Adapted from Question 22. Essentials of Genetics, 9th ed., Klug et al., Pearson.
Labrador retrievers may be black, brown, or golden in color. This coat color is controlled by an interaction
of two genes at independent loci. Gene B encodes dark color, where a dominant allele codes for black and
recessive codes for brown. Gene E programs the expression of color, where a dominant allele results in
expression (either black or brown, depending on the genotype at the B locus), and recessive prevents
expression (resulting in a yellow coat color, regardless of the genotype at the B locus).
Although each color may breed true, many different outcomes occur in different matings where the
parents are not true breeding. The following results show some of the possibilities.
Parent Mating Progeny Outcome
(a) Black x Brown all black
(b) Black x Golden all black
(c) Black x Brown 1/2 black, 1/2 brown
(d) Black x Brown 3/4 black, 1/4 golden
(e) Brown x Brown 3/4 brown, 1/4 golden
(f) Black x Golden 2/4 golden, 1/4 black, 1/4 brown
(g) Black x Golden 4/8 golden, 3/8 black, 1/8 brown
(h) Black x Black 9/16 black, 4/16 golden, 3/16 brown
Select two of these matings to discuss in your paper. One mating must be (a), (b), (c), or (d); the other
must be (e), (f), (g), or (h). For each mating, indicate the parental genotypes as completely as possible, and
how this produces the corresponding progeny outcome. Describe the basis for these results. (i.e. Explain
the reasoning you used to arrive at your conclusion). Although not required, it may be helpful to start with
keyouts to determine the genotypes that would breed true for each color.
Requirements
for Evaluation
Possible
Points
Content:
ï§ Discussion demonstrates mastery of related course concepts
o Genotypes, gamete types, and gene interactions are described with
sufficient detail to indicate their contribution to the corresponding
phenotypic ratio
o Appropriate terms are included and used correctly
ï§ Response is focused, well-developed, and includes specific detail
ï§ Overall structure is clear, logical, and well organized
ï§ Body of completed paper is 300-500 words
ï§ No references necessary
14
Style:
ï§ Clarity: clear, easy to understand
ï§ Structure: varied sentence pattern
ï§ Tone: professional and polished; personal style; maintains interest
3
Mechanics:
ï§ Correct grammar, syntax, spelling, and punctuation
3
