In rabbits, the allele for short hair (H) is completely dominant over the allele for long … — JAMB UTME

In rabbits, the allele for short hair ( $H$ ) is completely dominant over the allele for long hair ( $h$ ). A pure-breeding long-haired female rabbit is crossed with a heterozygous short-haired male rabbit to produce an $F_1$ generation. If two short-haired offspring from this $F_1$ generation are subsequently mated to produce an $F_2$ generation, what is the expected ratio of homozygous dominant to heterozygous genotypes strictly among the short-haired $F_2$ individuals?

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

Answer

The expected ratio of homozygous dominant to heterozygous genotypes among the short-haired

F_2

individuals is

1 : 2

Crossing the pure-breeding long-haired female (

hh

) with the heterozygous short-haired male (

Hh

) produces short-haired offspring that are all heterozygous (

Hh

). Intercrossing these short-haired

F_1

individuals (

Hh \times Hh

) generates

F_2

genotypes in the proportion

1\ HH : 2\ Hh : 1\ hh

. Among only the short-haired individuals (

HH

and

Hh

), the proportion of homozygous dominant (

HH

) to heterozygous (

Hh

) is

1 : 2

Step-by-Step Solution

Determine the genotypes of the parental generation and the

F_1

short-haired offspring.

Parental cross is

hh \times Hh

. Offspring genotypes are

50\%\ Hh

(short-haired) and

50\%\ hh

(long-haired). Therefore, all short-haired

F_1

individuals must be heterozygous (

Hh

Mendel's Law of Segregation dictates that the homozygous recessive parent contributes only

h

gametes while the heterozygous parent contributes

H

h

gametes.

Determine the genotypic distribution of the

F_2

generation from crossing two short-haired

F_1

rabbits (

Hh \times Hh

The

F_2

genotypic distribution is

1/4\ HH

1/2\ Hh

, and

1/4\ hh

(

1\ HH : 2\ Hh : 1\ hh

Random fertilization between gametes carrying

H

and

h

alleles yields the standard monohybrid

F_2

genotypic ratio.

Filter for the target phenotype (short-haired offspring) and calculate the ratio between

HH

and

Hh

genotypes.

Short-haired

F_2

individuals comprise

1/4\ HH

and

2/4\ Hh

. Comparing these two genotypes gives

1\ HH : 2\ Hh

, or a ratio of

1 : 2

The question specifies restricting the ratio calculation strictly to the short-haired individuals, excluding the

hh

(long-haired) individuals from the denominator.

Key Concept

Conditional probability and genotypic ratio determination in monohybrid inheritance

Estimated Time:1m 30s

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