228 ESSENTIALS OF BIOLOGY 



mare X male ass gives the hybrid mule, but the cross female ass X 

 stallion gives the hybrid Hinny, which is a different kind of 

 animal. Similar results may come from the crossings of cage 

 birds and the sign of the crossing may even affect the fertility itself. 



80. ON MEN DELI AN HYBRIDITY 



It will be convenient to speak of " Mendelian categories " and 

 these are best illustrated by the results of the classical pea- 

 experiments. The plant, Pisum sativum^ is a systematic species 

 defined by an ensemble of morphological characters which we 

 shall call E. But this specific category can be split up into finer 

 ones, each defined by certain special characters, or small ensembles 

 of characters, that distinguish it from the others. Thus the pea- 

 plants may be tall ones {i) or dwarf ones {d). They may bear peas 

 that are green {g), or yellow [y), or round (r), or wrinkled {w). Thus 

 we may have categories : pea-plants with the characters, E, t,g,r ; 

 E, t, y, r ; E, d, g, r ; E, dy jy, r, etc. That is, there are combina- 

 tions of the special characters {t, d, jy, g, r, w) that define each 

 category, but all the categories exhibit the ensemble E. Later on 

 we shall speak of the characters of the ensemble as being " in- 

 tegrated " ones and of the special characters as being " loose " 

 ones. Mendelian stud es deal, first, with the results of crossing 

 different Mendelian categories, and, second, with associated 

 cytological results. 



Pea-plants can be bred by cross-fertilization, the pollen of A 

 fertilizing the ovules of B, or vice versa. Or they can be self- 

 fertilized (" selfed ") when the pollen of A fertilizes the ovules 

 of A. 



Typical experiments. We consider only two characters, yellow- 

 ness of the peas and greenness of the peas, (i) When a plant that 

 is known to bear yellow peas only is crossed with a plant that 

 is known to bear green peas only a Mendelian hybrid is produced. 

 This hybrid bears only yellow peas, but it has also the potentiality 

 of green peas. The character (y) is said to be " dominant " over 

 the character (^), (which is called *' recessive "). 



(2) The yellow peas from this experiment are sown and plants 

 are raised from them. These plants are then selfed and allowed 

 to bear peas and it is found that about one-fourth of all the 

 peas that they bear are green and that about three-fourths 



