Definitions Every Breeder Should Know
by Debi Garvin
Even though you may not want to dive into the ins and outs of genetics, there are a few definitions and basic concepts that every breeder should know. By understanding these concepts, you will be better prepared for conscientious breeding, and have a better understanding of why certain characteristics are very difficult to predict while some are very easy. In the definitions below, I will use two examples – color in Angus cattle and color in flowers. In the cattle examples, black will be designated as dominant by a “B”, whereas red is recessive and designated by “b”. In the flower example, red is “R” and white is “r”, with red being dominant and white being recessive. If you want to know more about genetics – especially in camelids – I highly recommend purchasing “A Breeders Guide to Genetics – relax, it’s not rocket science” by Ingrid Wood and Denise Como.
- – The physical appearance of an animal.
- - The actual genetic makeup of an animal as determined by the genes it carries. Different genotypes may result in animals that have the same phenotype (i.e. look alike). For example, Angus cattle that have different genotypes (BB and Bb) are both black.
- – Rodlike or rounded bodies made up of DNA that contain the genes for an animal. Camelids have 74 chromosomes (37 pairs – 36 autosomal pairs and 2 sex chromosomes).
- - Chromosomes other than the sex chromosomes.
- - The unit of inheritance – part of the DNA molecule. Mammals have thousands of individual genes organized in gene pairs, one gene from each chromosome.
- – Alternative forms of genes. For example, Angus cattle carry a red color gene and a black color gene – these are two alleles of the same color gene.
- – Both alleles in the gene pair are the same (BB or bb).
- – The alleles in the gene pair are different (Bb).
- – An allele that expresses itself whether heterozygous or homozygous. In the above example, black is dominant to red. The phenotype of the animal will be that of the dominant allele, even if a recessive allele is present. Therefore, in Angus cattle both BB and Bb are black cattle.
- – A gene whose phenotype is covered up by its dominant allele. In the cattle example, red is recessive to black. For the phenotype to “show-up”, the animal must be homozygous recessive. In Angus cattle, all red cattle are bb.
- – A situation where neither allele is dominant over the other. In the flower example, RR would be a red flower, rr would be a white flower, but an Rr would be a pink flower, since neither allele is completely dominant over the other.
- – Traits that are determined by more than one pair of genes, such as height in mammals and most fiber characteristics in camelids. In the above color examples, the traits are determined by a single gene pair and therefore are not polygenetic. The majority of traits in livestock are controlled by more than one gene pair and therefore are polygenetic.
- – A defect acquired before birth and present at birth – it may be genetic or non-genetic.
- – A defect that is due to genetic inheritance. For a defect to be genetic, generally both parents, but in all cases at least one parent, must carry the responsible gene. Polydactyly in cats is a genetic defect.
- – These are usually congenital defects NOT caused by genetics. Numerous factors can be responsible for these defects, such as environment, diet, exposure to radiation, exposure to pesticides, viruses, drugs, and some vaccines. Perhaps the most widely publicized case of non-genetic defects was in babies whose mothers took the drug thalidomide. Birth defects found in ducks in California were caused by excessive selenium in the water. Addition of Folic Acid to the diet of women has dramatically reduced the incidence of the congenital defect spina bifida. Known causes of non-genetic congenital defects in farm animals include consumption of certain plants at critical points during gestation, such as Lupinas sericeus, Gutierezla, and Veratrum californicum. Researchers also believe that giving vaccines at certain points during the gestation period can lead to congenital defects.
- – Interaction of two or more gene pairs (different gene pairs, not different alleles of the same gene) which may even be located on different chromosomes to produce a phenotype that they don’t produce when they occur separately. This phenomenon results in offspring having “surprise” phenotypes that neither parent possesses. The most frequent cases of epitasis involve coat color, where one set of alleles can either mask or allow the expression of another set. For example, in horses black and chestnut are two basic coat colors, with black being dominant over chestnut. However, a separate dominant white gene at a different locus masks the expression of all other color genes. So if a horse has one dominant white gene, it is irrelevant what the other color genes are, the horse is white. If a heterozygous white horse is bred and the recessive form of the white gene is passed to the offspring, then the other color genes will express themselves. If the dominant white gene is passed on, the offspring will be white regardless of what other color genes are present.
- – These are the two chromosomes that determine the sex of an animal. Females have 2 “X” chromosomes, while males have one “X” and one “Y” chromosome. Therefore, only male offspring can inherit any trait carried only on the Y chromosome of the father, and all traits carried on the Y chromosome will be passed to male offspring. Male offspring will also inherit any trait carried on the non-homologous portion of the X chromosome.
- - Genes that are carried on the non-homologous (non-paired) portion of the sex chromosomes. Generally, these have been thought to be primarily on the larger X chromosome. However, recent research has shown that the Y chromosome may be more responsible for certain traits than what was once thought.
- – Situations where the trait is limited to one sex, such as milk and egg production.
- – This occurs when the sex of an animal influences the phenotypic expression of certain genes. Sex-influenced traits are carried on the autosomes. Some traits that are dominant in males but recessive in females are called sex-influenced traits. One example is the presence of horns in sheep. The heterozygous (Hh) sheep is horned if it is a male and hornless if it is a female.
Reference: Genetics of Livestock Improvement, John E. Lasley, Third Edition