The cross between RrTt and rrtt where R is a gene for red colour and T for tallness will result in______

Analyze the question: This is a dihybrid test cross. RrTt is crossed with the double recessive rrtt. We need the percentage of offspring that are both tall and red, meaning they show both dominant traits.

Concept: In a test cross, the recessive parent (rrtt) produces only one type of gamete, rt. So the offspring's phenotypes depend entirely on the gametes of the heterozygous parent (RrTt).

Step 1: Gametes

• RrTt produces: RT, Rt, rT, rt (each ¼, assuming independent assortment)
• rrtt produces: rt only

Step 2: Offspring

| Gametes from RrTt | Offspring | Phenotype | |---|---|---| | RT | RrTt | Red, Tall | | Rt | Rrtt | Red, short | | rT | rrTt | Not red, Tall | | rt | rrtt | Not red, short |

Step 3: Ratio The phenotypic ratio is 1:1:1:1, so each phenotype makes up 25%.

Red and tall (RrTt) = 25%

Answer: Option B, 25% tall with red fruits.

Exam tip: In a dihybrid test cross (heterozygote × double recessive), always expect a 1:1:1:1 ratio. Every phenotype class is 25%, so the "both dominant traits" class is always 25%. Options like "all" or "75%" can be eliminated quickly.