Genetic theory and practical animal breeding
How does genetics work?
Now it is going to be scientific and for those who like to have something to read in greater detail, a textbook such as Falconer and Mackay: Introduction to quantitative genetics; Longman paperback, 4th edition 1996, is recommended.
The molecular basis of inheritance is deoxyribonucleic acid (DNA) found in the chromosomes within the cell nucleus of mammals. DNA is a rather long molecule and consists of a sugar, called deoxyribose, and is linked via phosphate groups. Attached to the deoxyribose is an organic base. Four bases, usually adenine, thymine, guanine, and cytosine are possible and form the genetic code. The genetic code is made up from the sequence of the organic bases. Genes are sections of DNA that contain "blueprints" for biological active proteins.
In mammals, chromosomes come in pairs, this is the so-called diploid set of chromosomes. However, the sperm and the ovum only contain one set of chromosomes each so that the resulting fertilized egg again has a diploid set.
Every individual within a species has the same genes on its DNA; however, genes can come in variants (alleles), the gene shows polymorphisms. These variants can also be observed in a single individual if this individual carries two variants in its diploid set of chromosomes. In this case, the animal is heterozygous for the specific gene. If it carries the same alleles on both chromosomes, the animal is called homozygous. Other individuals within the same population may be homozygous for another allelel.
In genetics and hence within the field of scientific animal breeding, the general goal is to trace observable expressions of a trait, the phenotype, back to their molecular basis. We talk about qualitative traits if the phenotype only appears in distinct classes like in the coat colours black or red. Quantitative traits on the other hand vary continously; e.g. milk yield, daily gain. A general rule is that qualitative traits most often are determined by only a few genes if not by only a single gene. Within the group of qualitative traits and in an ideal situation we have: 1 variant of a gene = 1 specific protein = 1 phenotype (like coat colour).
It is important to note that the expression of a phenotype, especially for quantitative traits, is strongly influenced by the environment. Hence it is rather difficult to associate the actions of individual genes with the phenotype.
In any case, scientific genetics is always attempting to identify causal polymorphisms on the molecular level. This predominantly will be the existence of variants of organic bases at a specific location on the DNA. These variants are called single nucleotide polymorphisms (SNP).
It would mean hitting the jackpot if the project could identify SNP which would govern the coat colour and the markings of White Galloways.
The inheritance of coat colour
The inheritance of coat colour in mammals is influenced by a number of genes. One well known gene is the MC1R-gene, also called "Red" gene which is responsible for the basic colours red or black. However, not all specific cases of red or black are attributable to the MC1R gene. The red colour also can be caused by the "variant red gene" which is yet to be fully identified. Other cases of a "different red" is the "changeling" of colour, again presumably caused by yet another gene.
Other genes act as "colour diluters", and again others play a role in the inheritance of spots and colour patterns. Also, the environment, especially the prenatal environment can influence coat colour.
In different breeds, scientific knowledge of the inheritance of coat colour differs widely. In general, it may be stated that the number of clearly identified genes that play a role in coat colour inheritance is rather small.
Hypotheses
If one approaches the phenomenon of inheritance of coat colour, it is useful to separate the entire problem into smaller pieces since the inheritance of a complex phenotype such as the coat colour of White Galloways most likely is caused by more than one gene.
Following the hypothesis ...
... of the Canadian scientist Prof. Sheila Schmutz of the University of Saskatchewan the basic colour "white" of the the White Galloways is determined by a gene with a dominant allel for "white". This "white" presumably is a "white" different from other "whites" of other white cattle breeds.
This hypothesis in the first place would mean that two white parents may have fully black offspring if they are both heterozygous for the "White Galloway White". Following Mendel's rules, such matings would result in black offspring with a probability of 25 %.
A further hypothesis ...
... first of all points to an inheritance of the colour of the markings (ears, "socks") which is determined by the MC1R gene.
Any hypotheses concerning the degree of the markings are yet missing. However, most likely, the degree of the markings could be influenced by environmental (prenatal) influences. This would make things even more difficult.
Important Information
About the Project
Contact
Phone:
+49 (0) 345 55 22320
Email: hermann.swalve
@landw.uni-halle.de
