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