Question

If parents have free earlobes and the offspring has attached earlobes, then the parents must be
A. Homozygous
B. Heterozygous
C. Co-dominant
D. Nullizygous

Answer 1

Free earlobe is a dominant trait while attached earlobe is a recessive trait. Free ear lobe parents will be heterozygous for the trait if they have offspring with attached earlobes.

Complete answer: The allele which is responsible for the attached or free earlobe is U. If a person has a free earlobe, then its genotype would be either UU or Uu.
If a person has an attached earlobe then its genotype would be uu.
So, if parents (male as well as female) have free earlobes and offspring have attached earlobes, then surely both parents would be heterozygous for this trait.
That means the genotype of both the parents would be Uu.
Let us try to understand with the help of Punnett square.

Male/Female	U	u
U	UU Free earlobe	Uu Free earlobe
U	Uu Free earlobe	uu Attached earlobe

Additional information: Many times someone with a dominant trait has a hidden recessive allele. If they give that on (and the other parent does too), then the child will be with the recessive trait. So two parents with attached earlobes should not be eligible to have a child with unattached ones.

So the correct answer is B. Heterozygous

Note: Attached or free earlobe trait is an autosomal trait. If the lower part of the ear is attached directly to the head then it is attached to the earlobe, but if it has free-hanging it is called free earlobe. The ear lobe is made up of cartilage.