Z is for zy­gotes

An eclec­tic al­pha­bet of chicken facts with a dif­fer­ence. By Andy Cawthray

Your Chickens - - Contents -


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.


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

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