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#1 2012-02-15 14:09:26

poplar girl
Administrator
From: Athabasca, AB, Canada
Registered: 2011-06-30
Posts: 3159

Topic 2: SUMMARY of Genetics Terms and Definitions

Below is a summary of genetics terms and definitions along with some examples in relation to poultry.
Please see the following thread to read the discussion surrounding these definitions or to post questions or comments at any time
http://forums.euskaloiloas.com/viewtopi … 21&p=1

Allele- One of the possible variants of a gene, distinguished from other alleles by its phenotypic effect. There may be one, two or multiple alleles of a given gene. Where there is only one allele present in a population (e.g. breed), that gene is said to be “fixed” in that population. Alleles may be dominant, partially dominant or recessive. An individual carries exactly two alleles (which may or may not be identical to each other) at a given locus. Example: there are two alleles for skin (leg) colour - w (yellow) and and W (white). W is dominant over w. A bird that has two yellow alleles (ww) will have yellow legs, while birds with one or two W alleles (Ww or WW) will have white legs.

Autosomal- Autosomal chromosomes, or autosomes, are all those chromosomes that are not the sex chromosomes (sex chromosomes are W and Z in birds; males are ZZ, while females are ZW). Autosomes come in pairs, where both members of a given pair carry identical gene sequences. There are 38 pairs of autosomes (=76 total) in chickens, and one pair of sex chromosomes, for a total of 78 chromosomes. “Autosomal” can refer to the chromosomes themselves, or to any genes found on these chromosomes. Example: The gene for blue egg colour, O, is autosomal.

Chromosome - A long linear strand of DNA (deoxyribonucleic acid) containing a number of genes, as well as regulatory DNA and proteins. The chromosome provides the basic genetic "map" of the organism. Chickens have 78 chromosomes (39 pairs), varying enormously in length and amount of information carried. Example: THe genes for naked neck (Na), pea comb (P), white skin colour (W) and blue egg colour (O) are all carried on the same chromosome pair (known as autosomal linkage group 3).
Please also see:
http://forums.euskaloiloas.com/viewtopic.php?id=522

Gene - The specific portion of the chromosome, transmitted during reproduction, that governs the trait in question. It may (or may not) have several alternate forms (see allele). Each individual bird, for each autosomal gene, will have exactly two alleles (versions of the gene), one on each of the given chromosome pair; these alleles may be identical or may be different. The expression of a gene (i.e. the trait itself) may be more or less influenced by the environment; some traits are almost totally governed by genes (e.g. number of toes, or feather colour), while others are moderately to strongly influenced by environment (e.g. growth rate, or egg yolk colour).

Genotype - The genetic makeup of the organism; it includes all the instructions for creating and maintaining that organism, including physical, biochemical and behavioral traits. “Genotype” may refer to the organism’s whole genetic make-up, or (more commonly, but sometimes confusingly) to just one pair, or several pairs, of genes. Example: the [partial] genotype of a Buff Columbian Wyandotte is eb/eb Co/Co.

Heterozygote (heterozygous) - A heterozygote is an organism that has two different variants (alleles) of the given gene, one at that gene’s locus on each of a pair of homologous (or matching) chromosomes. When gametes (eggs and sperm) are formed, each gamete may have either allele, so either version may be passed to an offspring. “Heterozygous” means to be carrying two gene variants, or alleles. Birds that are heterozygous at the locus for a given trait will not “breed true” for that trait. Example: Blue chickens are all heterozygous at the Bl locus; the genotype of a blue chicken is Bl/bl.

Homozygote (homozygous)- A homozygote is an organism that has two copies of the same gene variant (allele) at that gene’s locus on each of a pair of homologous (or matching) chromosomes. When gametes are formed, all gametes have the same allele, so that all offspring inherit the same allele.
For an organism to “breed true” it is necessary that the individuals be homozygous for the genes in question. In general, all purebred individuals of a breed or variety are homozygous for the genes that define the breed or variety (“Blue” is an exception).


Raising red cuckoo (marraduna) Euskal Oiloak and self blue (lavender) & black Belgian Bearded d'Uccles.

