Reproduction of Angtcup^rms M 



MENDELISM modern experimental study of Heredky, based on the 



work of Gregor Mendd (1823-1869), Praelat of BrUnn, who worked on /* 

 4tt/nw*i in the monastery garden; paper pobBihad 1865, forgotten till 1900. The 

 int to record and interpret observation* on the progeny of hybrids of the first 

 generation (F,), and their successors by interbreeding to F,. F., etc 



BxperimenUU RctTUlU : Hybrids are not merely mixtures of parental charmderB, 

 but exhibit Domnuuut \ i.e. one chancier of a complementary pair is normally 

 presented to the exclusion of the other; such a pair is termed an Allelomorph ; the 

 prevailing character is said to be Dominant (D), and the one masked or really 

 wanting Reoatsive (H). 



The organism is thus conceived as built up of unit-characters as ' factors ', which 

 may be isolated for experimental purposes, though any such recognisable character 

 must imply a group of co-ordinated factors, and some may be more readily examined 

 experimentally than others. The 'factor* remains undefined except as it gives 

 experimental results. 



Numerical results for dominance in succeeding generations were 



explained by Mendel as suggesting that a gamete carries only one unit of an 

 allelomorphic pair ; or, in other words, has the character or has it not An organism 

 inheriting the same factor from both parents is so far pure for that character, or has 

 a ' double dose ' (kome*ygoU\ and produces gametes all bearing it, of one son only. 

 An organism inheriting a character from one parent only is httsrotygou, and produces 

 gametes of two kinds, one with the character and one without, assumed 50 X of each. 



Monohybridism. Taking one character only into consideration ; if D and R 

 are parental characters of the a* and 9 respectively, or met versa (reciprocal cross), 

 the gametes will be all D and R respectively ; the F, generation will be DR, with 

 the bodily appearance of the D. 



The gametes of F, as parents will be (D, R) o*, and (D, R) j. giving 4 possible 

 unions for F r as DD, DR RD, RR, in mathematically equal proportions (15 % 

 each) if the law were absolute; i. e. 75 % would carry the dominant character 

 (phenotypc), s$ % would be recessive and distinct Of the phenotype, 25 X would 

 be homozygote (genotype) and equally pure and fixed ; but 50 % being still mixtures 

 like the parents (DR), would be similarly liable to segregate according to thU 1:2:1 

 ratio in the F, generation. The homozygotes DD and RR, when interbred among 

 themselves, remain true. 



This ' Law of Mendelian Segregation ' was deduced by Mendel from his experi- 

 mental observations. It holds very approximately so long as the factors are reliable : 

 it so far explains the reversion of hybrids to the parent strains, but it makes no advance 

 in making anything new. A mixed strain, self-fertilized, ' Mendefizes out '. 



Dihybridism : Of much greater importance is the case of the consideration of 

 two allelomorphs simultaneously, since this gives new combinations, as new strains 

 differing in visible characters, and so far new forms, which may be valuable com- 

 mercially if they can be maintained pure ; i. e. come true to seed. 



If A. B. are two separate factors, present, their absence may be marked by SK b ; if 

 one such factor is characteristic of each parent, ile original given homozygote parents 

 may be described as Ab and Ba, either being the o^ or 9. These would produce 

 gametes as Ab and B*, respectively, both o" and $. The F, generation win be all 

 of the form ABab, in any case, with both dominant characters expressed. But the 

 gametes of this F, generation, taking any one expression of both pairs, will be A B. 

 Ab, aB, ab. both o" and ?. Combination of these in all possible unions gives 

 a schedule of 16 possibilities, included as: 



The Sixteen Square : From this it follows that A of the whole F, 



will carry the dominant characters of both strains ; A will be wholly recessive ; of the 



1 character, and the other A to 

 other parent, and will be readily recognized ; or the set will segregate according to a 



remainder ^ will be dominant to one parental character, and the other A to the 



9:3:3:1 ratio, this being again closely approximated in experimental work, to the 

 extent that if it does not come out it may be fairly assumed that there was something 

 with the ' factors ' dealt with, or some other law intervenes. 



of the whole in each set will be homozygote and so fiw true, as indicated 



at 



wrong wt 

 Also 



