In pea plants (Pisum\ sativum), seed shape and seed color inherit independently according … — JAMB UTME

In pea plants ( $Pisum\ sativum$ ), seed shape and seed color inherit independently according to Mendel's Second Law. If two plants heterozygous for both round seeds ( $R$ ) and yellow seeds ( $Y$ ) are crossed ( $RrYy \times RrYy$ ), what is the expected phenotypic ratio among the $F_2$ offspring?

$9 : 3 : 3 : 1$ (Round Yellow : Round Green : Wrinkled Yellow : Wrinkled Green)Answer
B
$3 : 1$ (Round Yellow : Wrinkled Green)
C
$1 : 2 : 1$ (Round Yellow : Round Green : Wrinkled Green)
D
$1 : 1 : 1 : 1$ (Round Yellow : Round Green : Wrinkled Yellow : Wrinkled Green)

Answer

The expected phenotypic ratio in the

F_2

generation of a dihybrid cross (

RrYy \times RrYy

) is

9 : 3 : 3 : 1

According to Mendel's Law of Independent Assortment, two pairs of traits segregate independently of each other during gamete formation. Crossing two dihybrids (

RrYy \times RrYy

) yields 16 total fertilization outcomes. Phenotypically, these break down into 9 dominant-dominant (Round Yellow), 3 dominant-recessive (Round Green), 3 recessive-dominant (Wrinkled Yellow), and 1 recessive-recessive (Wrinkled Green), giving the standard

9 : 3 : 3 : 1

ratio.

Step-by-Step Solution

Determine the gametes produced by each heterozygous parent (

RrYy

Each parent produces four distinct types of gametes in equal proportions:

RY

Ry

rY

, and

ry

Mendel's Law of Independent Assortment states that allele pairs separate independently during gamete formation.

Combine the gametes in a

4 \times 4

Punnett square to find the phenotypic combinations.

Out of 16 total offspring combinations: 9 express both dominant traits (Round Yellow), 3 express dominant seed shape and recessive seed color (Round Green), 3 express recessive seed shape and dominant seed color (Wrinkled Yellow), and 1 expresses both recessive traits (Wrinkled Green).

Dominant alleles (

R

and

Y

) mask the expression of recessive alleles (

r

and

y

) when present.

Write the resulting phenotypic ratio.

The final phenotypic ratio is

9 : 3 : 3 : 1

Combining independent monohybrid ratios

(3:1) \times (3:1)

yields the classic

9:3:3:1

dihybrid ratio.

Key Concept

Mendel's Law of Independent Assortment and Dihybrid Phenotypic Ratio

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic