The Cattleman’s Dictionary
The Science Behind Genetic Selection
By Dr. Jackie Atkins
Recently, the ASA has gathered terms for an online glossary in order to help our members understand the science behind genetic selection a little better. While this does not make for exciting reading, we hope these definitions will help our members have a clearer grasp on some basic principles in genetics. Here is a sample of definitions that clarify the difference between a gene and an allele, homozygous and heterozygous, genotype and phenotype, and recessive traits vs. varying degrees of dominant traits. If you have ideas on terms you would like to see defined by the ASA, please send them to firstname.lastname@example.org.
Gene: A specific section of DNA that typically codes for proteins or some other gene product. Each individual carries two copies of every gene, one from each parent.
Allele: Genes have variations called alleles. For instance, one gene that controls coat color is called the extension gene. This gene has three variations or alleles which are black (E), red (e), or wild type (Black-brown color, E+).
Homozygous: Homozygous means an individual has two of the same alleles for a certain gene. A homozygous animal for the extension gene (coat color) could have two black alleles (EE) or two red alleles (ee).
Heterozygous: Heterozygous means an individual has two different alleles of the same gene. An individual with one red allele and one black allele for coat color would be heterozygous (Ee).
Genotype: The genetic makeup of an individual (as opposed to physical characteristics). In the coat color example, an animal’s genotype would be what alleles for coat color they have (EE, Ee, or ee).
Phenotype: The observable characteristics of an individual. The phenotype is influenced by both the individual’s environment and their genotype. In the coat color example, animals with a genotype of EE will have a black coat phenotype. Environmental influences may alter the phenotype (nutrition, freeze branding, bleaching, etc.) but the genotype will remain the same.
Incomplete Penetrance: Refers to the phenomenon when some animals with the genotype for a certain trait do not show the correlating phenotype. In other words, penetrance describes the proportion of animals for a certain genotype that display the phenotype. You would think this should always be one but sometimes it takes a second trigger (a second gene or something in the environment) to cause the phenotype to be displayed. A recent example of incomplete penetrance is Developmental Duplication where some of the individuals that are homozygous for the recessive gene (and should display duplicated body parts) have no outward sign of the condition.
Dominant trait: A dominant trait means an animal needs only one copy of the allele to display the trait. An example of a dominant trait would be black coat color. If an animal gets one copy of the black allele and one copy of the red allele, the animal will be black.
Recessive trait: A recessive trait means an animal needs to inherit two copies of that allele in order to display the trait (phenotype). This is only possible if both parents have at least one copy of this allele. If the animal inherits just one copy, then the dominant allele will drive the phenotype.
Red coat color is a recessive trait. A calf must have two copies of the red coat allele (ee) to display a red coat. That does not mean both parents had to be red, they just had to carry at least one copy of the red coat allele. Mating two heterozygous black parents (Ee) would have a red calf (ee) approximately 25% of the time.
Incomplete Dominance: One allele is not completely dominant over the other allele. With incomplete dominance, the heterozygote has a phenotype intermediate between the homozygotes resulting in a third phenotype different than either of the homozygotes. An example is flower color in snapdragons where red is incompletely dominant over white and a heterozygote has pink flowers (in between the two homozygotes).
Co-dominance: In co-dominance, the heterozygote exhibits both characteristics of the homozygotes (both phenotypes are displayed). An example of this is blood type in humans. A heterozygote with A and B alleles produce both A and B antigens. Another example is roan coloring where an animals coat color contains two individual colors (red and white).
The difference between co-dominance, incomplete dominance, and complete dominance can been seen in the phenotypes of the heterozygotes. In Co-dominance each trait is seen (red and white hair shafts), with incomplete dominance there is a blending between the phenotypes (red and white make pink flowers), and with complete dominance just the dominant trait is observed when individuals are heterozygous (black coat color).