In guinea pigs (*Cavia porcellus*), the allele for rough coat (R) is completely dominant o… — JAMB UTME

In guinea pigs (*Cavia porcellus*), the allele for rough coat ( $R$ ) is completely dominant over the allele for smooth coat ( $r$ ). Match each parental cross listed in the left column with its correct expected offspring genotypic or phenotypic ratio in the right column.

Heterozygous rough coat ( $Rr$ ) $\times$ Heterozygous rough coat ( $Rr$ )3 : 1 phenotypic ratio of rough coat to smooth coat
Heterozygous rough coat ( $Rr$ ) $\times$ Homozygous smooth coat ( $rr$ )1 : 1 phenotypic ratio of rough coat to smooth coat
Homozygous rough coat ( $RR$ ) $\times$ Homozygous smooth coat ( $rr$ )100% rough coat phenotype (100% $Rr$ genotype)
Homozygous rough coat ( $RR$ ) $\times$ Heterozygous rough coat ( $Rr$ )100% rough coat phenotype (1 : 1 genotypic ratio of $RR : Rr$ )

Answer

Heterozygous cross (

Rr \times Rr

) matches 3 : 1 phenotypic ratio of rough coat to smooth coat; Heterozygous testcross (

Rr \times rr

) matches 1 : 1 phenotypic ratio of rough coat to smooth coat; Pure-breeding cross (

RR \times rr

) matches 100% rough coat phenotype (100%

Rr

genotype); Dominant backcross (

RR \times Rr

) matches 100% rough coat phenotype (1 : 1 genotypic ratio of

RR : Rr

Each parental genotype combination directly produces specific gamete combinations according to Mendel's Law of Segregation. Crossing two heterozygotes (

Rr \times Rr

) yields a 3 : 1 phenotypic ratio; crossing a heterozygote with a recessive homozygote (

Rr \times rr

) yields a 1 : 1 ratio; crossing pure-breeding opposite homozygotes (

RR \times rr

) produces 100% heterozygous offspring (

Rr

); and crossing a homozygous dominant with a heterozygote (

RR \times Rr

) produces 100% dominant phenotypically with equal proportions of

RR

and

Rr

genotypes.

Step-by-Step Solution

Determine gametes and Punnett square for

Rr \times Rr

Gametes are

R

and

r

from each parent. Offspring genotypes:

1\ RR : 2\ Rr : 1\ rr

Mendel's Law of Segregation states alleles segregate during gamete formation, resulting in a 3 : 1 phenotypic ratio of dominant (rough) to recessive (smooth).

Determine offspring for testcross

Rr \times rr

Gametes from heterozygous parent are

R

and

r

; gametes from homozygous recessive parent are all

r

. Offspring genotypes:

1\ Rr : 1\ rr

Half the offspring express the dominant phenotype and half express the recessive phenotype, giving a 1 : 1 ratio.

Determine offspring for

RR \times rr

All gametes from

RR

are

R

, all gametes from

rr

are

r

. All offspring are

Rr

100% of the F1 generation is heterozygous and displays the dominant rough coat phenotype.

Determine offspring for

RR \times Rr

Gametes are

R

(from

RR

) and

R, r

(from

Rr

). Offspring genotypes:

1\ RR : 1\ Rr

All offspring inherit at least one

R

allele, so 100% show the rough coat phenotype, with a 1 : 1 ratio between

RR

and

Rr

genotypes.

Key Concept

Mendel's First Law (Law of Segregation) and Monohybrid Cross Ratios

Question

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Step-by-Step Solution

Key Concept

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