Z is for zygotes
An eclectic alphabet of chicken facts with a difference. By Andy Cawthray
When an egg is fertilised, but prior to any form of incubation, it is known as a zygote. It contains genes from both the male and female chickens. This zygote has all the elements required for the development of a chick, but they do not activate until the incubation process is initiated. Once incubation is underway, cell division starts to occur and the zygote is then referred to as an embryo.
A hereditary unit on a chromosome. It transfers characteristics from either the male or female parent as an encoded element of the zygote. It will pair with the equivalent gene from the opposite sex parent (eg, eye colour, comb structure). The gene will then be decoded into the embryo once cell division and development proceeds.
DOMINANT AND RECESSIVE
In straightforward terms, some genes are stronger than others and are therefore capable of suppressing other genes from expressing themselves (and hence stopping the offspring from displaying a particular trait like plumage colour, for example). These genes are known as dominant genes. Examples include leg feathering, five toes and rose comb. The genes they suppress are known as recessive genes. These genes remain within the makeup of the chicken, but are unable to express themselves. They can, however, appear in subsequent matings if such a mating is made with a chicken that either carries the same recessive gene or one that acts recessively in its presence. Examples of recessive genes are silkie feathering, clean legs and single combs.
It is important to note that the dominant or recessive nature of the gene is relative to the gene it is paired with.
Genes that pair that are identical in their code, eg red eye or pea comb, are known as
homozygous. Chickens with homozygous feather colour genes are often sought when breeding for plumage to ensure that the birds breed is completely true and for the breeder to attain a level of predictability in the outcome. They do, however, tend to have a multitude of other matching pairs and can become overly inbred.
A pair of genes differing from each other is known as heterozygous. The genes can be recessive or dominant and the most dominant will invariably be the trait the embryo takes on. (It is worth noting that not all traits are controlled in this manner and in some instances, such as rumplessness, it is the gene pairing that dictates the trait as opposed to one of the gene’s dominance).
Heterozygous can also be used to describe an individual bird. For example, a bird that may be black in its plumage colour may be carrying a recessive colour gene that will appear in some of the offspring; black being the dominant gene in the original pairing. This can result in unpredictable matings if the provenance of the bird is unknown.
This is a term used to identify a gene whose presence will cause a chicken to die, usually during its embryonic development, prior to or soon after, hatching. There are over 50 lethal genes currently identified, most of which are recessive, although a small number are dominant. An example is the short legged gene present in Japanese bantams. As a breed they have short legs and, within the standard for the breed, the shorter the leg the better.
The genetic trait that causes this characteristic is sometimes known as the creeper gene or creeper allele and it becomes lethal when two copies are present within any offspring. This usually results in death before hatching.
The creeper gene ideally needs to be present in both the parents (eg, they both have short legs) when breeding, but crossing two such birds will result in around 25% inheriting both short legged alleles and therefore being unlikely to hatch; 25% will pick up both the long legged alleles, meaning that they are unsuitable for further breeding; while the remaining 50% carry one copy of each. As the short legged allele is dominant, these birds will have short legs. In a nutshell, this means that on average 75% of fertile eggs will hatch and only 50% of those that do will be suitable for further breeding.
Genetic speak often uses percentages — 50% or 25% of those will look like this or that. This is because the genetic crossing that is taking place states it is the case. This is only maths — an ‘on average’ based on the possible outcomes and not a precise prediction. Roll a dice 600 times and you might get a six on 100 occasions. Roll it only six times and you might get six number ones.
ABOVE: Once cell division begins, a zygote is referred to as an embryo
In all chickens some genes are more dominant than others INSET: A fertilised egg is a zygote