This same pair of genes may be used to illustrate the types of gametes produced by individuals of each genotype.
Genotype Gametes BB B & B = all B Bb B & b = '/ a of each type bb b & b = all b
The homozygous dominant and homozygous recessive genotypes are true breeding or produce but one kind of gamete.
When a single pair of alleles is taken into consideration, it is possible to have the three different kinds of genotypes as previously illustrated. With these three genotypes, six different kinds of matings may take place. These monohybrid mating possibilities are as follows. Parents
BB x BB BB x Bb BB x bb Bb x Bb Bb x bb bb x bb
Expected Genotypic Ratio Among Offspring all BB 1 BB: lBb all Bb 1 BB: 2 Bb: 1 bb 1 Bb: bb all bb
The phenotypic ratio among the offspring for the various matings can be easily computed by identifying the phenotype for each kind of genotype. It must be remembered that the heterozygote( Bb) differs for the situation of complete dominance as compared to incomplete dominance with reference to phenotype.
More than one pair of alleles may be considered simultaneously in computing the expected results from matings. The details of these will not be discussed; however one should remember that an individual produces two kinds of gametes for each heterozygous locus. An individual possessing 20 such independent loci would produce 220 kinds of gametes. The mating of two individuals of this kind would produce offspring possessing 320 different kinds of genotypes.
It is not possible to discuss total heredity as such because of its complexity. Instead v / e must regard it in terms of unit characters or traits and some of these may be influenced by many pairs of alleles.
Hereditary traits may be classified into two categories, qualitative and quantitative. Qualitative traits are those having a simple mode of inheritance. Often time but a single pair of alleles is involved. These traits are not appreciably influenced by environment. Coat color and certain hereditary anomalies, such as lethals, are examples. Quantitative traits have a complex mode of inheritance. Many pairs of alleles affect each of these and environment may be responsible for a large portion of the total variation we observe. General animal size, fertility, conformation, longevity, running ability, disposition, and temperament are examples.
COLOR INHERITANCE IN HORSES
The information available on coat color inheritance in horses is certainly not complete. There are several reasons for this. The generation interval for the horse is long and the general expense for planned experimental matings is prohibitive. A good deal of the research on the subject has been achieved by means of stud book information which undoubtedly contains some errors due to color misclassifications. In spite of the deficiency of material, many reliable investigations have been made, such as the works of W. E. Castle, F. Gremmel, G. W. Salisbury, and others. The author has summarized the basic information on the subject and questionable areas are identified. The basic foundation colors may be listed as follows;
1. Black— Black is a uniform color overall and is subject to bleaching by exposure to intense sunlight.
2. Bay— A bay horse possesses a black mane, tail, lower legs, and ear tips and a red colored body of variable intensity with regard to shade.
3. Brown r— The brown horse possesses the same black points as the bay but has a brown body. The shade of brown is variable, from an almost black seal brown to a light shade. The brown horse is sometimes confused with black and bay. The seal brown or dark brown horse possesses light points especially in the areas of the muzzle and eye.
4. Chestnut— This color is reddish and variable in shade. The mane and tail are commonly of the same color as the body but may be flaxen or white. The darker chestnut may be blackish in color; the intermediates, liver colored; and the lighter, definitely reddish or sorrel.
Most other colors are derivations of these by genetic modification.
The allelic pairs which determine coat color in horses may be summarized as follows;*
1. B = Black b = Chestnut
Complete dominance.
‘ Symbols other than these have been used by different authors.
2. E = restriction of black to mane & tail, bay. e = complete extension of black e’ = restriction of black to mane & tail, brown.
3. G = gray g = non-gray complete dominance( is epistatic or covers all other colors).
4. Ro = roan ro = non-roan complete dominance( associated mainly with chestnut, especially sorrel).
5. D = dilution of color d = non-dilution
Incomplete dominance
50 VOICE of The Tennessee Walking Horse