Petal coloration in foxgloves is determined by three genes:
1. M encodes an enzyme, which synthesizes anthocyanin, the purple pigment is seen in petals, m/m produces no pigment. This results in the phenotype of albino with yellowish spots.
2. D is an enhancer of anthocyanin, which results in darker pigments; d/d does not enhance. In contrast, at the third locus, w/w allows pigment deposition in petals, but W prevents pigment deposition except in the spots. So the results in the white, spotted phenotype is observed in the following crosses.
In the following two crosses:
Cross 1:
M/-; D/-; w/w (dark purple) X m/m; ?/?; ?/? (White with yellowish spots)
F 1 :
½ M/-; D/-; w/w dark purple
½ M/-; d/d; w/w light purple
The presence of color in the progeny indicates that there is an absence of W allele in the parents. And the ratio indicates that one gene in the parents is heterozygous. The presence of d/d indicates a light purple progeny giving the genotypes
Parent:
M/M; D/d; w/w X m/m; d/d; w/w
F 1 :
½ M/m; D/d; w/w
½ M/m; d/d; w/w
Cross 2:
When m/m; ?/?; ?/? (White with yellowish spots) X M/-; d/d; w/w (light purple)
F 1 :
½ M/-; ?/?; W/- white with purple spots
¼ M/-; D/-; w/w dark purple
¼ M/-; d/d; w/w light purple
The ratio of dark colored to the light purple colored plant indicates that one of the parents must be heterozygous, while a 1:1 ratio indicates that it is a test cross. The presence of purple color in all the plants shows that one parent is homozygous (M/M).
Therefore, the genotypes are:
If a cross between m/m; D/d; W/w X M/M; d/d; w/w
F 1 :
½ M/m; -/d; W/w
¼ M/m; D/d; w/w
¼ M/m; d/d; w/w