"Genetics of the Dog"
(1989) by Malcolm B. Willis ********************************************************************** PRINCIPLES OF GENETICS: Basic terms Most of the articles appearing in dog magazines relating to genetics are concerned with genetic defects. While this is and important subject, there is much more to the sciences of genetics and animal breeding that can be useful to dog breeders. A good place to start is with some definitions. Genes are carried on chromosomes and each gene carries the code to make a specific protein or a part of a protein. Some proteins are structural, for example the proteins that make up muscle fibres. Other proteins are enzymes that act as regulators of biochemical processes like growth, reproduction, determination of colour, etc. Dog have 39 pair of chromosomes, made up of 38 pairs of autosomes and one pair of sex chromosomes (two X's for females or an X and a Y for males). Each member of the same pair appear identical when examined under a microscope. In common usage the term "gene" can refer to either locus or allele, depending on how and/or when it is used. When we refer to a gene's position on a chromosome, we talk about a locus (plural = loci). We may also distinguish between genes at two different locations by referring to "locus A vs. locus B". When we discuss different products or forms of a gene, the more accurate term is allele. One of the genes affecting coat colour produces either black coats or brown coats. This coat colour gene has only two alleles, the black allele and the brown allele. To put these terms into perspective, consider the following: there are 10 seperate loci that determine the basic coat colour in dogs. At any particular locus, two to four alleles are present, depending on which locus and the particular breed. Even though more than two alleles may be available in the population, each individual only possesses two alleles (one on the chromosome inherited from the sire and one on the chromosome from the dam). When we look at a litter of puppies, why don't they look identical, they have the same parents? Remember the dog as 39 pairs of chromosomes. When the sire's sperm cells are produced, one chromosome from each of the 39 pair goes into each sperm cell. Which chromosome from each pair goes into a particular sperm is purely a matter of chance - like tossing a coin. Either heads or tails. Take 39 coins, toss them 4 times and record which ones were head and which were tails for each toss. You can imagine the probability of exactly the same order of heads and tails happening all four times is quite small. The same process happens with the bitch's chromosomes. Much of the variation found between littermates is due to this chance assortment (segregation) of chromosomes into the parental sperm cells, or gametes, and the additional random union of gametes at conception - which sperm unites with which egg. When the genes for two or more traits lie on the same chromosome, the genes are linked, that is, they are inherited together as long as the chromosome is not broken or modified prior to segregation into the germ cells. The genetic make-up of an individual, for a single trait or in total is the genotype. The physical expression of that genotype is the phenotype. When we consider one locus with only two alleles possible (A or a), there are three possible genotypes: AA, , or aa. If a pup inherits the same allele from both parents (either AA or aa), its genotype is homozygous, but if it inherits a different allele from each parent (Aa), it has a heterozygous genotype. Some traits exhibit three separate phenotypes corresponding to the three genotype, but many traits only display two phenotypes. This is because one allele is dominant (commonly designated by a capital letter) to the other, the recessive allele (designated by a lower case letter). In the presence of complete dominance, the phenotypes exhibited by the homozygous dominant (AA) genotype and heterozygous (Aa) genotype are identical and indistinguishable. In less scientific terms, the heterozygote is called a "carrier", because the recessive allele is carried undetected until revealed in the progeny in a subsequent generation. If the phenotype of the heterozygote (Aa) can be distinguished from the phenotype of the homozygote (AA), we call this incomplete dominance. Remember, we can only see the phenotype, but with incomplete dominance, there are three separate phenotypes, whereas, with complete dominance there are only two phenotypes. From "CANINE TOPICS", June 1989. |