INHERITANCE ON CROSSING 55 



combinations TT and Tt in equal numbers, in a large 

 number of cases. Similarly, ^^-ith a pollen-grain t, it is 

 an even chance whether any particular combination NviU 

 be tT or U. We have then the combinations TT, Tt, tT, 

 and tt in equal numbers. But the second and third 

 combinations are the same, so that we have really 

 only three possible combinations, which occur in the 

 ratio — 



i, or 25 per cent. T T . . . " pure '* tails. 

 I, or 60 „ Ti . . . hybrid tails. 

 J, or 25 „ 1 1 . , . dwarfs. 



Tills is our MendeHan ratio. 



The case is analogous to a series of tossings of two 

 coins. In a hundred of such tossings we may expect 

 to get fairly nearly the result — 



2 heads 25 times. 



head and tail 50 ?) 



2 tails 25 „ 



Each male or female reproductive ceU may be, as it 

 were, either a head or a tail. The " two heads " corre- 

 spond to the pure tall plants, the head and tail to the 

 hybrid or impure tails, and the two tails results are the 

 dwarfs. 



Such is the generally accepted theory of MendeHan 

 inheritance. We are able to test its value by other 

 matings. Suppose, for instance, that we were to cross 

 our intermediate form of primula with the pure " star " 

 type. If A represent the presence of a factor which 

 produces the wavy, " Chinese " type of petal, and a its 

 absence, the pure Chinese type wiU be AA, and the 

 " star " type aa. The intermediate, hybrid form Aa 

 being crossed with the star form aa, the possible com- 

 binations are Aa and aa, with equal chances of each. 

 We might therefore expect 50 per cent, of the inter- 

 mediate type and 50 per cent, star, which is what 

 actually occurs. We can now give a hst of all the 

 possible matings, and the results expected and found. 

 We may represent the dominant condition generally by 



