In an isolated bird population, the allele for a crested head (C) is completely dominant o… — Armed Services Vocational Aptitude Battery (ASVAB)

In an isolated bird population, the allele for a crested head ( $C$ ) is completely dominant over the allele for a smooth head ( $c$ ). Field biologists observe that only 2% of the population possesses a crested head. A researcher mates a heterozygous crested bird with a smooth-headed bird. Which of the following identifies the expected phenotypic probability for their offspring and correctly explains the low prevalence of the dominant trait in the wild?

50% crested, 50% smooth; The $C$ allele has a low frequency in the gene pool, demonstrating that dominance controls individual expression rather than population prevalence.Answer
B
75% crested, 25% smooth; Because the trait is dominant, the $C$ allele will inherently multiply and eventually become the most common phenotype in the population.
C
50% crested, 50% smooth; Dominant traits always appear in the majority of a population, meaning the 2% field observation must be scientifically inaccurate.
D
100% crested, 0% smooth; The dominant $C$ allele completely suppresses the recessive $c$ allele in the population, making the smooth-headed phenotype mathematically impossible in this cross.

Answer

50% crested, 50% smooth; The

C

allele has a low frequency in the gene pool, demonstrating that dominance controls individual expression rather than population prevalence.

The correct answer accurately calculates the 50% crested and 50% smooth phenotypic ratio from a

Cc \times cc

cross. It also correctly states that dominant alleles only determine phenotypic expression in a heterozygote, not the frequency of the allele in the population's gene pool.

Step-by-Step Solution

Determine the genotypes of both parent birds based on the provided text.

The crested parent is heterozygous (

Cc

). The smooth-headed parent expresses the recessive phenotype, so it must be homozygous recessive (

cc

Genotypes are required to set up a genetic cross and predict offspring probabilities.

Calculate the expected phenotypic ratio using a Punnett square for the cross

Cc \times cc

The offspring have a 50% chance of inheriting

Cc

(crested) and a 50% chance of inheriting

cc

(smooth).

Mendelian segregation dictates that the heterozygous parent contributes the

C

allele 50% of the time and the

c

allele 50% of the time, while the homozygous recessive parent always contributes the

c

allele.

Evaluate the conceptual statements regarding the low population frequency (2%) of the dominant crested trait.

Dominance describes how alleles interact within a single organism's genotype, not how frequently that allele appears in the entire population. Therefore, a dominant trait can be rare if the dominant allele itself is rare in the gene pool.

This resolves the apparent contradiction in the prompt and eliminates options relying on the misconception that dominant traits must be common.

Key Concept

Mendelian Inheritance and Allele Frequency

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