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 ( $S$ ) is dominant over long fruit shape ( $s$ ). A pure-breeding plant with solid green, short fruits is crossed with a plant having striped, long fruits. The resulting $F_1$ plants are self-pollinated to produce an $F_2$ generation of 800 offspring. How many of the $F_2$ plants are expected to produce striped, short watermelons?

150Answer
B
450
C
200
D
50

Answer

150

In a classical dihybrid cross involving two heterozygous parents (

GgSs

), independent assortment yields an F2 phenotypic ratio of 9:3:3:1. The phenotype with one recessive trait and one dominant trait (striped, short; genotype

ggS\_

) represents

3/16

of the total population. Multiplying this fraction by 800 gives

(3/16) \times 800 = 150

plants.

Step-by-Step Solution

Identify parental and F1 genotypes

Parental cross is

GGSS \times ggss

, yielding an

F_1

genotype of

GgSs

Pure-breeding parents pass one dominant or recessive allele per gene to the offspring.

Determine the F2 phenotypic ratio using Mendel's Law of Independent Assortment

Selfing

GgSs \times GgSs

yields a 9:3:3:1 phenotypic ratio (9 solid/short : 3 solid/long : 3 striped/short : 1 striped/long).

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

Calculate expected offspring count for the target phenotype

Expected count =

(3 / 16) \times 800 = 150

plants.

The target phenotype (striped, short) represents 3 out of 16 total phenotypic combinations.

Key Concept

Mendel's Law of Independent Assortment and Dihybrid Phenotypic Ratios

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