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2012-02-15 14:09:26

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#2 2012-02-19 14:50:18

poplar girl
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From: Athabasca, AB, Canada
Registered: 2011-06-30
Posts: 3159

Re: Topic 2: SUMMARY of Genetics Terms and Definitions

Linkage - Genes that lie on the same chromosome are said to be linked; traits governed by these genes tend to be inherited together. The linkage can be loose or tight, depending on where they are located on the chromosome. The closer together they are on the chromosome, the tighter the linkage; the farther apart they are on the chromosome the looser the linkage. Linkage is measured in “map units”, which reflect the likelihood of the genes being separated by crossing over during gamete formation.
This is useful, in that the appearance of one trait can be used to predict a second trait, if the genes governing the traits are linked.

Example: The blue egg locus (O, o) is linked to (on the same chromosome as) the pea comb locus (P, p). The distance between them is about 4.3 map units. A purebred Ameraucana will have genotype OP/OP. If mated to, say, a barred rock (genotype op/op), the genotype of the offspring (for these loci) will be OP/op. Gametes produced by this bird will be mostly OP or op, but 4.3% of the gametes will be Op or oP (due to crossing over).
Thus, if the granddaughter of a pure Ameraucana has a pea comb, there is a (100-4.3=) 95.7%  probability that she will lay blue eggs. (How cool is that?!?)

Locus - The specific physical location of a gene (or DNA sequence) on a chromosome. (The plural of locus is loci)

Modifier or Modifying gene - A gene at one locus that alters or influences the expression or function of another gene at a different locus. This effect is called epistasis.


Raising red cuckoo (marraduna) Euskal Oiloak and self blue (lavender) & black Belgian Bearded d'Uccles.

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#3 2012-03-03 00:59:06

poplar girl
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From: Athabasca, AB, Canada
Registered: 2011-06-30
Posts: 3159

Re: Topic 2: SUMMARY of Genetics Terms and Definitions

Mutation - A sudden structural alteration in DNA. These changes can be either spontaneous (natural) or induced, by mutagenic chemicals or radiation. In most cases, DNA changes either have no effect or cause harm, but occasionally a mutation can improve an organism's chance of surviving and passing the beneficial change on to its descendants. Most mutations that presist In populations are recessive to the wild type and will not be expressed unless two birds are mated which carry the recessive mutation. One quarter of the offspring can be expected to express the mutation. Birds expressing the changes are referred to as sports; natural and artificial selection will likely affect their survival, as well as the persistence of the mutated gene (allele) in the gene pool.

Outcross – Mating of birds that are unrelated (or, more realistically, quite distantly related). It is a relative term, since all birds are related to each other. It is generally applied to the breeding together of animals that are members of the same breed, variety or strain, but not closely related in pedigree.
The term “outcross” can also refer to the progeny of an outcross.

Phenotype- Phenotype is what we 'see' when we look at a chicken, but it also includes structure (body shape and size), function (egg laying abilities, meat qualities, fertility) physiology, disease resistance, and behavior (friendliness, self preservation skills). It includes all expressed traits. The phenotype and the pedigree are the main methods of judging the chicken's genetic makeup.


Raising red cuckoo (marraduna) Euskal Oiloak and self blue (lavender) & black Belgian Bearded d'Uccles.

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#4 2012-03-10 18:36:59

poplar girl
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From: Athabasca, AB, Canada
Registered: 2011-06-30
Posts: 3159

Re: Topic 2: SUMMARY of Genetics Terms and Definitions

Quantitative - A quantitative trait is one that exhibits a continuous range of values, and is known to be under the control of multiple genes. Examples of quantitative traits: Brown egg colour, body size, egg weight.

Recessive - Describes an allele (variant of a gene) whose expression is suppressed by the other allele at the same locus in the given individual; the other allele is said to be “dominant” over the first.
Example: o (non-blue egg) is recessive to O (blue egg); a bird carrying one O and one o allele (so genotype is O/o) will lay blue (or green) eggs.

