In watermelon plants (*Citrullus lanatus*), solid green rind color (G) is dominant over st… — JAMB UTME

In watermelon plants (*Citrullus lanatus*), solid green rind color ( $G$ ) is dominant over striped rind color ( $g$ ), and short fruit shape ( $R$ ) is dominant over long fruit shape ( $r$ ). If a watermelon plant heterozygous for both traits ( $GgRr$ ) is self-pollinated and yields $800$ offspring, how many of these offspring are expected to possess solid green rind and long fruit shape?

A
$450$
$150$ Answer
C
$50$
D
$600$

Answer

150 offspring exhibit the solid green rind and long fruit shape phenotype.

In a dihybrid cross between two heterozygous parents (

GgRr \times GgRr

), the genes assort independently, yielding an

F_2

phenotypic ratio of

9 : 3 : 3 : 1

(9 solid green short : 3 solid green long : 3 striped short : 1 striped long). The proportion of offspring showing solid green rind (

G\_

) and long fruit (

rr

) is

\frac{3}{16}

. Among

800

offspring, the expected number is

\frac{3}{16} \times 800 = 150

Step-by-Step Solution

Determine the parental genotypes and set up the cross.

Parental cross is

GgRr \times GgRr

Self-pollination of a dihybrid plant involves crossing two organisms heterozygous for both genes.

Calculate the expected phenotypic ratio for the

F_2

generation according to Mendel's Second Law.

The standard dihybrid phenotypic ratio is

9 : 3 : 3 : 1

Genes for rind color and fruit shape assort independently during gamete formation.

Identify the probability of the target phenotype (solid green rind, long fruit).

Target phenotype genotype is

G\_rr

, which corresponds to a probability of

\frac{3}{16}

Dominant for rind color (

G\_

) has probability

\frac{3}{4}

, and recessive for fruit shape (

rr

) has probability

\frac{1}{4}

. Thus,

\frac{3}{4} \times \frac{1}{4} = \frac{3}{16}

Multiply the phenotype probability by the total offspring count.

\frac{3}{16} \times 800 = 150

Multiplying total progeny count by the expected probability yields the expected phenotype frequency.

Key Concept

Mendel's Law of Independent Assortment and Dihybrid Phenotypic Calculations

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