On the zygotic constitution of dominant and recessive whites etc. 239 
in their white-yellow crosses of the silkworm, and, as was expected, 
both F, whites and yellows segregated into whites and yellows in Fo, 
the white being dominant in the white series and recessive in the 
yellow series. These are the results described by KELLOGG as “reversal 
of dominance, splitting of colours equally in first cross, impure recessive 
behaviour in the second hybrid generations”. 
A parallel case has been observed by KEEBLE and PELLEW (9) in 
their Primula crosses. They crossed the white Primula, “Snow King” 
with various colour-flowered varieties, such as ‘Crimson King”, 
“Reading pink”, “2A”, and “Pink Siellata”. Some of them (Snow 
king =< Crimson Pink, or Reading Pink >< Snow King) gave mixed F, 
broods consisting of whites and coloureds in the ratio 1:1, some 
(2A >< Snow King) all coloured F, and the remainder (Pink Sted/ata = 
Snow King) all white Fy. Both white and yellow F, thus produced 
segregated into whites and yellows in F,, in the white matings the 
proportion being 13 W:3 Y and in the yellow, 1 W:3 Y. 
From the facts above described, and the deductions drawn from 
them we may safely say that the cause of the varying dominance 
of certain white breeds of the silkworm is due to the presence of 
certain breeds in which various whites differing in their zygotic con- 
stitutions are mixed together. Such mixed breeds are practically 
white, but zygotically are heterozygous or mixed forms. Therefore, 
mated with the yellow they will give both mono- and dihybrid off- 
spring whose outward appearance is the same. For instance, of the 
ratio 3 W:1 Y or 1 W:1 Y there are two kinds, one being monohybrid 
and the other dihybrid forms as is shown in former pages. Such 
cases cause great confusion to persons engaged in experiment of 
this kind. 
Summary. 
I. Among white breeds of the silkworm, which ToyAma (10a) 
has already described, there are two kinds, one being dominant and 
another recessive towards yellows. There are others in which both 
dominant and recessive whites are mingled together. 
2. The cause of dominant whites may be due to the presence of 
some factor which prevents the appearance of colour, even in the 
case where the colour factor is present, as has been already observed 
in plants and animals by many biologists. 
3. If we represent the presence and the absence of the suppressing 
factor by S and s and the presence and the absence of the yellow 
