasm among mitochondria and glycogen granules, the chromosomes are found within the nucleus. These structures, which become clearly visible under a microscope before they split, otherwise rest as chromatin granuals. These control the organisms characteristics by means of genes. Lined up on every chromosomes are genes which control egg colour, eye colour and other traits. Chickens have 78 chromosomes, some being sex chromosomes (so called due to determining gender and sex linked characteristics) micro and some being macro chromosomes.
Cells divide by meiosis and mitosis. Mitosis is where the chromosomes replicate and line vertically up the middle. The chromosome and its replica are called chromatids. They cross over each other in the centre and are held together by centromere. The cell splits vertically and the daughter cells have the same amount of chromosomes as the first. Mitosis is used for growth in plants and animals and asexual reproduction in plants.
In meiosis the chromosomes replicate and line up in two vertical lines in the centre of the cell. One row moves right, the other left and the cell splits down the middle. The chromosomes arrange themselves across the middle of the daughter cell. The chromatids seperate and travel to opposite ends of the cell, and the cell divides again. After the dividing has finished, the result is four cells, each with half the original number of chromosomes. Meiosis is used for sexual reproduction, so that when the egg is fertilized, the chromosomes in the males sperm complete the set of chromosomes. If this didnt happen, then the number of chromosomes each individual posesses would double with every generation.
A cell with the full number of cells is diploid, while one formed by meiosis is always haploid.
The gender of the individual is determined by certain kinds of chromosomes and their genes. In humans, females are homogametic, meaning that the two letters that represent their gene type are the same but with chickens the cockerel is homogametic. A ZZ chromosome is male, while a ZW chromosome is female.
Sex linked traits are carried in one copy on non-homogametic genes, while the homogametic gene carries two copies. As the male carries the larger amount of copies, he is mostly responsible for traits such as egg colour in his chicks.
Principles; Basic poultry genetics
These are just some basics that are essential for hobby breeders, covering basic DNA and genes.
1) There are two strands of DNA, twisted into a double helix. At each ‘point’ in the DNA that we call genes, the gene is equal on both sides. This means that there can be one or two doses of the gene. One dose is known as heterozygous, two are known as homozygous.
2) The ‘ locus’ is a name given to the point where the gene falls on the DNA. Sometimes, several related genes are found on the same locus, these are known as allelles. If there are two different genes on the same locus, this is called a heterozygote. A homozygote is where two doses of one gene are found on the same locus (one on each strand).
3) ‘Sex linked’ is a term used to describe a gene found on the z-chromosome. The z chromosome from hens only affects her sons and is known as sex-linkage.
4) If there is one dose of dominant gene it will make an effect on the phenotype ( The way genes act and the effect they produce). One dose of recessive gene makes them a ‘carrier’ for the gene, but its effect is not displayed.
5) A multi-interaction gene is where several genes are needed to produce an effect. A good example is the gene for single lacing. Pg, Co and Ml are needed for the plumage to show that marking.
This is useful information, especially if you plan to breed or study avian genetics. I hope you found this feature interesting and educative.