34 



THE AGRICULTURAL NEWS. 



Febeuaey 6, 1909. 



the next generation, give plants with coloured Howers 

 only, and (2) those which, on breeding among them- 

 selves, behave like the original hybrids, and produce 

 plants, some of which have coloured dowers and others 

 white, the two kinds being present at the rate of three 

 of the former to one of the latter. 



Now to turn for a moment to the second case, which 

 is somewhat similar, but relates to animals instead 

 of plants. Rose-combeil bantam fowls are of two 

 kinds, black and white. On crossing a pure-bred 

 black bird with a pure white, the offspring are all 

 black, similar to the black parent. In the second 

 generation, bred from these hybrids, both black and 

 white birds are present, the former being three times 

 as numerous as the latter. The case here is exactly 

 similar to that of the peas already mentioned. The 

 white birds breed true, while the blacks are of two 

 classes : (1) those which breed true, and (2) those which 

 resemble the original hybrids, in that, when mated 

 together, they give blacks and whites in the ratio of 

 three to one. 



(|)ualities which have been shown by experiment 

 to be transmissible in the manner described, as colour 

 and whiteness in the Howers of the pea, and blackness 

 and whiteness of plumage in the case of rose-combed 

 bantams, are known as Mendelian characters. That 

 quality or characteristic which alone is apparent in the 

 first generation produced from crossing the original 

 parents, e.g., colour in the pea flowers, and blackness 

 in the plumage of the bantams, is known as the ' domi- 

 nant ' character, while the alternative characteristic, 

 which disappears in the first generation, but again 

 reappears in a stable form in a definite proportion of 

 the individuals comprising the second generation, 

 i.e., whiteness of the pea Howers, or whiteness of 

 plunjage in the bantams, is referred to as the 

 ' recessive ' ciualily 



The .-ibtive facts having been observed, the ne.xt 

 step is to Hud a theory which shall satisfactorily account 

 for them and for similar j)henomena. Mendel was able 

 to put forward a simple explanation, which has since 

 been proved many times over. The formation of a new 

 individual (animal or plant) is the result of the union 

 of two germ-cells, the spermatazoon or pollen grain in 

 the case of the male, and the ovum or egg-cell from the 

 female. In these single cells are necessarily contained 

 the characteristics contributed to the offspring by the 

 male and female parents, respectively. Now in the 

 examples already given, we are dealing with the 

 iidieritance of alternative characters, i.e., colour or 

 whiteness in the pea flower, .-uid ])lackness or whiteness 



of plumage in the bantam fowls. The central idea of the 

 Mendelian theor^is that any given germ-cell can contain 

 only one of theslt-alternative characteristics, or ' unit 

 characters ' as thfey are termed. To return to the case of 

 the garden peas :, a germ-cell contributed by a plant of 

 pure strain with coloured flowers will contain the 

 character of ' colour ' only, while germ-cells from 

 a similarly pure plant with white blooms will contain 

 the quality of whiteness ' only. When in the process of 

 crossing a ' coloured ' germ meets a ' coloured ' germ, 

 the result is a coloured flower. Similarly, when two 

 ' white ' germs meet, a white-flowered plant necessarily 

 results. 



If now a ' coloured ' germ meets a ' white ' germ, as 

 in the hybridization experiments referred to, the 

 resulting plant bears coloured flowers, because colour is 

 dominant to whiteness in this case. .Such a plant 

 possesses both alternative characters, but cannot trans- 

 mit them in a blended form, and in the germ cells 

 formed by this plant the ' unit characters ' separate 

 out. Half the germ cells of each se.x will possess the 

 quality of colour, and the remaining halves, that of 

 whiteness. When the resulting hybrids are bred among 

 themselves, therefore, the two sets of germ-cells come 

 together, and, according to a simple mathematical law, 

 this can only lead to the production of a number of 

 ])lants, one quarter of which result from the union of 

 two 'coloured ' cells, one quarter from two ' white 'cells, 

 and two quarters by the union of a 'coloured' and 

 a ' white.' The first and second classes are in each 

 case pure bred plants, breeding true to type, and yield- 

 ing respectively coloured and white flowers only. But 

 the remaining two quarters are similar to the hybrid 

 j)lants, possessing the qualities of both colour and 

 whiteness, but appearing with coloured flowers because 

 'colour' is dominant to whiu-ness. These will 

 necessarily breed in an ex.nctly similar manner to the 

 j)arent hybrids. 



Such is Mendel's explanation of the manner in 

 which simple ' unit characters ' are said to be inherit- 

 ed. During tlie past few years this has been 

 demonstrated to be true for such varied morphological 

 and physiological characters as structure, size, shape, 

 cdIoui'. and Icrtility among plants, as well as for 

 a luimber of (jualities in animals. For examj)le, it has 

 been shown that tallness and dwarfness are JMendelian 

 ch.-iracters in sweet peas, the former being dominant ; 

 ami that in horses, bay or brown colour is dominant 

 to chestnut. 



(A concliulinL,'- articlo dealing with the .same .-^ubjoLt will 

 fiiriii tlu' eilitnriiil in the next issue.) 



