Animal Breeding the Relative Influence of Parents
Animal Breeding the Relative Influence of Parents No question pertaining to breeding has given rise to more controverted opinions than that which relates to the relative influence of parents as male and female, in determining the characteristics of the off- spring.
Many have claimed, and with, much positiveness, that certain characters are derived chiefly from the male and certain other characters are derived chiefly from the female. But, since there is not much agreement between the leading advocates of these theories, even when the same in some leading essentials, and since the arguments presented in support of them are chiefly of a negative character, they fall short of incontrovertible demonstration.
The Relative Influence of Parents in Breeding Defined. — By the relative influence of parents in breeding is meant the influence which they exert as male or female in determining the character of the progeny. It differs from prepotency in drawing the contrast between the influence of the parents as male and female in determining transmitted characters, whereas prepotency has a regard to the influence exerted by either parent without inquiring as to whether any peculiarities of transmission belong to one sex or the other.
And it may be mentioned here that if the contention were true that one parent because of its sex influences certain features of transmission, then such transmission would be a disturbing factor Antagonistic at least in some instances to prepotency.
Such disturbance would complicate the laws that govern transmission to such an extent as to seriously hinder successful breeding. At the outset, therefore, it would seem improbable that influences so antagonistic should be inherited in the same animals.
Sex Alone Does Not Affect Transmissive Power. —Much of what will be said in the remaining portion of this chapter will have a bearing on the affirmation just made.
The correctness of any theory that would assign a greater relative influence on one parent as such in determining the characteristics of the off- spring has not yet been established. It has been claimed that there is a preponderance in resemblance in the offspring to the male parent. It has also been claimed that there is a preponderance in resemblance to the female parent.
But more commonly both claims have reference to certain characters in the progeny rather than to the whole being, otherwise •their absurdity would be so manifest that it would not be necessary to consider them.
Some have said that the male parent transmits certain features of form, function, or of disposition, while others have said that the female parent transmits like properties. But the theory that the male parent exerts overall the greater influence because it is a male has long been popular. That it does exert the greater influence, overall, is true, as will be shown be- low, but not in virtue of its sex.
Were it true that this greater influence was exerted because of its sex, there would not be so many instances in which there is a preponderance in resemblance to the female. In the human family children very frequently resemble the mother more than the father in form, in features and in mental powers.
This preponderance in resemblance to the female parent among domestic animals is also frequent, though not so frequent relatively as in the human family, and because in the latter there is no selection in breeding as with domestic animals, hence the average female is likely to be as prepotent as the average male.
Why the Male Parent Exerts the Greater Influence in Transmission. —The theory that the male parent exerts the greater influence in virtue of its sex has arisen probably from the greater number of in- stances, in which, in breeding domestic animals a preponderance of resemblance may be traced to the male parent. But this may be owing first, to the greater care used ordinarily in breeding males, which renders them more prepotent, and to the greater pains taken in choosing them, and second, to the larger number of the progeny tracing to one male.
Males are usually more purely bred than females, and they are usually possessed of a greater average individual vigor. They are in consequence more prepotent than females as shown in Chapter IX. It could not be otherwise then, but that the resemblance to the males would preponderate in each of the individual progeny. And since the progeny of one male is in nearly all instances in practical breeding much more numerous than the progeny of each female, the sum of the resemblance in the progeny to the male is greater than the sum of the same to all the females combined.
The Offspring Resemble Most the Parent Most Highly Bred. —The probability is strong that there will be a preponderance in resemblance to the parent most highly bred, whether male or female. It has been shown above why the progeny more frequently resemble the male. But suppose the conditions of choice were reversed that more pains were taken in breeding and choosing females, then it would doubtless follow that in the progeny of each female there would be more of resemblance to the female than to the male parent. This is well brought out in crossing a well-established breed with one but recently established, and in mating a pure breed with an animal of mixed breeding.
If a male chosen from a well-established breed is mated with a female of a breed but recently established, other things being equal, there will be a pre- ponderance of resemblance in the progeny to the male parent. Reverse the process and there will be a preponderance in resemblance to the female parent.
Both results are due to the greater potency of the breed that has been long established. Similarly, if a pure-bred male is mated with a female of mixed breeding, there will be more of resemblance in the progeny to the male. Reverse the process and there will be more of resemblance in the progeny to the female. Both results are due to the greater potency of pure blood as compared with that from mixed blood.
