In a case of complete dominance, what is the phenotypic ratio of the cross Bb X Bb; where B =black and b = white?

Step 1: Analyze the question We are crossing two heterozygous parents (Bb × Bb). Complete dominance means the dominant allele (B, black) fully masks the recessive allele (b, white) in heterozygotes.

Step 2: The concept This is a Mendelian monohybrid cross. Each parent produces two types of gametes, B and b, in equal numbers. Offspring genotypes are found with a Punnett square.

Step 3: Punnett square

| | B | b | |---|---|---| | B | BB | Bb | | b | Bb | bb |

Genotypic ratio: 1 BB : 2 Bb : 1 bb

Step 4: Phenotypes

• BB → black
• Bb → black (B is completely dominant)
• bb → white

So 3 black : 1 white.

Checking the other options:

• A and C include "grey" or "blue" phenotypes. These would only appear with incomplete dominance or codominance, which the question rules out.
• B (1 black : 3 white) reverses the correct ratio.

Answer: D. 3 blacks : 1 white

Exam tip: A cross of two heterozygotes (Aa × Aa) with complete dominance always gives a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio. If the question mentions intermediate colours such as grey or blue, it's incomplete dominance, and the phenotypic ratio becomes 1:2:1.