Pigeon Genetics Game Answers

Pigeon Genetics Game Answers Questions And Answers
Pigeon Genetics Game Answers Answer
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1. Pigeon Genetics‎ > ‎

1.1 Genetics Terminology

Basic Terminology
Chromosome	Carriers of the genetic information in the cell. More formally, a chromosome is an organized structure of proteins and a single strand of DNA that makes up the genes.
Locus	The position of the gene on the chromosome. It is possiblethrough breeding and statistics to calculate how far apart genes are onthe same chromosome (though in this age of microbiology, this'classical' view of chromosome mapping is a little simplified, but more than sufficient for the purposes of this site.)
Alleles	Different mutations of the gene at a specific locus on achromosome. A pigeon example: barless, bar (wild-type), check and T-patterncheck are alleles at the pattern locus.
Genotype	The genetic make up of the organism. There are many waysto visualize and annotate the genotype of an organism. See the section on symbols for details of the method used on this site.
Phenotype	The traits that the organism displays or alternately, the physical attributes.
Zygosity (Similarity of genes at a locus)
Homozygous	The organism contains the same allele on both chromosomes of a pair.
Heterozygous	The organism contains two different alleles on a pair of chromosomes. These birds are called split for the recessive factor or carrying the recessive factor.
Hemizygous	When an allele is located on the sex chromosome, thegender that only has different sex chromosomes (ZW in the case ofpigeons). Female pigeons cannot carry two different alleles on the Zchromosome. This means that a hen can only ever be hemizygous for thesegenes, while the male pigeon can be either homozygous or heterozygousbecause they have two Z chromosomes.
Use of Symbols Symbols are used to denote genes and genotypes. This allows us to write down the known and unknown genetics make-up of the organisms we study. The rules for symbols in genetics is very simple and can be used as a mnemonic device for remembering dominance of genes and their other interrelations. This will be discussed in more detail in the next section. Rules for genetics symbols. Symbols for genes are usually letters or combinations of letters and numbers. The community who studies the genetics of an organism usually choose these symbols amongst themselves (the honor of naming an mutation usually falls upon the person who discovers the mechanism of inheritance of the mutation). For instance, pigeons that have checkered wing shields carry a gene called 'check'. This gene is symbolized as C. Alleles always have the same symbol, but are distinguished by adding superscripts or changing an upper case letter to a lower case letter or vice versa. The C (checker) gene has an allele called T-pattern checker and this is symbolized by *C^T. There is also the barless allele of the C gene, and it is symbolized with a lower case c. The allele that is present in the wild-type pigeon is always symbolized using a plus sign (+). Some geneticists use the plus on its own, but on this site, the wild-type allele is denoted using the letters for the locus with a superscript plus sign. The wild-type allele for the C locus is then symbolized as c⁺. When a gene is on the sex chromosome, the hemizygous genotype is denoted with a dash (-). These symbols can then be combined to describe the genetic makeup of a whole organism. For instance we could write A//a bc//bC d//-*. This describes a bird that has the alleles A and a on one chromosome pair, the alleles b and c as well as b and C on the other chromosome pair and is hemizygous for the d allele. (This means that this is a hen). NOTE: Don't worry if you are not completely comfortable with the use of symbols, it becomes easier the more one is exposed to the system, and later becomes second nature.
Expression, Dominance and Epistasis The influence of the genes on the phenotype as well as their interrelations of phenotypical expressions
Interrelations at the same locus
Dominant	A dominant allele A totally masks the expression of allele a. This means that homozygotes (A//a) and heterozygotes (A//a) have the same phenotype, a heterozygote and a homozygote will be indistinguishable. An example in pigeon genetics is the Spread factor (S).
Recessive	A recessive allele a is totally masked by the presence of a dominant allele A. This means that in order for a recessive allele to affect the phenotype, the organism requires two copies of the recessive allele (a//a). The barless factor (c) is an example of a recessive mutation in the pigeon.
Incomplete Dominant	Often the dominance of allele A is not expressed totally when combined with a in a heterozygote (A//a). In such cases the phenotype of the homozygote (A//A) and the heterozygote (A//a) differs. Usually the heterozygote has a phenotype intermediate between the phenotypes of the dominant homozygote (A//A) and the recessive homozygote (a//a). When considering pigeon genetics, the grizzle (G) and indigo (In) factors are incomplete dominant factors. Note: This kind of dominance is sometimes called semi-dominant or co-dominant (though geneticists distinguish these terms - for the purposes of this site, a single differentiation is sufficient).
A short general discussion of allele interactions can be found lower down on this page.
Interrelations at different loci
Epistasis	When the expression of a gene at one locus alters the expression of genes located at a completely different locus.
Recessive Epistasis	The expression of recessive allele at one locus changes the expression of genes at another locus. The prime example n pigeon genetics is the recessive white gene (z). A homozygous recessive white pigeon will be completely white, no matter which other genes the pigeon carries. Thus recessive white can be considered to be recessive epistatic to the alleles of the colour series, the pattern series, pearl and orange eye, the dilution locus and pretty much every other mutation that is related to pigeon coloration.
Note: For the interaction of genes at 2 loci, 14 different classes of epistatic relationships are defined by geneticists, but they are beyond the scope of this general introduction.

A note on gene interactions:

A very important part of genetics (in a classical sense) is understanding the interrelations between genes. The way that the presence of one gene influences the expression of another gene.
There are different levels of interaction between genes. The simplest of these are dominant and recessive. Dominant implies that the expression of a gene dominates in expression over the more recessive alleles at the locus, while recessive implies that the expression of this gene does not happen in the presence of a more dominant allele. As stated before, he symbols for dominant alleles are usually upper case while the recessive alleles are denoted using lower case.

Here is a simple example:

Assuming we have two alleles A and a at a locus, we have three possible genotypes: AA, Aa and aa. If AA and Aa birds are indistinguishable (their phenotypes are the same), the A allele is called dominant and the a allele is called recessive. This means that to see the a in action, one would need a bird that carries two copies of the gene.

Sex Linkage

Genes that exist on the sex chromosome (Z) in birds are called sex linked. Since the male pigeon has two Z chromosomes he can be heterozygous for the alleles at that locus while females are always hemizygous, they can only carry a single copy of a gene at that locus.
Sometimes the expression on a hemizygous female differs from either the heterozygous or homozygous expression in the males (as is the case for the Faded allele (St^F) at the almond locus).
Certain matings involving sex linked genes allow birds to be sexed at birth. More details will be given in the sections relating to sex linked mutations.