Hybrid vigor - The extra vigor, exceeding that of their parents, frequently shown by the crosses (hybrids or outcrosses) between genetically dissimilar parents. Parents can be from different species, breeds, varieties, strains, or inbred lines. Hybrid vigour may be expressed as more rapid growth, larger size, better viability, higher fertility, disease resistance, and/or other traits we think of as related to survivability and productivity.
Many consider hybrid vigour to be merely the absence of inbreeding depression, particularly when applied to crosses between more closely related parents (e.g. from different inbred lines, strains, breeds and varieties).
Examples: production strains of chickens, such as Cornish crosses and white leghorns, are four-way crosses between highly inbred lines of the same (in the case of leghorns) or different (in the case of Cornish crosses and brown production layers) breeds or varieties.
References: Hutt, Genetics of the Fowl (pg 552-553)
Previous thread on this topic: http://forums.euskaloiloas.com/viewtopic.php?id=332

Heterosis - Generally considered to be to be synonymous with hybrid vigor.


Raising red cuckoo (marraduna) Euskal Oiloak and self blue (lavender) & black Belgian Bearded d'Uccles.

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#5 2012-03-23 01:35:24

poplar girl
Administrator
From: Athabasca, AB, Canada
Registered: 2011-06-30
Posts: 3159

Re: Topic 2: SUMMARY of Genetics Terms and Definitions

Epistasis - An interaction between genes at different loci in which an allele at one locus modifies the expression of alleles at one or more other loci.

Inbreeding - The result of mating (closely) related individuals. An individual is said to be inbred if its parents were (closely) related. Can be quantified using the inbreeding coefficient (=F; also called COI). There are many different crosses and breeding strategies that involve inbreeding and are used in animal and plant breeding programs. These include back-crosses, sib matings and line-breeding. Note that the progeny of two highly inbred individuals who are not related to each other will (perhaps surprisingly) have F=0; they are not inbred.

Line breeding – Selective inbreeding to perpetuate certain desired qualities or characteristics in a strain of livestock. The term “line breeding” commonly carries a positive connotation, suggesting care and deliberate effort, while "inbreeding" is generally viewed negatively.


Raising red cuckoo (marraduna) Euskal Oiloak and self blue (lavender) & black Belgian Bearded d'Uccles.

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#6 2012-04-15 12:41:04

poplar girl
Administrator
From: Athabasca, AB, Canada
Registered: 2011-06-30
Posts: 3159

Re: Topic 2: SUMMARY of Genetics Terms and Definitions

Recombination - is the result of crossing over of homologous chromosomes during meiosis (which results in eggs and sperm). During this process, the paired chromosomes (one from each parent) line up together and exchange bits. This results in the separation of linked alleles. For example, the blue egg locus (O;o) is tightly linked to the Pea comb locus (P;p). If an individual inherited OP from one parent, and op from the other, its genotype (for these loci) would be OP/op. Without crossing over, it would only generate two types of gametes, OP and op. With crossing over (and thus recombination) it can generate Op and oP gametes as well, which is why you sometimes get a bird with a straight comb laying blue eggs; it got the Op allele form its half-Ameraucana parent and an op (or possibly Op) allele from its other parent.
This process allows for a huge increase in the amount of diversity in a population. Interestingly, Mendel didn’t figure this out; he thought all loci assorted independently.

Meiosis - The process of two consecutive cell divisions that produces haploid sex cells from diploid progenitor cells. Meiosis results in four “daughter” cells, each with a haploid set of unduplicated chromosomes. These haploid sex cells, or gametes, are the eggs and sperm in animal species. Each gamete carries just one of the two alleles at each locus in the parent’s chromosomes, and each gamete is genetically distinct (unique).

Mitosis – strictly speaking, mitosis refers to the division of the chromosomes of the cell to form two identical sets, but in common usage mitosis generally refers to the entire mitotic phase, which includes cytokinesis (the division of the rest of the cell). The two new cells formed will be identical to the parent cell unless errors are made during cell replication.

Sex linkage - A sex-linked trait is one controlled by a locus on the Z (or W) chromosome in birds (X or Y in mammals). Since the W chromosome carries few loci, most sex-linked loci are on the Z chromosome. In certain crosses sex-linked traits manifest themselves only in the heterogametic sex, which is very useful, as it allows sexing of new hatchlings. This requires that the male parent be homozygous for the recessive allele and the female to have (a single copy of) the dominant allele.
An example of a sex-liked gene is B, for barring. Crossing a barred female (B/-) with a non-barred male (b/b) will give all barred (but heterozygous) male offspring (B/b) and all unbarred (b/-) female offspring. 


Raising red cuckoo (marraduna) Euskal Oiloak and self blue (lavender) & black Belgian Bearded d'Uccles.

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