In tomato plants, the allele for red fruit (R) is completely dominant over the allele for … — JAMB UTME

In tomato plants, the allele for red fruit ( $R$ ) is completely dominant over the allele for yellow fruit ( $r$ ). If a heterozygous red-fruited plant is self-pollinated and produces $400$ offspring in the $F_2$ generation, how many of these plants are expected to be heterozygous red-fruited?

$200$ Answer
B
$300$
C
$100$
D
$400$

Answer

The expected number of heterozygous red-fruited plants is 200.

Crossing two heterozygous individuals (

Rr \times Rr

) produces genotypes in a

1\ RR : 2\ Rr : 1\ rr

ratio according to Mendel's First Law. Therefore, half (

50\%

) of the

400

total offspring are expected to have the heterozygous genotype (

Rr

), which equals

200

plants.

Step-by-Step Solution

Determine the parental genotypes and set up the monohybrid cross.

The self-pollinated plant is heterozygous (

Rr

), so the cross is

Rr \times Rr

Mendel's First Law (Law of Segregation) states that paired alleles segregate during gamete formation so each gamete carries only one allele.

Calculate the genotypic ratio of the F2 generation.

The offspring genotypic ratio is

1\ RR : 2\ Rr : 1\ rr

, meaning

1/4

homozygous dominant,

2/4

(or

1/2

) heterozygous, and

1/4

homozygous recessive.

Random combination of parental gametes (

R

and

r

) yields probabilities of

25\%\ RR

50\%\ Rr

, and

25\%\ rr

Compute the expected count of heterozygous offspring out of 400 total plants.

Heterozygous count =

\frac{2}{4} \times 400 = 200

plants.

Multiplying the probability of the heterozygous genotype (

1/2

) by the total number of offspring gives the expected number.

Key Concept

Mendel's Law of Segregation and Monohybrid Genotypic Ratios

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