Questions: Dominant and Recessive Traits Introduction

Question 1

Two parents both have brown eyes, but they have a child with blue eyes. How is this possible?

• A. It is not possible — at least one parent must have blue eyes
• B. Both parents carry a hidden recessive allele for blue eyes (Bb), and the child inherited the recessive allele from each parent (bb)
• C. The child's eye color was determined by the environment, not genes
• D. Blue eyes are dominant over brown

Answer: (B)

If brown (B) is dominant over blue (b), both parents can have the genotype Bb — they appear brown-eyed because they have a dominant B allele, but they each also carry a recessive b allele. When both parents happen to pass their b allele to a child, the child is bb and has blue eyes. This occurs with a 25% probability for each child of two Bb parents.

Question 2

A dominant allele is typically more common in a population than a recessive allele.

• T. True
• F. False

Answer: False

Dominant and recessive describe how alleles interact when paired — not how common they are. A dominant allele can be rare (like the allele for polydactyly, or extra fingers), and a recessive allele can be very common. The terms describe gene expression, not population frequency.

Question 3

What is the difference between homozygous and heterozygous?

Model answer: Homozygous means having two identical alleles for a gene (BB or bb). Heterozygous means having two different alleles (Bb). A heterozygous individual shows the dominant trait but carries the recessive allele and can pass it to offspring.

This distinction is crucial for predicting trait inheritance. Homozygous dominant (BB) and heterozygous (Bb) individuals both show the dominant trait, but only heterozygous individuals carry the recessive allele. Two heterozygous parents can produce homozygous recessive offspring — this is how 'hidden' traits appear in families.