Question

In Andalusian fowl, B is the gene for black plumage (head feathers) and B' (pronounced "B prime") is the gene for white plumage. These genes, however, show incomplete dominance. The heterozygous (BB') condition results in blue plumage. List the genotypic and phenotypic ratios expected from the following crosses: a) black x blue b) blue x blue c) blue x white

Answer 1

INCOMPLETE DOMINANCE INHERITANCE:

5.  In Andalusian fowl, B is the gene for black plumage (head feathers) and B' (pronounced "B prime") is the gene for white plumage.  These genes, however, show incomplete dominance.  The heterozygous (BB') condition results in blue plumage.  List the genotypic and phenotypic ratios expected from the following crosses:  a) black x blue  b) blue x blue  c) blue x white

6. In snapdragons, petal color is determined by a single gene locus with two alleles making the "red" allele (R) incompletely dominant to the "white" allele (r).  Heterozygotes have petals, which are neither red nor white, but pink.  a) If a true-breeding red flower is pollinated with pollen from a white flower: What fraction of the seeds (F1 generation) would be expected to produce red-flowered plants?  What fraction of the gametes produced by the F1 plants would be expected to bear the R allele?  b) If two pink flowered plants are crossed, what genotypic and phenotypic ratios are expected among the offspring (F1 generation)?

Answer 2

Answer:

a. Phenotypic and Genotypic ratio= 1:1

b. Phenotypic and Genotypic ratio= 1:2:1

c. Phenotypic and Genotypic ratio= 1:1

Explanation:

The crosses are monohybrid involving a single gene coding for plumage color in Andalusian fowl. The alleles for this gene exhibit incomplete dominance, a genetic scenario whereby an allele does not completely mask the expression of another, instead it blends with it forming an intermediate phenotype.

In this case, allele B is for black plumage

allele B' is for white plumage

In an heterozygous state i.e. combined allelic state (BB'), an intermediate blue plumage is formed.

A black fowl will possess a genotype BB

A blue fowl will possess a genotype BB'

A white fowl will possess a genotype B'B'

In the crosses between these fowls as highlighted in the question above (see attached image), the following genotypic and phenotypic ratio will be produced by the resulting offsprings.

a) Black (BB) × Blue (BB') : 1:1 (2 Black, 2 Blue)

b) Blue (BB') × Blue (BB') : 1:2:1 (1 Black, 2 Blue, 1 White)

c) Blue (BB') × White (B'B') : 1:1 (2 Blue, 2 White)