MECHANISM OF INHERITANCE

Hawke's Bay Tribune, Volume XVII, 3 December 1927, Page 12

MECHANISM OF INHERITANCE

Counting The Culls

By

“Sundowner"

JN a lecture given to the Dundee branch of the British Medical Association, and published at' length in the British Medical Journal, F. E. A. Crew, M.D., DSc., Ph.D., F.S.R.Ed., Animal Breeding Research Dept. University of Edinburgh, gave a most interesting record of his findings in connection with what he terms the "mechanism of inheritance.” The lecture bristles with scientific and technical terms which are more or less unintelligible to the layman, but boiled down to everyday language, the paper deals with the transmission of "dominant” and “recessive” hereditary characteristics to offspring, more especially where crossing of individuals having these traits is undertaken. The lecturer, for the sake of illustration, took as an example the results of crossing a species of fruit fly with which he had conducted his exepriments, but stressed the fact that the same results were obtainable right up, through the insect and animal kingdoms.

EXAMPLES IN CATTLE. For the sake of making his findings clearer to the average farmer we have chosen cattle, in which dominant and recessive colouring is a breed peculiarity, as a medium through which to illustrate the results of crossing. A dominant characteristic is one which is a breed feature which has become fixed through many generations of transmission, or as in a wild state; thus it might b» said that black is the "dominant colour of the Polled Angus breed as is also the polled or hornless head. Black, again, is a dominant colour when compared with red, which latter is therefore termed “recessive,” and in a cross in which a black polled male and a red polled female were mated we would expect a black colouring to dominate the offspring. As all farmers who breed cattle are aware, this is so in the first generation, when the progeny of black and red polled beasts are uniform ly black and polled. Mate, however, a male and a female of this first cross, and their offspring would follow a definite average in colouring, states Dr. Grew. Three out of every four on an average would combine the black colouring with the polled head, but the fourth (or 25 per cent, of the progeny) would be red polled. Again, take a horned breed, such as the Holstein, which combines the dominant colours white and black in its coat, and cross this with a Red Polled beast (Norfolk). The whole of the resultant first generation would be black and white polled cattle. -This would be so because the polled characteristic is dominant to the horned in cattle. Again, if a male and female of this generation (black and white polled) were crossed the result would average: Black and white polled 9, red polled 3, black and white horned 3, and horned red 1, out of every 16 of the progeny. Should the breeder use a male of the first generation (black and white polled) and cross him Avith a horned red cow such as the Devon (in which both horns and red colour are dominant )the resultant calves would be 25 per cent, black and white polled, 25 per cent, polled red, 25 per cent, horned black and white, and 25 per cent, horned red. Still again, should a Red Devon (horned) male be mated with a Black Polled female, the whole of the calves would exhibit the dominant characteristics of black colour and polled head, but if a male from these were again crossed with a horned red female, the progeny would be black polled and horned red in equal proportions. CALCULABLE RESULTS. The doctor goes much further than this in tabulating the various percentag-ss of true crosses and what w» are used to calling "throwbacks” where various combinatii »s of dominant and recessive cLwrcteristics are blended, but trie above are sufficient to show lfir-1 he claims that the results of crossing are not haphaz» id as so many of us have thought but with almost any com-

MORE LIGHT ON STOCK BREEDING

(Written for the Tribune. All Rights Reserved.)

bination of characteristics can be calculated mathematically. There is no necessity to go deeply into the functions of the chromatin in inheritance, the theory of which is known as the chromosome theory of hereditary, but it is sufficient to state that it is claimed that all hereditary characteristics are transmitted to the offspring via the chromatin, and just as dominant traits, such as colour, and hornlessness, may be calculated with certainty from a dross, so can other dominant features of conformation or con stitution. It will readily be seen how valuable this knowledge could be in the hands of an intelligent breedei of stock. New types of animals combining the good qualities of different breeds, could be evolved with a practical certainty of per petuating and fixing the dominant and valuable characteristics without the disheartening loss of time and money spent in pursuing experiments in crossing which so often prove to be but culs-de-sac. PRESENT CROSSING DIFFICULTIES. All farmers who have dabbled in the interesting and disappointing business of cross breeding with stock know how simple it frequently appears with any two breeds of animal to get a first cross apparently embodying the advantages of both types. This has been demonstrated to almost every North island farmer who has crossed pure Romney and Lincoln sheep ; but having secured a comparatively uniform type of first cross animals, the problem of maintaining their excellence in their offspring is a very different matter. The qualities which we describe as excellent are the "dominant” characteristics of the two breeds which have been crossed. Thus a half-breed MerinoEnglish Leicester sheep would inherit the fineness and density of the wool of the Merino, but it would also inherit the hornless head of the English Leicester, the "polled” condition being dominant, and the horned characteristic of the Merino recessive. Besides this it would inherit a compact, meaty carcase from the English Leicester, this quality having become dominant in the breed from generations of breeding with this sole object in view. Fineness of bone is dominant in both breeds hence we would expect to find this intensified in the progeny, and as all breeders know this is so, and has to be carefully guarded against in breeding hulfbreds. To revert to the cross RomneyLincoln again. If a male and fe male of the first cross are mated, the lambs next generation should consist of 75 per cent, embodying the dominant characteristics of the two breeds and 25 per cent, with recessive characters. It is here where the flock become mixed in type, and these "recessives” must be ruthlessly culled if ultimate uniformity and “fixing” of type is to be attained.

In practice many farmers attempt to secure uniformity by mating these recessives to a pure ram of the opposite type; thus those ewes which had inherited the Lincoln features would be mated to a Romney, and vice versa. But it can be seen how hopeless this effort would ultimately prove, for the resultant lambs would inherit an infinitely greater tendency towards the charactertistics both dominant and recessive, of the ram used. LINE BREEDING. It is here where line breeding comes to the aid of the breeder for he choses a male displaying the dominant features which he wishes to perpetuate, and mates him with a similar female from the same sire. In other words, a half-sister or perhaps a full-sister if he intends inbreeding; in which case there is still greater probability pf securing true-type progeny. By closely following the examples given by Dr. Crew, the dominant features of each type could be ultimately esta fished in the offspring, and the number of “re-

cessives” or culls which would have to be rejected could be accurately calculated. Indiscriminate crossing in both cattle and sheep has been, and is to-day, the bane of New Zealand, and though we badly need new types or breeds suitable for our varying climates *nd pastures, these cannot be arrived at haphazard, but only through a thorough knowledge of the “mechanism of inheritance,”