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 two heterozygous red-fruited plants ( $Rr$ ) are crossed, what is the expected genotypic ratio of their offspring?

A
$3 : 1$
$1 : 2 : 1$ Answer
C
$1 : 1$
D
$9 : 3 : 3 : 1$

Answer

The expected genotypic ratio among the offspring is

1 : 2 : 1

(

1\ RR : 2\ Rr : 1\ rr

Crossing two heterozygous parents (

Rr \times Rr

) segregates alleles into gametes such that

25\%

of offspring receive

RR

(homozygous dominant),

50\%

receive

Rr

(heterozygous), and

25\%

receive

rr

(homozygous recessive). This results in a

1 : 2 : 1

genotypic ratio.

Step-by-Step Solution

Identify parental genotypes and alleles

Both parents are heterozygous (

Rr

), producing gametes containing either the

R

allele or the

r

allele with equal probability (

50\%

R

50\%

r

According to Mendel's Law of Segregation, alleles separate during gamete formation.

Construct a Punnett square for

Rr \times Rr

The possible genetic combinations are

1\ RR

2\ Rr

, and

1\ rr

Combining male and female gametes yields

1/4\ RR

2/4\ Rr

, and

1/4\ rr

Determine the genotypic ratio

The genotypic ratio is

1\ RR : 2\ Rr : 1\ rr

, which simplifies to

1 : 2 : 1

Genotypic ratio accounts for actual allele combinations regardless of physical appearance.

Key Concept

Mendel's Law of Segregation and Monohybrid Genotypic Ratios

Estimated Time:45s

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