WAEC SSCE - Biology - 2019 (Objective)

Question 1 Report

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

all the offspring being tall with red fruits

25 % tall with red fruits

50 % tall with red fruits

75 % tall with red fruits

Answer Details

The cross between RrTt and rrtt will result in 50% of the offspring being tall with red fruits. The capital R and T represent dominant alleles for red colour and tallness, respectively, while the small r and t represent recessive alleles for white colour and shortness, respectively. When RrTt (heterozygous for both traits) is crossed with rrtt (homozygous recessive for both traits), the possible gametes for RrTt are RT, Rt, rT, and rt, while the gametes for rrtt are rt only. When the gametes combine, we get the following possible genotypes for the offspring: RT, Rt, rT, and rt. Of these, RT and rT will result in tall plants because they contain at least one dominant allele for tallness, while Rt and rt will result in short plants. Similarly, the genotypes RT and Rt will result in red fruits, while rT and rt will result in white fruits because red color is dominant over white. So, the possible genotypes for the offspring are: RT, Rt, rT, and rt, with the phenotypic ratio being 1:1:1:1 for tall with red fruits, tall with white fruits, short with red fruits, and short with white fruits, respectively. Therefore, 50% of the offspring will be tall with red fruits, which is the correct answer.

Read lesson note on Genetic Terminologies (WAEC)

Read lesson note on Chromosomes: The Basis Of Heredity (WAEC)






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