Ordinarily therefore the progeny will bear the closer resemblance to the parent of the more ancient lineage in the one instance, and to that of the purest breeding in the other. But there may be some exceptions for reasons that will now be given.
Unexpected Variations in Transmission. —Although the predominant influence of the best-bred parent is the rule in transmission, the intensity of other conditions may. interfere so as to produce unexpected variations.
For instance, where high breeding is practiced, with reference to securing a single quality only, or a limited number of desirable qualities, in securing these strength and constitution may have been so neglected as to result in transmission that is variable. Much depends upon the strength and constitution of each parent, as well as upon the composition of the blood. Under normal conditions the best-bred parent would almost certainly transmit a preponderance in properties to the off- spring.
But a weakened constitution, sometimes at least and, weakens potency in transmission. Diminished strength of constitution including present vigor may therefore tend to counteract potency in transmission, the result of pure breeding.
The antagonism may become so strong even, that its influence in producing variation may be stronger than that of good breeding in perpetuating likeness in transmission. It is possible, therefore, that in many instances as much depends upon the strength and constitution of each parent as upon the composition of the blood.
This variableness in transmission may arise, in part at least, from the inheritance of variable characters represented in the ancestral line, and it may be that impaired vigor enables these to assert themselves in a way which would be hardly possible where much vigor is present, since the latter would prove a controlling influence running counter to them.
Whatever may be said of the explanation, the fact remains, that in both sexes, animals possessing blood precisely similar have shown a marked difference in their powers of transmission, whether male or female.
The Influence of Age on Transmission. —The ability of either parent as male or female in trans mitting characteristics to the progeny is influenced by old age and consequently by bodily vigor. As the bodily vigor of an animal decreases with advancing age, its prepotency in many instances would seem to suffer. In such instances the decrease in prepotency is charged up to a decrease in bodily vigor.
As this decrease in bodily vigor will affect alike male and female, it follows that it will affect the ability of to transmit characters. If it were true therefore that sex as such were capable of certain kinds of transmission, because of sex, advancing age with its decrease in bodily vigor would step in and form a disturbing factor, that is to say, an animal declining in vigor would have less power than one of the opposite sex in the zenith of bodily vigor, to transmit properties when mated with the same.
Such mating would therefore so far disturb transmission in virtue of the sex, if such trans- mission did exist. But advancing old age and diminished’ bodily vigor are not always accompanied by diminished prepotency, as in some instances animals deficient in strength and vigor are highly prepotent. Such transmission is oftentimes readily apparent in the progeny of animals with an inclination to certain diseases or already suffering from the same. In this fact lies the great hazard in breeding from purely bred animals deficient in these qualities.
Transmission When Prepotency is Not Marked. •—When there is no marked prepotency on the part of either parent it has been claimed that the male offspring frequently resemble the sire and the female offspring the dam. Such resemblances have been noticed in the transmission of disease.
Carefully gathered statistics have shown, as quoted by Miles, that in a certain number of cases of consumption and of insanity, the instances of inheritance of these respective diseases from the male parent were more numerous in males, and of inheritance of the same from the female parent were more numerous in females.
This would seem to favor the view that it is possible or a female in virtue of its sex to transmit certain peculiarities to the progeny. But the force of such an argument is weakened by what is said in the succeeding paragraph by the power of transmission which one sex sometimes possesses to transmit peculiarities which affect only the other sex.
The principle involved, however, tends to emphasize the importance of careful selection in the sires introduced into the stud, herd or flock.
Transmission of Peculiarities Through the Opposite Sex. —Instances are not infrequent wherein disease and other peculiarities are limited to one sex and transmitted by the other. This has already been referred to in Chapter VII., but will now be further enlarged upon, because of its bearing on the subject that is being discussed.
Such transmission has been observed in the inheritance of certain forms of ichthyosis. There have been instances in which the disease was confined to one sex and transmitted through the other, that is to say, it would affect only males though transmitted by females in which it was not apparent.
But the opposite of this has happened with the same forms of disease, that is to say, the disease was apparent only in females though inherited from males.
It has been observed in the inheritance of a tendency to obesity when only one sex will be thus affected. But as with the inheritance of skin diseases, such a tendency has at one time manifested itself in one sex, at another time in the other sex, and in yet other instances the transmission is variable and mixed.
It has also been observed in the influence of the dairy sire in transmitting form and functional activity to the udder of the same. It is claimed, however, that such transmission is more marked when the females are grades. This is just what may be looked for, and it is doubtless the outcome of that greater prepotency which a pure-bred sire has when mated with a female of mixed breeding. The greater pre- potency of the male affects the whole organism of the female though of the other sex.
