vi] MENDELIAN HEREDITY 79 



coloured head and shoulders and a coloured stripe 

 along the spine. The self and hooded factors are an 

 allelomorphic pair independent of colour, so that 

 a hooded rat may be black or grey. The factors may 

 also be borne by albinos, and when very young an 

 albino bearing the factor for the hood may be dis- 

 tinguished by the different texture of the hair on 

 the head and shoulders, giving the appearance of 

 a water-mark or ' ghost-hood/ The heterozygote 

 between self-coloured and hooded patterns differs from 

 either parent, being black above and white below 

 the so-called ' Irish ' type of the fancy. Such ' Irish ' 

 rats bred together always give both self-coloured and 

 hooded rats in addition to Irish among their progeny. 

 A similar case in rabbits is that of the well-known 

 Dutch marking, which seems to correspond with the 

 hooded condition in rats. In flowers the number of 

 such characters determining the nature and distribu- 

 tion of colour may be considerable, so that among 

 the offspring of a cross between two varieties of 

 Chinese primulas or snapdragons a very large 

 colour-series may be produced, which on first in- 

 spection may seem a continuous series from the 

 darkest to the palest ; but careful analysis of these 

 cases has shown that the different factors may be 

 recognised and isolated, and the series of colours 

 falls strictly within the rules of Mendelian inherit- 

 ance when each factor is considered alone. 



