Every cell contains two copies of each gene, one contributed at conception by the mother and one by the father. The gene sets contain similar, but not necessarily identical, information in each gene. For example, both genes in a set may contain instructions that determine hair structure, but one gene may contain the instructions for straight hair and the other for curly hair.
The alternative forms of each gene are called alleles.
If both alleles are identical, then the individual is Homozygous at that gene locus (location). If the alleles are dissimilar, then the individual is said to be Heterozygous for that gene set. Information about the homozygosity or heterozygosity for various genes can be inferred from information about parents and/or progeny and can be used for predicting the outcome of matings.
Often when the gene pair is heterozygous, one allele may be visibly expressed but the other is not. The expressed allele in a heterozygous pair is known as the Dominant allele and the unexpressed one as the Recessive allele. The term dominant is given an allele only to describe its relationship to related alleles, and is not to be taken as an indication of any kind of physical or temperamental strength of the allele or the individual possessing it. Likewise, possession of a recessive allele does not connote weakness.
In any individual expressing the dominant allele of a gene, it cannot be determined by looking at the individual whether the second allele is a dominant or recessive. The presence of a recessive allele may be masked by a dominant allele, which leads to the expression Hidden Recessive. A dominant alleles is never hidden by it’s recessive allele.
For simplicity in constructing models, geneticists symbolize genes by letters such as A, B, C and so on. A dominant allele of a gene can be symbolized by an italicized capital letter, e.g., A, and the recessive by an italicized lower case letter, e.g. a…
A workable genetic formula for equine hair structure:
- C = dominant Curly gene
- c = recessive straight allele in same location as dominant Curly gene
- R = dominant Straight gene unrelated to straight gene at the C locus
- r = recessive curly allele at same location as the dominant Straight gene
CC RR – Homozygous dominant curly, looks curly, always throws dominant curly foals. Has normal straight genes at recessive loci.
Cc RR – Heterozygous dominant curly, looks curly, has normal straight genes at recessive loci. Throws curly gene 50% of time.
CC Rr – Homozygous dominant, heterozygous recessive. Looks like a curly, can throw recessive curly if bred to another recessive carrier or recessive curly 50% of the time. Won’t be able to tell when since all foals will be curly due to 100% transmission of a dominant curly gene.
Cc Rr – Heterozygous dominant and heterozygous recessive. Is dominant curly visibly, and can produce dominant curly foals off straight mares 50% of the time and can transmit recessive gene 50% of the time.
CC rr – Homozygous dominant & homozygous recessive. Curly coat. Will always throw curly foals and will always transmit one recessive curly gene.
Cc rr – Heterozygous dominant curly and homozygous recessive curly. Always curly coated, and always transmits a recessive gene, but only throws dominant gene 50% of the time, so can throw straights off non recessive curlies, or straight horses.
cc RR – Normal straight horse. No curly signs visible. Don’t know about hypoallergenicity. Can’t throw any curly gene.
cc Rr – Straight coated horse. Carries one recessive gene. If paired with a recessive curly or recessive curly carrier can throw a curly.
cc rr – Recessive curly. No dominant Curly gene, but two recessive curly ones. Will always throw a recessive curly gene, but will only parent a curly if the other parent also throws in a recessive curly gene.
Adapted from the UC Davis website about coat color.