In guinea pigs, the allele for a rough coat (R) is completely dominant over the allele for… — JAMB UTME

In guinea pigs, the allele for a rough coat ( $R$ ) is completely dominant over the allele for a smooth coat ( $r$ ). A monohybrid cross is performed between two heterozygous rough-coated parents ( $Rr \times Rr$ ). What is the expected ratio of homozygous dominant individuals to heterozygous individuals among the offspring possessing the rough coat phenotype?

$1 : 2$ Answer
B
$3 : 1$
C
$1 : 3$
D
$1 : 1$

Answer

The expected ratio of homozygous dominant individuals to heterozygous individuals among the rough-coated offspring is 1 : 2.

In a cross between two heterozygous parents (

Rr \times Rr

), the expected offspring genotypic breakdown is

1\,RR : 2\,Rr : 1\,rr

. The rough coat phenotype comprises both the

1\,RR

and

2\,Rr

individuals. Comparing homozygous dominant (

RR

) to heterozygous (

Rr

) within this rough phenotype class gives a ratio of

1 : 2

Step-by-Step Solution

Determine the parental genotypes and set up the monohybrid cross.

The cross between two heterozygous parents is represented as

Rr \times Rr

Both parents carry one dominant allele (

R

) and one recessive allele (

r

Determine the genotypic ratio of the F1 generation using Mendel's Law of Segregation.

The resulting offspring genotypes are

1\,RR : 2\,Rr : 1\,rr

Gametes segregate independently during meiosis, giving a

25\%

chance for

RR

50\%

chance for

Rr

, and

25\%

chance for

rr

Identify the subset of offspring that display the rough coat phenotype.

Rough-coated offspring include genotypes

RR

(homozygous dominant) and

Rr

(heterozygous), totaling

3

out of

4

expected outcomes.

Because allele

R

is completely dominant, both

RR

and

Rr

express the rough coat phenotype.

Calculate the ratio of homozygous dominant (

RR

) to heterozygous (

Rr

) within the rough-coated phenotype group.

Ratio of

RR : Rr = 1 : 2

For every

1

homozygous dominant (

RR

) rough-coated individual, there are

2

heterozygous (

Rr

) rough-coated individuals.

Key Concept

Mendel's Law of Segregation and Monohybrid Genotypic Ratios

Estimated Time:1m 0s

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