84 Journal of Agriculture. [lo Feb., 1909. 



HEREDITY. 



It has been shown that in the fertilised ovum and therefore in every cell 

 in the adult body the chromosomes are derived half from the mother and 

 half from the father. If we regard the chromosomes as conveying the 

 special characters of each parent we might expect to find that the offspring 

 is a blend of its parents and ancestors. In fact Galton has framed a noted 

 law of ancestral inheritance which states that the two partnts contribute 

 between them on the average one half of the total heritage of the offspring ; 

 the four grandparents one quarter ; the eight great grandparents one eighth, 

 and so on. It should be noted, however, that this law would only hold 

 true if a large number of cases were taken and the average computed. 

 Elaborate statistical investigation has shown that such qualities as size, 

 duration of life, and fertility, follow Galton's law. But it must 

 be admitted that certain qualities do not, and here we enter on controversial 

 matter. In 1865 Gregor Johann Mendel read a paper on the results he had 

 obtained with crossing peas. His work excited no interest until 1900, 

 when the experiments were repeated and confirmed and MendeV s law, as 

 it is called, gaining a footing in biological circles. In Mendel's law there 

 are two distinguishing features. One is that certain characters may he 

 dominant and others recessive. Thus a grey mouse mated with a_ white 

 mouse will have grey offspring. Greyness here is dominant and whiteness 

 recessive ; but the offspring are true hybrids for they will have some white 

 among their offspring. The second feature in Mendel's law is that it 

 allows us to calculate the distribution of certain characters amongst the off- 

 spring. An example or two will make this point clear. A black fowl of a 

 certain tvpe mated with a splashed white of a certain type will give a 

 hybrid which the fancier in his ignorance has termed pure-bred Andalusian. 

 If we mate Andalusian with Andalusian the result is that the offspring 

 arise in the following ratios — one black, two Andalusians, one splashed 

 white. If the parental characters are represented by D (dominant) and R 

 (recessive) then if D is mated with R the offspring will all be DR. If DR 

 be mated with DR then the offspring will be DD, DR, RD, RR, which 

 may be written DD, 2DR, RR ; if the dominance be very m.arked the pro- 

 portion will appear as three dominants to one recessive. Take as another 

 example the susceptibility of wheat to " rust." The quality of immunity 

 is recessive, that of predisposition is dominant. When an immune strain 

 and a predisposed strain are crossed the resulting hvbrids are all sus- 

 ceptible. But if these hybrids are allowed to self-fertilize then dominant 

 susceptibles and recessive immune plants are produced in the calculated 

 ratio of three to one. A more complex example is that in which a pea 

 plant vielding green and round peas is crossed with one yielding yellow 

 and wrinkled peas. Now round and yellow are both dominant, whilst 

 wrinkled and green are both recessive. The hybrid therefore will be 

 yellow and round. If such a hybrid pea plant be self-fertilized the 

 progeny will be found to follow the proportions which can be calculated, 

 namely 9 yielding yellow and round peas, 3 yielding yellow and wrinkled, 

 3 yielding green and round, and one yielding green and wrinkled. 



It is a matter of the greatest importance that future research in this 

 field should give such results that the breeder will know what characters 

 follow Mendel's law and what do not. As an example of an exception 

 mav be mentioned the crossing of Border Leicester rams and Cheviot ewes 

 giving a hybrid which breeds true to its type. Also the various characters 

 may, in the future, be classified according to their degree of dominance or 

 recessiveness. Thus, in the case of poultry, rose comb, white plumage, 

 feathered shanks, and brown eggs are supposed to be dominant as against 

 the recessive leaf comb, black plumage, bare shanks, and white eggs. 