Nor has it been proved to a demonstration that one sex as male or female has the power of transmitting those peculiarities in a greater degree than the other. And the whole question is still further obscured by the preponderance in resemblance to one parent which is observable at one period of development, and to the other parent at another period of development.
Theories Regarding Transmission by Parents as Male and Female. —Various theories have been advanced to the effect that in generation the male progeny determines the character of certain organs, and of other features of the organization, and that the female parent likewise determines the nature of yet other features and characteristics of the organization.
Chief among those theories is the following:
- That the male parent influences chiefly the external characters of the offspring and the female the internal characters of the same. According to this theory the male parent chiefly determines the nature of the bony framework, its covering and locomotion, and consequently its appearance, while the female parent chiefly determines the internal structures, as the vital and digestive organs, thus controlling very largely the stamina and growth of the animal. Such propagation is done as it were in parts, one parent determining certain characters of the organization and the other parent determining other characters of the same.
- That one parent will chiefly determine the character of the forehead and organs of sense along with the vital and nutritive organs, while the other parent chiefly determines the character of the back of the head and also the locomotive organs. This theory claims also that which parent will produce these peculiarities will depend somewhat on sameness of blood, difference in blood, and closeness of relationship.
- That propagation is clone, as it were, by halves, that is to say, that each parent gives to the offspring the shape of one half of the body more or less. According to this theory as propounded by certain of its advocates the male parent generally determines the character of the back, loins and hindquarters, the size, skin and general shape, while the female chiefly determines the character of the forequarters, head, vital and nervous system.
In other words, this theory virtually claims that the female parent determines chiefly the nature of the anterior part of the body including what may be termed the higher features of the organization, while the male parent determines chiefly the nature of the posterior parts, and what may be termed the lower features of the same
Orton propagated the first theory, the second by Walker and the third by Spooner and others. They all agree in claiming that in transmission certain features of the organization are more influenced by one parent than by the other, but when they attempt to particularize regarding the organs affected, the disagreement is most marked. Other theories have been propounded which only tend to further complicate and obscure the question.
Objections to Theories Advanced Above. —The probable if not indeed the absolute incorrectness of the theories just submitted may be shown without great difficulty.
- It is evidenced in the marked lack of agreement in the theories themselves and in the advocates of what is practically the same theory. There is a wide gap between the theories as enunciated. While they all rest on a substratum of the idea that propagation is done by halves, they differ most widely as to what constitutes the half. For instance, Orton is positive that size is governed chiefly by the female parent, and Spooner is equally positive that it is governed by the male parent. Nor have the advocates of any of those theories sustained them by arguments strong and convincing.
- It is evidenced in the influence of a prepotent male on the whole organization, that is to say, on internal structure as well as external form, on the higher as well as the lower parts of the organization, and on the anterior as well as the posterior parts of the being. Mate a vigorous pure-bred sire with a grade female whose blood elements are much mixed, and the whole being of the progeny will bear the stamp of the male upon it. The same will be manifest in the external form, in the color, size and locomotion of the progeny, and in vital, digestive, and nervous action.
Reverse the process and there will be a like preponderance of resemblance to the female in all the avenues of the being of the progeny. This one argument alone should prove fatal to any theory that claims that one parent, in virtue of its sex, influences only certain characters in the progeny.
- It is further evidenced in the fact of the antagonism of several of those theories to what has been ascertained regarding the progress of development in the embryo. But the discussion of this phase of the question cannot be considered here.
Practical Deductions. —From what has been advanced the conclusion is inevitable that at the present time it would not be safe to attribute a preponderance of influence in transmission to either male or female in virtue of its sex. From the whole ground gone over it is apparent: —
- That the relative influence of parents upon the offspring depends upon conditions that cannot always be determined. Potency is sometimes absent when all the conditions would seem to favor its presence, and in other instances it is present when the conditions are against it.
- The transmission of characters, resembling the parent in which they have become dominant are likely to prevail. This is but another way of saying that the most prepotent parent is likely to have the greater influence in determining the character of the offspring. The guaranties of prepotency, as purity of blood and superior individual vigor, will therefore ordinarily be the strongest guaranties of likeness in transmission by either parent in the progeny.
- On the other hand, this will not exclude the inheritance of peculiarities from either or both parents other than those which are dominant. Particularly will this be true in cross breeding. The unexpected will then happen more frequently than in other lines of breeding.
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