Normal length of fur in rabbits is controlled by the dominant allele R, and a short type of fur called "rex" is determined by the recessive allele r. The dominant allele B is responsible for black fur colour, while the recessive allele b determines brown colour. What are phenotypic ratios resulting from a cross between a homozygous rabbit with normal length of black fur and rex rabbit with brown fur?

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Question: Normal length of fur in rabbits is controlled by the dominant allele R, and a short type of fur called "rex" is determined by the recessive allele r. The dominant allele B is responsible for black fur colour, while the recessive allele b determines brown colour. What are phenotypic ratios resulting from a cross between a homozygous rabbit with normal length of black fur and rex rabbit with brown fur?

Genetic Information Given:

R = dominant allele for normal fur

r = recessive allele for rex fur (short fur)

B = dominant allele for black fur

b = recessive allele for brown fur

Parent Genotypes:

Genotype of homozygous normal black fur rabbit Genotype: RRBB

Genotype of rex brown fur rabbit (both recessive traits): rrbb

Cross:

RRBB × rrbb

By applying Mendel's Law of Independent Assortment, we make a Punnett square using gametes:

Gametes from RRBB → Only one type: RB
Gametes from rrbb → Only one type: rb

When crossed:

RB × rb → RrBb

So, all offspring (F1 generation) will have genotype RrBb.

Phenotype of RrBb:

Rr = expresses normal fur (R is dominant)

Bb = expresses black colour (B is dominant)

Final Phenotypic Ratio:

Since all F1 offspring have the same genotype (RrBb), the phenotypic ratio is 100% normal black fur.

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